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Full text of "Airplane photography"

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FACULTY OF FORESTRY 
UNIVERSITY OF TORONTO 



AIRPLANE PHOTOGRAPHY 



Digitized by the Internet Archive 

in 2007 with funding from 

Microsoft Corporation 



http://www.archive.org/details/airplanephotograOOivesuoft 



AIRPLANE 
PHOTOGRAPHY 



BY 

HERBERT E. IVES 

MAJOR, AVIATION SECTION, SIGNAL OFFICERS RESERVE CORPS, UNITED STATES 
army; lately officer in charge of EXPERIMENTAL DEPART- 
MENT, PHOTOGRAPHIC BRANCH, AIR SERVICE 



208 ILLUSTRATIONS 




PHILADELPHIA AND LONDON 
J. B. LIPPINCOTT COMPANY 






COPTBIQHT, 1920, BT J. B. LIPPINCOTT COMPANY 



PBINTKD BY J. B. LIPPINCOTT COMPANY 

AT THE WASHINGTON SQUARE PRESS 

FHIXjADELiFHIA, XT. S. A. 



TO MY WIFE 

A HELPFUL CRITIC, EVEN THOUGH SHE 
NEITHER PHOTOGRAPHS NOR FLIES 



PREFACE 

Airplane photography had its birth, and passed through 
a period of feverish development, in the Great War. Prob- 
ably to many minds it figures as a purely military activity. 
Such need not be the case, for the application of aerial 
photography to mapping and other peace-time problems 
promises soon to quite overshadow its military origin. It 
has therefore been the writer's endeavor to treat the sub- 
ject as far as possible as a problem of scientific photog- 
raphy, emphasizing those general principles which will 
apply no matter what may be the purpose of making photo- 
graphs from the air. It is of course inevitable that who- 
ever at the present time attempts a treatise on this newest 
kind of photography must draw much of his material from 
war-time experience. If, for this reason, the problems and 
illustrations of this book are predominantly military, it 
may be remembered that the demands of war are far more 
severe than those of peace; and hence the presumption is 
that an account of how photography has been made suc- 
cessful in the military plane will serve as an excellent guide 
to meeting the peace-time problems of the near future. 

It is assumed that the reader is already fairly conversant 
with ordinary photography. Considerable space has indeed 
been devoted to a discussion of the fundamentals of photog- 
raphy, and to scientific methods of study, test, and speci- 
fication. This has been done because aerial photography 
strains to the utmost the capacity of the photographic 
process, and it is necessary that the most advanced methods 
be understood by those who would secure the best results 
or contribute to future progress. No pretence is made that 

7 



8 PREFACE 

the book is an aerial photographic eneyclopsedia; it is not a 
manual of instructions; nor is its appeal so popular as it 
would be were the majority of the illustrations striking 
aerial photographs of war subjects. It is hoped that the 
middle course steered has produced a volume which will be 
informative and inspirational to those who are seriously 
interested either in the practice of aerial photography or in 
its development. 

The writer is deeply in debt to many people, whose 
assistance of one sort or another has made this book possible. 
First of all should be mentioned those officers of the English, 
French and Italian armies through whose courtesy it is 
that he can speak at first hand of the photographic prac- 
tices in these armies at the front. It is due to Lieutenant 
Colonel R. A. Millikan that the experimental work of which 
the writer has had charge was initiated in the United States 
Air Service. To him and to Major C. E. Mendenhall, under 
whom the work was organized in the Science and Research 
Division of the Signal Corps, are owing the writer's thanks 
for the opportunities and support given by them. A similar 
acknowledgment is made to Lieutenant Colonel J. S. Sullivan, 
Chief of the Photographic Branch of the Army Air Service, for 
his interest and encouragement in the compilation of this 
work, and for the permission accorded to use the air service 
photographs and drawings which form the majority of 
the illustrations. 

The greatest debt of all, however, is to those officers who 
have formed the staff of the Experimental Department. To 
mention them by name: Captain C. A. Proctor, who was 
charged with our foreign liaison, and who acted as deputy 
chief during the writer's absence overseas; Captain A. K. 
Chapman, in charge of the work on optical parts, and later 
chief of our Rochester Branch; Captain E. F. Kingsbury, 



PREFACE 9 

who had immediate charge of camera development; Lieu- 
tenant J. B. Brinsmade and Mr. R. P. Went worth, who 
handled the experimental work on camera mountings and 
installation; Lieutenant A. H. Nietz, in charge of the 
Langley Field Laboratory of the Experimental Department; 
Mr. R. B. Wilsey and Lieutenant J. M. Hammond, who, with 
Lieutenant Nietz, carried on the experimental work on 
sensitized materials. A large part of what is new and what is 
ascribed in the following chapters to "The American Air 
Servce " is the work of this group of men. Were individual 
references made, in place of this general and inclusive one, 
their names would thickly sprinkle these pages. It has been 
a rare privilege to have associates so able, enthusiastic, 
and loyal. 

THE AUTHOR 

November, 1919 



CONTENTS 

I. INTRODUCTORY 

chapter page 

1. General Survey 15 

2. The Airplane Considered as a Camera Platform 20 

II. THE AIRPLANE CAMERA 

3. The Camera^ — General Considerations 39 

4. Lenses for Aerial Photography 44 

5. The Shutter 68 

6. Plate-Holders and Magazines 87 

7. Hand-Held Cameras for Aerial Work 95 

8. Non-Automatic Aerial Plate Cameras 102 

9. Semi-Automatic Aerial Plate Cameras ^ . . . 116 

10. Automatic Aerial Plate Cameras 124 

11. Aerial Film Cameras 130 

12. Motive Power for Aerial Cameras 145 

13. Camera Auxiliaries 163 

III. THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS 

It. Theory and Experimental Study of Methods of Camera Suspension . 179 

15. Practical Camera Mountings , 193 

16. Installation of Cameras and Mountings in Planes 208 

IV. SENSITIZED MATERIALS AND CHEMICALS 

17. The Distribution of Light. Shade and Color in the Aerial View .... 221 

18. Characteristics cf Photographic Emulsions 227 

19. Filters 239 

20. Exposure cf Aerial Negatives 247 

21. Printing Media 252 

22. Photographic Chemicals 257 

V. METHODS OF HANDLING PLATES, FILMS AND PAPERS 

23. The Developing and Drying of Plates and Films 267 

24. Printing and Enlarging 279 



n CONTENTS 

VI. PRACTICAL PROBLEMS AND DATA 

25. Spotting 291 

26. Map Making 304 

27. Oblique Aerial Photography 320 

28. Stereoscopic Aerial Photography 329 

29. The Interpretation of Aerial Photographs 351 

30. Naval Aerial Photography 368 

VII. THE FUTURE OF AERIAL PHOTOGRAPHY 

31. Future Developments in Apparatus and Methods 383 

32. Technical and Pictorial Uses 388 

33. Exploration and Mapping , 401 



I 



INTRODUCTORY 



AIRPLANE PHOTOGRAPHY 

CHAPTER I 
GENERAL SURVEY 

Aerial Photography from Balloons and Kites. — Photog- 
raphy from the air had been developed and used to a limited 
extent before the Great War, but with very few exceptions 
the work was done from kites, from balloons, and from 
dirigibles. Aerial photographs of European cities had 
figured to a small extent in the illustration of giliidebooks, 
and some aerial photographic maps of cities had been made, 
notably by the Italian dirigible balloon service. Kites had 
been employed with success to carry cameras for photo- 
graphing such objects as active volcanoes, whose phenomena 
could be observed with unique advantage from the air, 
and whose location was usually far from balloon or 
dirigible facilities. 

As a result of this pre-war work we had achieved some 
knowledge of real scientific value as to photographic condi- 
tions from the air. Notable among these discoveries was 
the existence of a veil of haze over the landscape when seen 
from high altitudes, and the consequent need for sensitive 
emulsions of considerable contrast, and for color-sensitive 
plates to be used with color filters. 

The development of aerial photography would probably 
however have advanced but little had it depended merely 
on the balloon or the kite. As camera carriers their limita- 
tions are serious. The kite and the captive balloon cannot 
navigate from place to place in such a way as to permit the 

15 



16 AIRPLANE PHOTOGRAPHY 

rapid or continuous photography of extended areas. The 
kite suffers because the camera it supports must be manipu- 
lated either from the ground or else by some elaborate 
mechanism, both for pointing and for handling the exposing 
and plate changing devices. The free balloon is at the 
mercy of the winds both as to its direction and its speed of 
travel. The dirigible balloon, as it now exists after its 
development during the war, is, it is true, not subject to the 
shortcomings just mentioned. Indeed, in many ways it is 
perhaps superior to the airplane for photographic purposes, 
since it affords more space for camera and accessories, and 
is freer from vibration. It is capable also of much slower 
motion, and can travel with less danger over forests and 
inaccessible areas where engine failure would force a plane 
down to probable disaster. But the smaller types as at 
present built are not designed to fly so high as the airplane, 
and the dirigibles, both large and small, are far more ex- 
pensive in space and maintenance than the plane. For this 
one reason especially they are not likely to be the most 
used camera carriers of the aerial photographer of the future. 
Inasmuch as the photographic problems of the plane are 
more difficult than those of the dirigible and at the same time 
broader, the subject matter of this book applies with equal 
force to photographic procedure for dirigibles. 

Development of Airplane Photography in the Great War. 
— The airplane has totally changed the nature of warfare. 
It has almost eliminated the element of surprise, by render- 
ing impossible that secrecy which formerly protected the 
accumulation of stores, or the gathering of forces for the 
attack, a flanking movement or a "strategic retreat." To 
the side having command of the air the plans and activities 
of the enemy are an open book. It is true that more is 
heard of combats between planes than of the routine task 



GENERAL SURVEY 17 

of collecting information, and the public mind is more apt 
to be impressed by the fighting and bombing aspects of 
aerial warfare. Nevertheless, the fact remains that the chief 
use of the airplane in war is reconnaissance. The airplane is 
"the eye of the army." 

In the early days of the war, observation was visual. It 
was the task of the observer in the plane to sketch the out- 
lines of trenches, to count the vehicles in a transport train, 
to estimate the numbers of marching men, to record the 
guns in an artillery emplacement and to form an idea of 
their size. But the capacity of the eye for including and 
studying all the objects in a large area, particularly when 
moving at high speed, was soon found to be quite too small 
to properly utilize the time and opportunities available in 
the air. Moreover, the constant watching of the sky for the 
"Him in the sun" distracted the observer time and time 
again from attention to the earth below. Very early in the 
war, therefore, men's minds turned to photography. The 
all-seeing and recording eye of the camera took the place 
of the observer in every kind of work except artillery fire 
control and similar problems which require immediate 
communication between plane and earth. 

The volume of work done by the photographic sections 
of the military air service steadily increased until toward 
the end of the war it became truly enormous. The aerial 
negatives made per month in the British service alone 
mounted into the scores of thousands, and the prints dis- 
tributed in the same period numbered in the neighborhood 
of a million. The task of interpreting aerial photographs 
became a highly specialized study. An entirely new activity 
— that of making photographic mosaic maps and of main- 
taining them correct from day to day — usurped first place 
among topographic problems. By the close of the war 

2 



/^ 



18 AIRPLANE PHOTOGRAPHY 

scarcely a single military operation was undertaken without 
the preliminary of aerial photographic information. Photog- 
raphy was depended on to discover the objectives for 
artillery and bombing, and to record the results of the sub- 
sequent "shoots" and bomb explosions. The exact configu- 
rations of front, second, third line and communicating 
trenches, the machine gun and mortar positions, the "pill 
boxes," the organized shell holes, the listening posts, and 
the barbed wire, were all revealed, studied and attacked 
entirely on the evidence of the airplane camera. Toward the 
end of the war important troop movements were possible 
only under the cover of darkness, while the development 
of high intensity flashlights threatened to expose even 
these to pitiless publicity. 

Limitations to Airplane Photography Set by War Con= 
ditions. — The ability of. the pilot to take the modern high- 
powered plane over any chosen point at any desired altitude 
in almost any condition of wind or weather gives to the plane 
an essential advantage over the photographic kites and 
balloons of pre-war days. There are, however, certain dis- 
advantages in the use of the plane which must be overcome 
in the design of the photographic apparatus and in the 
method of its use. Some few of these disadvantages are 
inherent in the plane itself ; for instance, the necessity for 
high speed in order to remain in the air, and the vibration 
due to the constantly running engine. Others are peculiar 
to war conditions, and their elimination in planes for peace- 
time photography will give great opportunities for the 
development of aerial photography as a science. 

Chief among the war-time limitations is that of economy 
of space and weight. A war plane must carry a certain 
equipment of guns, radio-telegraphic apparatus and other 
instruments, all of which must be readily accessible. Many 



GENERAL SURVEY 19 

planes have duplicate controls in the rear cockpit to enable 
the observer to bring the plane to earth in case of accident 
to the pilot. Armament and controls demand space which 
must be subtracted from quarters already cramped, so that 
in most designs of planes the photographic outfit must be 
accommodated in locations and spaces wretchedly inade- 
quate for it. Economy in weight is pushed to the last ex- 
treme, for every ounce saved means increased ceiling and 
radius of action, a greater bombing load, more ammunition, 
or fuel for a longer flight. Hence comes the constant pres- 
sure to limit the weight of photographic and other apparatus, 
even though the tasks required of the camera constantly call 
for larger rather than smaller equipment. 

To another military necessity is due in great measure 
the forced development of aerial photographic apparatus 
in the direction of automatic operation. The practice of 
entrusting the actual taking of the pictures to observers 
with no photographic knowledge, whose function was 
merely to "press the button" at the proper time, necessi- 
tated cameras as simple in operation as possible. The 
multiplicity of tasks assigned to the observer, and in particu- 
lar the ever vital one of watching for enemy aircraft, made 
the development of largely or wholly automatic cameras 
the war-time ideal of all aerial photographic services. 
Whether the freeing of the observer from other tasks will 
relegate the necessarily complex and expensive automatic 
camera to strictly military use remains to be seen. 



CHAPTER II 



THE AIRPLANE CONSIDERED AS A CAMERA 

PLATFORM 

An essential part of the equipment of either the aerial 
photographer or the designer of aerial photographic appara- 
tus is a working knowledge of the principles and construction 
of the airplane, and considerable actual experience in the air. 




Upper .,...y 



ir n^fu 



^itr W'i:i.i 




Sfyjts 



& Qear--- 







\-\(.. 1. 



ck'nuTiLs of the plaiu". 



Conditions and requirements in the flying plane are far 
different from those of the shop bench or photographic 
studio. As a preliminary to undertaking any work on air- 
plane instruments a good text -book on the principles of 
flight should be studied. Such general ideas as are necessary 
for understanding the purly photographic problems are, 

however, outlined in the next paragraphs. 
20 



CAMERA PLATFORM 21 

Construction of the Airplane. — The modern airplane 
(Fig. 1) consists of one or more planes, much longer across 
than in the direction of flight {aspect ratio). These are 
inclined slightly upward toward the direction of travel, and 
their rapid motion through the air, due to the pull of the 
propeller driven by the motor , causes them to rise from the 
earth, carrying the fuselage or body of the airplane. In the 
fuselage are carried the pilot, observer, and any other load. 
Wheels below the fuselage forming the under-carriage or 
landing gear serve to support the body when running along 
the ground in taking off or landing. The pilot, sitting in 
one of the cockpits, has in front of him the controls, by means 
of which the motion of the plane is guided (Figs. 2 and 3). 
These consist of the engine controls — the contacts for the 
ignition, the throttle, the oil and gasoline supply, air pressure, 
etc., and the steering controls — the rudder bar, the stick 
and the stabilizer control. The rudder bar, operated by the 
feet, controls both the rudder of the plane, which turns the 
plane to right or left in the air, and the tail skid, for steering 
on the ground. The stick is a vertical column in front of the 
pilot which, when pushed forward or back, depresses or 
raises the elevator and makes the machine dive or climb. 
Thrown to either side it operates the ailerons or wing tips, 
which cause the plane to roll about its fore and aft axis. 
The stabilizer control is usually a wheel at the side of the 
cockpit, whose turning varies the angle of incidence of the 
small stabilizing plane in front of the elevator, to correct 
the balance of the plane under different conditions of loading. 

The fuselage consists usually of a light hollow framework 
of spruce or ash, divided into a series of bays or compart- 
ments by upright members, connecting the longerons, which 
are the four corner members, running fore and aft, of the 
plane. Diagonally across the sides and faces of these bays 



22 



AIRPLANE PHOTOGRAPHY 







CAMERA PLATFORM 23 

are stretched taut piano wires, and the whole structure is 
covered with canvas or Unen frabic. Cross- wires and fabric 
are omitted from the top of one or more bays to permit their 
being used as cockpits for pilot and observer. In later designs 
of planes the wire and fabric construction has been super- 
seded by ply-wood veneer, thereby strengthening the fuse- 
lage so that many of the diagonal bracing wires on the inside 



Fig. 4. — Biplane in fiight. 

are dispensed with. This greatly increases the accessibility 
of the spaces in which cameras and other apparatus must 
be carried. 

The fuselage differs greatly in cross-section shape and in 
roominess according to the type of engine. In the majority 
of English and American planes, with their vertical cylinder 
or V type engines, the fuselage is narrow and rectangular 
in cross-section. In many French planes, radial or rotary 
engines are used and the fuselage is correspondingly almost 
circular, and so is much more spacious than the English 



24 



AIRPLANE PHOTOGRAPHY 



and American planes of similar power. The shape and size 
of the plane body has an important bearing on the question 
of camera installation. 

Types of Planes. — The most common type of plane is 
the biplane (Fig. 4), with its two planes, connected by struts 
and wires, set not directly over each other, but staggered, 
usually with the upper plane leading. Monoplanes were in 
favor in the early days of aviation, and triplanes have been 




Fig. 5. — ^A single-seater. 

used to some extent. According to the position of the 
propeller planes are classified as tractors or pushers, tractors 
being at present the more common form. Planes are further 
classified as single-seaters (Fig. 5) , two-seaters, and three-seaters. 
These motor and passenger methods of classification are 
now proving inadequate with the rapid development of 
planes carrying two, three, and even more motors, divided 
between pusher and tractor operation, and carrying increas- 
ingly large numbers of passengers. Aside from structure, 
planes may be further classified according to their uses, as 



CAMERA PLATFORM 25 

scout, combat, reconnaissance, bombing, etc. Planes equipped 
with floats or pontoons for alighting on the water are called 
seaplanes (Fig. 182), and those in which the fuselage is boat- 
shaped, to permit of floating directly on the water, are 
flying boats (Fig. 183). 

The Plane in the Air. — The first flight of the photographic 
observer or of the instrument expert who is to work upon 
airplane instruments is very profitably made as a "joy ride," 
to familiarize him with conditions in the air. His experience 
will be somewhat as follows: 

The plane is brought out of the hangar, carefully gone 
over by the mechanics, and the engine "warmed up." The 
pilot minutely inspects all parts of the "ship," then climbs 
up into the front cockpit. He wears helmet and goggles, 
and if the weather is cold or if he expects to fly high, a heavy 
wool-lined coat or suit, with thick gloves and moccasins, or 
an electrically heated suit. The passenger, likewise attired, 
climbs into the rear cockpit and straps himself into the seat. 
He finds himself sitting rather low down, with the sides of 
the cockpit nearly on a level with his eyes. To either side 
of his knees and feet are taut wires leading from the controls 
to the elevator, stabilizer, tail skid and rudder. If the 
machine is dual control, the stick is between his knees, the 
rudder bar before his feet. None of these must he let his 
body touch, so in the ordinary two-seater his quarters are 
badly cramped. 

At the word "contact" the mechanics swing the pro- 
peller, and, sometimes only after several trials, the motor 
starts, with a roar and a rush of wind in the passenger's face. 
After a moment's slow running it is speeded up, the inter- 
mittent roar becomes a continuous note, the plane shakes 
and strains, while the mechanics hold down the tail to 
prevent a premature take-off. When the engine is properly 



26 AIRPLANE PHOTOGRAPHY 

warmed up it is throttled to a low speed, the chocks under 
the wheels are removed, the mechanics hold one end of the 
lower wing so that the plane swings around toward the field. 
It then "taxis" out to a favorable position facing into the 
wind with a clear stretch of field before it. After a careful 
look around to see that no other planes are landing, taking 
off, or in the air near by, the pilot opens out the engine, the 
roar increases its pitch, the plane moves slowly along the 
ground, gathers speed and rises, smoothly into the air. Near 
the ground the air is apt to be "bumpy," the plane may 
drop or rise abruptly, or tilt to either side. The pilot in- 
stantly corrects these deviations, and the plane continues to 
climb until steadier air is reached. 

At first the passenger's chief impressions are apt to be the 
deafening noise of the motor, the heavy vibration, the 
terrific wind in his face. If he raises his hand above the 
edge of the cockpit he realizes the magnitude of wind resist- 
ance at the speed of the plane, and hence the importance 
of the stream-line section of all struts and projecting parts. 

When he reaches the desired altitude the pilot levels off 
the plane and ceases to climb. Now his task is to maintain 
the plane on an even keel by means of the controls, correct- 
ing as soon as he notes it, any tendency to "pitch," 
to "roll" or to "yaw" off the course. The resultant path 
is one which approximates to level straight flying to a 
degree conditioned by the steadiness of the air and the skill 
of the pilot. If he is not skilful or quick in his reactions he 
may keep the plane on its level course only by alternately 
climbing and gliding, by flying with first one wing down and 
then the other, by pointing to the right and then to the left. 
The skilled photographic pilot will hold a plane level in 
both directions to within a few degrees, but he will do this 
easily only if the plane is properly balanced. If the load 



CAMERA PLATFORM ^7 

on the plane is such as to move the center of gravity too far 
forward with respect to the center of lift the plane will be nose- 
heavy, if the load is too far back it will be tail-heavy. Either 
of these conditions can be corrected, at some cost in efficiency, 
by changing the inclination of the stabilizer. When the plane 
reaches high altitudes in rare air, where it can go no further, 
it is said to have reached its ceiling. It here travels level 
only by pointing its wings upward in the climbing position, 
so that the fuselage is no longer parallel to the direction of 
flight. An understanding of these pecularities of the plane 
in flight is of prime importance in photographic map making, 
where the camera should be accurately vertical at all times. 

The direction and velocity of the wind must be carefully 
considered by the pilot in making any predetermined course 
or objective. The progress of the plane due to the pull of 
the propeller is primarily with reference to the air. If this 
is in motion the plane's ground speed and direction will be 
altered accordingly. In flying with or against the wind the 
ground speed is the sum or difference, respectively, of the 
plane's air speed (determined by an air speed indicator) and 
the speed of the wind. If the predetermined course lies 
more or less across the wind the plane must be pointed into 
the wind, in which case its travel, with respect to the earth, 
is not in the line of its fore and aft axis. The effect of "crab- 
bing, " as it is called, on photographic calculations is discussed 
later (Figs. 136 and 138). 

When the plane has reached the end of its straight 
course and starts to turn, its level position is for the moment 
entirely given up in the operation of hanking (Fig. 6). 
Just as the tracks on the curve of a railroad are raised on 
the outer side to oppose the tendency of the train to slip 
outward, so the plane must be tilted, by means of the 
ailerons, toward the inside of the turn. A point to be 



^8 



AIRPLANE PHOTOGRAPHY 



clearly kept in mind about a bank is that if correctly made 
a plumb line inside the fuselage will continue to hang 




Fig. 6.-r— Banking. 



vertical with respect to the floor of the plane, and not with 
respect to the earth, for the force acting on it is the combina- 



CAMERAPLATF OR M 29 

tion of gravity and the acceleration outward due to the turn. 
Only some form of gyroscopically controlled pointer, keep- 
ing its direction in space, will indicate the inclination of the 
plane with respect to the true vertical. If the banking is 
insufficient the plane will side slip outward or skid; if too 
great, it will side slip inward. 

As part of the " joy ride " the pilot may do a few " stunts," 
such as a "stall," a "loop," a " tail spin," or an "Immel- 
man." From the photographic standpoint these are of 
interest in so far as they bear on the question of holding the 
camera in place in the plane. The thing to be noted here is 
that (particularly in the loop), if these maneuvers are 
properly performed, there is little tendency toward relative 
motion between plane and apparatus. In a perfect loop it 
would, for instance, be unnecessary, due to the centrifugal 
force outward, for the observer to strap himself in. It is, 
however, unwise to place implicit confidence in the perfection 
of the pilot's aerial gymnastics. No apparatus should be 
left entirely free, although, for the reason given, compara- 
tively light fastenings are usually sufficient. 

When nearing the landing field the pilot will throttle 
down the engine and commence to glide. If he is at a con- 
siderable altitude he may come down a large part of the dis- 
tance in a rapid spiral. As the earth is approached the air 
pressure increases rapidly, and the passenger, if correctly 
instructed, will open his mouth and swallow frequently to 
equalize the air pressure on his ear drums. Just before the 
ground is reached the plane is leveled off, it loses speed, and, 
if the landing is perfect, touches and runs along the ground 
without bouncing or bumping. Frequently, however, the 
impact of the tail is sufficiently hard to cause it to bump 
badly, with a consequent considerable danger to apparatus 
of any weight or delicacy. This is especially apt to occur 



30 AIRPLANE PHOTOGRAPHY 

in hastily chosen and poorly leveled fields such as must often 
be utilized in war or in cross-country flying. 

Appearance of the Earth from the Plane. — The view 
from the ordinary two-seater is greatly restricted by the 
engine in front and by the planes to either side and below 
(Figs. 7, 8, and 9). By craning his neck over the side, or 
by looking down through an opening in the floor, the passen- 
ger has an opportunity to learn the general appearance of 
the subject he is later to devote his attention to photograph- 
ing. Perhaps the most striking impression he receives will 
be that of the flatness of the earth, both in the sense of 
absence of relief and in the sense of absence of extremes of 
light and shade. The absence of relief is due to the fact 
that at ordinary flying heights the elevations of natural 
objects are too small for the natural separation of the eyes 
to give any stereoscopic effect. The absence of extremes of 
light and shade is in part due to the fact that the natural 
surfaces of earth, grass and forest present no great range of 
brightness; in part to the small relative areas of the parts 
in shadow; in considerable part to the layer of atmospheric 
haze which lies as an illuminated veil between the observer 
and the earth at altitudes of 2000 meters and over (Figs. 
10 and 11). Due to the combination of these factors the 
earth below presents the appearance of a delicate pastel. 

As the gaze is directed away from the territory directly 
below, the thickness of atmosphere to be pierced rapidly 
increases, until toward the horizon (which lies level with 
the observer here as on the ground) all detail is apt to be 
obliterated to such an extent that only on very clear days 
can the horizon itself be definitely found or be distinguished 
from low lying haze or clouds (Fig. 4). 

Airplane Instruments. — ^Mounted on boards in front of 
the pilot and observer are various instruments to indicate 



CAMERA PLATFORM 



31 




Fig. 7. — The view ahead- 




FiG. 8. — The view aatern. 



32 



AIRPLANE PHOTOGRAPHY 




CAMERA PLATFORM 



33 



the performance of engine and plane (Fig. 2). Those of 
interest to the photographic observer are the compass, the 
altimeter, the air speed indicator, the inclinometers. 

The compass is usually a special airplane compass, with 
its "card" immersed in a damping liquid. Like most of the 
direction indicating instruments on a plane its indications 




I'iG. 10. — Appearance of the earth from a low altitude — 3000 feet or less. 

are only of significance when the plane is pursuing a steady 
course. On turns or rapid changes of direction of any sort 
perturbations prevent accurate reading. 

The altimeter is of the common aneroid barometer type. 
On American instruments it is usually graduated to read in 
100-foot steps. While somewhat sluggish, it is quite satis- 
factory for all ordinary determinations of altitude in photo- 



34 AIRPLANE PHOTOGRAPHY 

graphic work. Were primary map making to be undertaken, 
where the scale was determinable only from the altitude and 
focal length of the lens, the ordinary altimeter is hardly 
accurate enough. 

The air speed indicator consists of a combination of 
Venturi and Pitot tubes, producing a difference of pressure 




Fig. 11. — Appearance of the earth from a high altitude — 10,000 feet or more. 

when in motion through the air which is measured on a 
scale calibrated in air speed. This instrument is important 
for determining, in combination with wind speed, the 
ground speed of the plane, on the basis of which is calcu- 
lated the interval between exposures to secure overlapping 
photographs. Its accuracy is well above that necessary 
for the purpose. 



CAMERAPLATFORM 35 

Inclinometers for showing the lateral and fore and aft 
angle of the plane with the horizontal, are occasionally used, 
and have also been incorporated in cameras. The important 
point to remember about these instruments is that they are 
controlled not alone by gravity but as well by the acceler- 
ation of the plane in any direction. They consequently 
indicate correctly only when the plane is flying straight. 
On a bank the lateral indicator continues to indicate "ver- 
tical " if the bank is properly calculated for the turn. 



II 

THE AIRPLANE CAMERA 



CHAPTER III 
THE CAMERA— GENERAL CONSHDERATIONS 

Chief Uses of an Airplane Camera. — The kinds of 
camera suitable for airplane use and the manner in which 
they must differ from cameras for use on the ground are 
determined by consideration of the nature of the work they 
must do. Four kinds of pictures constitute the ordinary 
demands upon the aerial photographer. These are single 
objectives or pin points, mosaic maps of strips of territory'- 
or large areas, oblique views, and stereoscopic views. Each 
of these presents its own peculiar problems influencing 
camera design. 

Pinpoints consist of such objects as gun emplacements, 
railway stations, ammunition dumps, and other objects of 
which photographs of considerable magnification are desired 
for study. Here the instrumental requirements are suffi- 
cient focal length of lens to secure an image of adequate size; 
means for pointing the camera accurately; enough shutter 
speed to counterbalance the speed of the plane; sufficiently 
wide lens aperture to give adequate exposure with the shutter 
speed required; means of supporting the camera to protect 
it from the vibration of the plane. 

Mosaic maps are built up from a large number of photo- 
graphs of adjacent areas. In addition to the above require- 
ments, mosaic maps demand lenses free from distortion and 
covering as large a plate as possible, in order to keep to a 
minimum the number of pictures needed to cover a given 
area; means for keeping the camera accurately vertical, and 
means for changing the plates or films and resetting the 
shutter rapidly enough to avoid gaps between successive 

39 



40 AIRPLANE PHOTOGRAPHY 

pictures. At low altitudes and high ground speeds the interval 
between exposures becomes a matter of only a few seconds. 

Oblique views are made at angles of from 12 to 35 degrees 
from the horizontal, usually from comparatively low alti- 
tudes. They have been found to be particularly suitable for 
the use of men who have no training in photographic inter- 
pretation, being more like the pictures with which the men 
are familiar. Distributed among the infantry before an 
attack, they have proved indispensable aids to the proper 
knowledge of the ground to be covered. The additional re- 
quirement here is for high shutter speed to elminate the effect 
of the relatively very rapid movement of the foreground. 

Stereoscopic views are among the most useful of all air- 
plane pictures. They are made from successive exposures, 
the separation of the points of view being obtained not by 
two lenses at the distance of the eyes apart, but by the 
motion of the plane. For this purpose the views should 
overlap by at least 60 per cent; this, therefore, requires a very 
short interval between exposures. For stereo-oblique views 
this may mean that they are taken at intervals as short as 
one or two seconds. 

Chief Differences between Ground and Air Cameras. — 
Certain definite differences are thus seen to stand out be- 
tween airplane cameras and the ordinary kind. It is essential 
that the apparatus for use in the air shall have high lens and 
shutter speed, means for rapid changing of plates, and anti- 
vibration suspension. Without these features a camera is 
of little use for aerial work. These requirements lead inevi- 
tably to greater complexity of design. One simplification 
over ground cameras, however, is brought about by the fact 
that all exposures are made on objects beyond the practical 
infinity point of the lens; consequently, all cameras are fixed 
focus. This fixed focus feature is a positive advantage in 



THECAMERA 41 

construction, since it permits of the simple rigid box form, 
desirable and necessary to withstand the strains due to the 
weight of the lens and the stresses from the plane. But with 
the abandonment of all provision for focussing in the air 
must go special care that the material used in constructing 
the camera body is as little subject as possible to expansion 
and contraction with temperature, since there is often a 
drop of 30 to 40 degrees Centigrade from ground to upper 
air. The effect of change of temperature on focus will be 
treated in the discussion of lenses. 

In addition to these differences, we must keep in mind cer- 
tain requirements which are conditioned by the nature and 
place of aerial navigation. Thus all mechanical devices which 
will fail to function at the low temperatures and pressures met 
at high altitudes are entirely unsuitable. Experience has 
shown, too, that we must avoid all mechanism depending 
primarily on springs and on the action of gravity. Vibration, 
and the motion of the plane in all three dimensions, conspire 
to render mechanical motions unreliable when actuated by 
these agencies. All plate changing, shutter setting, and 
exposing operations should be as nearly as possible positively 
controlled motions. Because of the cold of the upper air 
all knobs, levers and catches must be made extra large and 
easy to handle with heavy gloves. Circular knurled heads 
to such parts as shutter setting movements are to be avoided 
in favor of bat- wing keys or levers. Grooves for the recep- 
tion of magazines must be as large and smooth as possible, 
and guides to facilitate the magazines' introduction should 
be provided (Fig. 50). No releases or adjustments which 
depend upon hearing or upon a delicate sense of touch are 
feasible in airplane apparatus. Wherever possible, large 
visible indicators of the stage of the cycle of operations should 
be provided. Loose parts are to be shunned, as they are 



42 AIRPLANE PHOTOGRAPHY 

invariably lost in service. Complete operating instruc- 
tions should be placed on the apparatus wherever pos- 
sible, to minimize the confusion due to changing and 
uninstructed personnel. 

The Elements of the Airplane Camera. — Disregarding 
its means of suspension, the airplane camera proper consists 
essentially of lens, camera body, shutter, and plate or film 
holding and changing box. 

In certain of the aerial cameras developed early in the 
war all of these elements were built together in a common 
enclosure. Later it was generally recognized that a unit 
system of interchangeable parts is preferable. In the case 
of the lens there arose various requirements for focal length, 
from ^ to 120 centimeters, according to the work to be 
done. Rather than use an entirely different camera for each 
different kind of work, it is better to have lenses of various 
focal lengths, mounted in tubes or cones, all built to attach 
to the same camera body. In the case of the shutter it is 
desirable to be able to repair or calibrate periodically. By 
making the shutter a removable unit, the provision of a few 
spares does away with the need for putting the Whole 
camera out of commission. Similar considerations hold 
with reference to other parts. 

A further material advantage that comes from making 
airplane cameras in sections is the greater ease with which 
they are inserted in the plane, usually through the openings 
between diagonal cross-wires. It is in fact only by virtue 
of this possibility of breaking up into small elements that 
some of the larger cameras could be inserted in the common 
types of reconnaissance plane. Illustrations of the building 
up of cameras from separate removable elements are given 
in the detailed discussion of the individual type^ 

Types of Airplane Cameras. — During the course of 



THE CAMERA 43 

the war airplane cameras have been classified on various 
bases, in different services. In the French service, where 
the de Maria type of camera was standardized early in the 
war, the usual classification was based on focal length; 
thus the standard cameras were spoken of as the 26, the 50 
and the 120 (centimeter). A further distinction was then 
made according to the size of plate, this being originally 
13X18 centimeters for the 26 centimeter, and 18X24 
centimeters for the larger cameras. In the English service 
the 4X5 inch plate was used almost exclusively, and their 
various types of cameras were known by serial letters — C, E, 
L, etc. Both these modes of classification became inade- 
quate with the ultimate agreement to standardize on the 
18x24 centimeter size for all plates, and to carry lenses of 
all focal lengths in interchangeable elements. 

For purposes of description and discussion, it is most 
convenient to classify cameras according to their method of 
operation and the sensitive material employed. On this 
basis we may distinguish among cameras using plates 
three kinds — non-automatic cameras, semi-automatic cameras, 
and automatic cameras. We may similarly discuss film 
cameras, but having treated the plate cameras comprehen- 
sively, it will be found that the discussion of all types of film 
camera can be handled most conveniently by studying the 
differences in construction and operation introduced by the 
characteristics of film as compared to plates. 



CHAPTER IV 
LENSES FOR AERIAL PHOTOGRAPHY 

General Considerations. — The design and selection of 
lenses for aerial photography present on the whole no 
problems not already encountered in photography of the 
more familiar sort. Indeed, the lens problem in the airplane 
camera is in some particulars more simple than in the ground 
camera. For instance, there is no demand for depth of focus 
— all objects photographed are well beyond the usually 
assumed "infinity focus" of 2000 times the lens diameter. 
Such strictly scientific problems of design as pertain to 
aerial photographic lenses are ones of degree rather than of 
kind. Larger aperture, greater covering power, smaller 
distortion, more exquisite definition — these always will be 
in demand, and each progressive improvement will be 
reflected in advances in the art of aerial photography. But 
many lens designs perfected before the war were admirably 
suited, without any change at all, for aerial cameras. 

Of the utmost seriousness, however, with the Allies, was 
the problem of securing lenses of the desired types in suffi- 
cient numbers. The manufacture of the many varieties of 
optical glass essential to modern photographic lenses was 
almost exclusively a German industry, which h^^d to be 
learned and inaugurated in Allied countries since 1914. In 
consequence of this entirely practical problem of quantity 
production without the glasses for which lens formulae were 
at hand, some new lens designs were produced. Whether 
any of these possess merits which will lead them to be pre- 
ferred over pre-war designs, when the latter can again be 
manufactured, remains to be seen. 

44 



LENSES 45 

While the glass problem was still unsolved, aerial cameras 
had to be equipped with whatever lenses could be secured 
by requisition from pre-war importation and manufacture, 
and later, with lenses designed to utilize those glasses whose 
manufacture had been mastered in the allied countries. It 
is important that the historical aspect of this matter be well 
understood by the student of aerial photographic methods, 
for the use of these odd-lot lenses reacted on the whole 
design of aerial cameras and on the methods of aerial photog- 
graphy, particularly in England and the United States. 
Almost without exception the available lenses were of short 
focus, considered from the aerial photographic standpoint; 
that is, they lay between eight and twelve inches. This set 
a limit to the size of the airplane camera, quite irrespective of 
the demands made by the nature of the photographic 
problem. Lenses of these focal lengths produced images 
which, for the usual heights of flying, were generally con- 
sidered too small, and which were, therefore, almost always 
subsequently enlarged. Such was the English practice, 
which was followed in the training of aerial photographers 
in America, where exactly similar conditions held at the 
start with respect to available lenses. French glass and lens 
manufacturers did succeed in supplying lenses of longer 
focus (50 centimeters), in numbers sufficient for their own 
service, although never with any certainty for their allies. 
The French, therefore, almost from the start, built their 
cameras with lenses of long focus, and made contact prints 
from their negatives. 

Practices adopted under pressure of an emergency to 
meet temporary practical limitations often come to dominate 
the whole situation. This is particularly true of aerial 
photography in the British and American services. The 
small apparatus built around the stop-gap short focus lenses 



46 AIRPLANE PHOTOGRAPHY 

fixed the plane designer's idea of an airplane camera, and the 
space it should occupy. This was directly reflected in the 
designs of the English planes, and the American planes 
copied after them. Meanwhile the American photographic 
service in France associated itself with the French service, 
adopting its methods and apparatus, and using French 
planes whose designs were not being followed in American 
construction. The task of harmonizing the photographic 
practice as taught in America, following English lines, 
with French practice as followed in the theater of war, and of 
adapting planes built on English designs so that they could 
carry French apparatus, was a formidable one, not likely to 
be soon forgotten by any who had a part in it. 

Photographic Lens Characteristics. — ^Whole volumes have 
been written on the photographic lens, and on the optical 
science utilized and indeed brought into being by its prob- 
lems. Such works should be consulted by those who intend 
to make a serious study of the design of lenses for aerial 
use. No more can be attempted, no more indeed is relevant 
here, than an outline review of the chief characteristics and 
errors of photographic lenses, considering them with special 
reference to aerial needs. 

The modern photographic lens is, broadly speaking, a 
development of the simple convex or converging lens. Its 
function is the same : to form a real image of objects placed 
before it. But the difference in performance between the 
simple lens and the modern photographic objective is enor- 
mous. The simple lens forms a clear image only close to 
its axis, for light of a single color, and as long as its aperture 
is kept quite small as compared to the distance at which 
the image is formed. The photographic lens, on the other 
hand, is called upon to produce a clear image with light of 
a wide range of spectral composition, sharply defined over 



LENSES 



47 



a flat surface of large area, and it must do this with an aper- 
ture that is large in comparison with the focal length, 
whereby the amount of light falling on the image surface 
shall be a maximum. This ideal is approximated to a really 
extraordinary degree by the scientific combination and 
arrangement of lens elements made from special kinds of 
glass in the best photographic lenses of the anastigmat type. 
The result is of necessity a set of compromises, whereby the 




Fig. 12. — Diagrammatic representation of spherical aberration. 

outstanding errors are reduced to a size judged permissible 
in view of the work the lens is to do. These errors or aberra- 
tions are briefly reviewed below, in order that the reader 
may readily grasp the terms in which the performance and 
tolerances in aerial lenses are described. 

Spherical Aberration and Coma. — Suppose we focus on a 
screen, by means of a simple convex lens the image of a 
distant point of light. Suppose for simplicity that this 
image is located on the axis of the lens and that light of only 
one color is used, such as yellow. It will be found that the 
smallest image that can be obtained is not a point, but a 
small disc. This is due to the fact that the rays of light 



48 AIRPLANE PHOTOGRAPHY 

passing through the outer portions of the lens are bent more 
than those passing through the lens in the region near the 
center. This effect is shown in Fig. 12 by the usual mode of 
representing it graphically. Here the figures 1, 2, 3, 4, 
represent distances from the axis of the lens, and the letters 
Ai, A2, A3, A4, the points of convergence of the rays from 
1, 2, 3, 4, etc. These distances projected upward on to the 
produced lens points form a curve which shows at a glance 
the extent and direction of the error due to each part of the 
leils. This information is of value where the lens is fitted 
with an adjustable diafram. With some types of correction 
sharper definition may be obtained by reducing the aperture. 
With others, however, diaframing impairs definition, by de- 
stroying the balance between under and over correction 
which averages to make a good image. In aerial lenses it 
is not customary to use diaframs, as all the light possible is 
desired. Consequently the reduction of spherical aberration 
must be accomplished by proper choice of lens elements and 
their arrangement. 

Off the axis of the lens the image of a point source takes 
on an irregular shape, due to oblique spherical aberration 
or coma. 

Chromatic Aberration. — ^Because of the inherent proper- 
ties of the glass of which it is made, a simple collective lens 
does not behave in the same way with respect to light of 
different colors. If one attempts, with such a lens, to focus 
upon a screen the image of a distant white light, it will be 
found that the blue rays will not focus at the same point 
as the red rays, but will come together nearer the lens. 
Modern photographic lenses are compounded of two or 
more kinds of glass in such a way as to largely eliminate 
this defect, the presence of which is detrimental to good 
definition. Such lenses are called achromatic, and the 



LENSES 49 

property of a lens by virtue of which this defect is eliminated 
is called its chromatic correction. 

Chromatic correction is never perfect, but two colors 
of the spectrum can be brought to a focus in the same 
plane, and to a certain extent the departure of other 
colors from this plane can be controlled. Off the axis of 
the lens outstanding chromatic aberration results in a 
difference in the size of images of different colors, known as 
lateral chromatism. 

Like spherical aberration, chromatic aberration is a 
contributing factor to the size of the image of a point source, 
which determines the defining power of a lens. It is, however, 
an error whose effect is to some extent dependent on the 
kind of sensitive plate used. Two lenses may give images 
of the same size (in so far as it is governed by chromatic 
aberration), if a plate of narrow spectral sensitiveness is 
used, while giving images of different size on panchromatic 
plates of more extended color sensibility. The choice of the 
region of the spectrum for which chromatic correction is to 
be made is thus governed by the color of the photographi- 
cally effective light. While in ordinary photography the blue 
of the spectrum is most important, in aerial work where 
color filters are habitually used with isochromatic plates 
the green is most important, and color correction centered 
about this region constitutes a real difference of • design 
peculiar to aerial lenses. Similarly the general use of deep 
orange or red filters with red sensitive plates, for heavy mist 
penetration, would call for a shift of correction to that part 
of the spectrum. 

Astigmatism and Covering Power. — Suppose the lens 

forms at some point off its axis an image of a cross. Suppose 

one of the elements of the cross to be on a radius from the 

center of the field, the other element parallel to a tangent. 

4 



50 AIRPLANE PHOTOGRAPHY 

The rays forming the images of these two elements of the 
cross are subject to somewhat different treatment in their 
passage through the lens. The curvature of the lens sur- 
faces is on the whole greater with respect to the rays from 
the radial element than to those from the tangential element. 
They are therefore refracted more strongly and come to a 
focus nearer the lens. The arms of the cross are conse- 
quently not all in focus at once. This error, termed astig- 
matism, is rather well shown in Fig. 15, where the images of 
the outlying concentric circles are sharp in the radial, but 
blurred in the tangential direction. 

Astigmatism can be largely compensated for, and its 
character controlled. The most usual correction brings 
the two images in focus together both at the axis, and on a 
circle at some distance out. This second locus of coincidence 
may or may not be in the same plane as the first, depending 
on which disposition produces the best average correction. 
The mean between the two foci determines the focal plane 
of the lens, which is in general somewhat curved. The 
covering power of a lens is given by the size of the field which 
is sufiiciently flat and free from astigmatism for the purpose 
for which the lens is used. This is largely determined by 
the astigmatism, but the other aberrations are also important. 

Illumination. — The amount of light concentrated by the 
lens on each elementary area of the image determines its 
brightness or illumination. The ideal image would, of course, 
be equally bright over its whole area of good definition, and 
for lenses of narrow angle this is approximately true. But 
when it is desired to cover a wide angle the question of illumi- 
nation becomes serious. The relationship between angle 
from the axis and illumination is that illumination is pro- 
portional to the fourth power of the cosine of the angle. 
This relationship is shown in the following table: 



LENSES 



51 



Angle 
0° 
10° 
20° 
30° 
40° 
50° 



Image brightness 
100 per cent. 

94.1 per cent. 

78.0 per cent. 

56.2 per cent. 
34.4 per cent. 

17.1 per cent. 



If the field of view is 60°, which corresponds to an 18X24 
centimeter plate with a lens of 25 centimeter focus, the 
brightness is only 5Q per cent., and the necessary exposure 
at the edge approximately 1.8 times that at the center. 
This effect is shown in Fig. 15. It is very noticeable if the 
exposure is so short as to place the outlying areas in the 
under-exposure period. 



1 1 1 1 1 1 a / u 




-j-j 



Fig, 13. — Barrel and pin-cushion distortion. • 

Distortion, — Sometimes a lens is relatively free from all 
the aberrations, mentioned above, so that it gives sharp, 
clear images on the plate, yet. these images may not be 
exactly similar to the objects themselves as regards their 
geometrical proportions; in other words, the image will 
show distortion. Lens distortion assumes two typical 
forms, illustrated in Fig. 13, which shows the result of 
photographing a square net-work with lenses suffering in 
the one case from "barrel" distortion and in the other 
from "pin-cushion" distortion. In the first the corners are 
drawn in relative to the sides; in the latter case the 



52 



AIRPLANE PHOTOGRAPHY 



sides are drawn in with respect to the corners. Either sort 
is a serious matter in precision photography, such as aerial 
photographic mapping aspires to become. It must be 
reduced to a minimum and its amount must be accurately 
known if negatives are to be measured for the precise location 
of photographed objects. In general symmetrical lenses 
give less distortion than the unsymmetrical (Fig. 14). 

Lens Testing and Tolerances for Aerial Work. — Simple 
and rapid comparative tests of lenses may be made by 






Fig. 14. — Arrangement of elements in two lenses suitable for aerial work: a, Zeiss Tessar; 
two simple and one cemented components (unsymmetrical); b. Hawk-eye Aerial; two positive 
elements of heavy barium crown, two negative of barium flint, uncemented (symmetrical). 



photographing a test chart, consisting of a large flat surface 
on which are drawn various combinations of geometrical 
figures — Klines, squares, circles, etc. — calculated to show up any 
failures of defining power. For testing aerial lenses the 
chart should be as large as possible, so that it may be photo- 
graphed at a distance great enough for the performance of 
the lens to be truly representative of its behavior on an 
object at infinite distance. This means in practice a chart 
of 4 or 5 meters side, to be photographed at a distance 20 
to 30 times the focal length of the lens. 

A typical photograph of such a chart is shown in Fig. 15. 
It reveals at a glance the more conspicuous lens errors. 



LENSES 



53 





54 AIRPLANE PHOTOGRAPHY 

At the sides and corners the concentric circles show the 
lens's astigmatism, by the clear definition of the lines radial 
to the center of the field and their blurring in the tangential 
direction. The falling off in illumination with increasing 
distance from the center is also exhibited; and the blurring 
t)f all detail outside the rectangle for which the lens was 
calculated shows that spherical, chromatic, and other 
aberrations have become prohibitively large. 

But the only complete test of a lens is the quantitative 
measurement of errors made on an optical bench. A point 
source of light, which may at will be made of any color of 
the spectrum, is used as the object and its image formed by 
the lens in a position where it can be accurately measured 
for location, size, and shape by a microscope. A chart 
giving the results of such a test is shown in Fig. 16. In the 
upper left-hand corner is shown the position of the focus for 
the different colors of the spectrum. Below this is recorded 
the lateral chromatism at 21 degrees, in terms of the differ- 
ence in focus for a red and a blue ray. Below this again 
comes the distortion, or shift of the image from its proper 
position, for various angles (plotted at the extreme right) 
from the lens axis. To the right of this is the image size, at 
each angle, and finally, to the right of the diagram, are 
plotted the distances of the two astigmatic foci from the 
focal plane, together with the mean of the two foci, which 
practically determines the shape of the field. 

An important point to notice is that these data are 
uniformly plotted in terms of a lens of 100 millimeters focal 
length irrespective of the actual focal length of the lens 
measured. Thus this particular chart is for a 50 centimeter 
lens but would be plotted on the same scale for a 25 or a 
100 centimeter lens. Underlying this practice is the assump- 
tion that all the characteristics of lenses of the same design 



LENSES 



55 



and aperture are directly proportional to their focal length. 
If this were so, then a 50 centimeter lens would give double 
the size of image that a 25 centimeter does, and so on. 
As a matter of fact, test shows that the size of the image 
does not increase so rapidly as the focal length; so that 
while the image size for a 25 centimeter lens would be, say, 



yh/0t 

X45 


Blue 




Yelhw Orange f^ed 


V;tl 


3 . 






tm 


1 


r 








m 




/ 



y 



TiJ 



Cola 



b!€fa! CUiyfom Si' .Vis-Xfc-S 



„.„;r«e!S_.rllt5a„ 



\ njBmtw 






Jsi^jmtJ^ 



. 4- 






4m^ 



- ^ 

- 3 

- 15 
' i5;- 

- ei 

'- a? 
-SO' 



Fig. 16. — Chart recording measurements of lens characteristics. 



.05 millimeters per 100 millimeters focal length, it will be 
only .03 or .04 millimeters per 100 millimeters focal length 
for a 50 centimeter lens. The actual size of a point image 
will therefore be greater, though not proportionately greater. 
The chart presents tests on a good quality lens, and so 
gives a good idea of the permissible magnitude of the various 
errors. In many ways the most important figure is that for 
image size, including as it does the result of all the aberra- 



56 AIRPLANE PHOTOGRAPHY 

tions. In the example given, this varies from .075 to .15 
mm. actual size. For the same type of lens of 25 centi- 
meters focus this range will be from .05 to .10 mm. Since 
these are commonly used focal lengths, a good average 
figure for image size, commonly used in aerial photographic 
calculations, is 1/10 mm. In regard to astigmatic tolerances, 
the two astigmatic foci should not be separated at any point 
by more than 6 to 7 millimeters, and the mean of these 
should not deviate from the true flat field by more than Y^ 
millimeter, in each case the figures being based on the con- 
ventional 100 millimeters focal length. Distortion should 
not be over .08 millimeter at 18° or .20 milli^ieter at 24° 
from the axis (per 100 millimeters focal length). 

Lens Aperture. — In the simple lens the aperture is 
merely the diameter. In compound lenses the aperture 
is not the linear opening but the effective opening of an 
internal diafram. Photographically, however, aperture has 
come to have a more extensive meaning. While in the 
telescope the actual diameter of an objective is perhaps 
the most important figure, and in the microscope the focal 
length, in photography the really important feature is the 
amount of light or illumination. This is determined by 
lens opening and focal length together; specifically, by the 
ratio of the lens area to the focal length. The common 
system of representing photographic lens aperture is by the 
ratio of focal length to lens diameter, the lens being assumed 
to be circular. Thus 1^5 (often written r.5) indicates that 
the diameter is one-fifth the focal length. 

Two points are to be constantly borne in mind in con- 
nection with this system of representation. First, all lenses 
of the same aperture (as so represented) give the same 
illumination of the plate (except for differences due to loss 
of light by absorption and reflection in the lens system). 



LENSES 57 

This follows simply from the fact that the illumination of 
the plate is directly proportional to the square of the lens 
diameter, and inversely as the square of the focal length. 
Secondly, the illumination of the plate is inversely as the 
square of the numerical part of the expression for aperture. 
That is, lenses of aperture r/4.5 and F/G give images of 

relative brightness! —y =1.78. 

What lens aperture, and therefore what image brightness, 
is feasible, is determined chiefly by the angular field that 
must be covered with any given excellence of definition. 
The largest aperture ordinarily used for work requiring good 
definition and flat field free from distortion is F/4.5. An- 
astigmatic lenses of this aperture cover an angle of 16° 
to 18° from the axis satisfactorily, which corresponds to an 
18X24 centimeter plate with a lens of 50 centimeters focus. 
Lenses with aperture as large as F/3.5 were used to some 
extent in German hand cameras of 25 centimeters focal length, 
with plates of 9X12 centimeters. English and American 
lenses of this latter focal length were commonly of aperture 
F/4.5, designed to cover a 4X5 inch plate. 

As a general rule the greater the focal length the smaller 
the aperture — a relationship primarily due to the difficulty 
of securing optical glass in large pieces. Thus while 50 
centimeter lenses of aperture F/4.5 are reasonably easy to 
manufacture, the practicable aperture for quantity produc- 
tion is F/6, and for 120 centimeter lenses, F/10. This 
means that a very great sacrifice of illumination must be 
faced to secure these greater focal lengths. As is to be 
expected from the state of the optical glass industry, the 
German lenses were of generally larger aperture for the same 
focal lengths than were those of the Allies. Besides the 
F/3.5 lenses already mentioned, their 50 centimeter lenses 



58 AIRPLANE PHOTOGRAPHY 

were commonly of aperture F/4.8, their 120 centimeter 
lenses of aperture F/T, or of about double the illuminating 
power of the French lenses of the same size. 

Demands for large covering power also result in smaller 
aperture. The 26 centimeter lenses used on French hand 
cameras utilizing 13X18 centimeter plates were commonly 
of aperture F/6 or F/5.6. The lens of largest covering power 
decided on for use in the American service was of 12 inch 
focus, to be used with an 18X24 centimeter plate (extreme 
angle 26 ); the largest satisfactory aperture for this lens 
is F/5.6. 

Ordinarily the question of aperture is closely connected 
with that of diaframs, whereby the lens aperture may be 
reduced at will. Diaframs have been very little used in 
aerial photography. All the aperture that can be obtained 
and more is needed to secure adequate photographic action 
with the short exposures required under the conditions of 
rapid motion and vibration peculiar to the airplane. Any 
excess of light, over the minimum necessary to secure proper 
photographic action, is far better offset by increase of shutter 
speed or by introduction of a color filter. For this reason 
American aerial lenses were made without diaframs. In 
the German cameras, however, adjustable diaframs are 
provided (Fig. 43), controlled from the top of the camera 
by a rack and pinion. In the camera most used in the Italian 
service an adjustable diafram is provided, but this is occa- 
sioned by the employment of a between-the-lens shutter of 
fixed speed, so that the only way exposure can be regulated 
is by aperture variation, a method which has little to 
recommend it. 

The Question of Focal Length.— In aerial photography 
the lens is invariably used at fixed, infinity, focus. Under 
these conditions the simple relationship holds that the size 



LENSES 59 

of the image is directly proportional to the focal length and 
inversely proportional to the altitude. If any chosen scale 
is desired for the picture the choice of focal length is deter- 
mined by the height at which it is necessary to fly. This 
at least would be the case were there no limitation to the 
practicable focal length — which means camera size — and 
were one limited to the original size of the picture as taken; 
that is, were the process of enlargement not available. But 
the possibility of using the enlarging process brings in other 
questions: Is the defining power of a short focus lens as 
good in proportion to its focal length as that of a long focus 
lens.f^ If so a perfect enlargement from a negative made by 
a short focus lens would be identical with a contact print 
from a negative made with a lens of longer focus. Is defining 
power lost in the enlarging process with its necessary em- 
ployment of a lens which has its own errors of definition 
and which must be accurately focussed.^ 

Certain factors which enter into comparisons of this 
sort in other lines of work, such as astronomical photography, 
play little part here. These are, first, the optical resolving 
power of the lens, which is conditioned by the phenomena of 
diffraction, and is directly as the diameter; and, second, 
the size of the grain of the plate emulsion. The first of these 
does not enter directly, because the size of a point image 
on the axis of the lens, due merely to diffraction, is very 
much less than that given by any photographic lens which 
has been calculated to give definition over a large field, instead 
of the minute field of the telescope. Yet it may contribute 
toward somewhat better definition with a long focus lens 
because of the actually larger diameter of such lenses. The 
second factor is not important, because, as will be seen later, 
the resolving power of the plates suitable for aerial photog- 
raphy is considerably greater than that of the lens. The 



60 AIRPLANE PHOTOGRAPHY 

emulsion grain is in fact only a quarter or a fifth the size of 
the image as given by a 25 centimeter lens, and enlargements 
of more than two or three times are rarely wanted. 

A series of experiments was made for the U. S. Air Service 
to test out these questions, using a number of representa- 
tive lenses of all focal lengths, both at their working aper- 
tures and at identical apertures for all. With regard to lens 
defining power, as shown by the size of a point image, the 
answer has already been reported in a previous section. 
Lenses of long focus give a relatively smaller image than 
lenses of the same design of short focus. In regard to the 
whole process of making a small negative and enlarging it, 
the loss of definition is quite marked, as compared to the 
pictures of the same scale made by contact printing from 
negatives taken with longer focus lenses. 

This answer is clear-cut only for lenses calculated to 
give the same angular field. Thus a 10 inch lens covering a 
4X5 inch plate has about the same angle as a 50 centimeter 
lens for an 18X24 centimeter plate. When, however, it 
comes to the longer foci, such as 120 centimeters, the practi- 
cal limitation to plate size (18X24 cm.) has been passed, 
and the angular field is less than half that of the 50 centi- 
meter lens. The 120 centimeter lens need only be designed 
fpr this small angle, with consequent greater opportunities 
for reduction of spherical aberration. It is therefore an 
open question whether a 50 centimeter lens designed to 

50 . ^ , . , 

cover a plate of linear dimensions — — times that used with 

120 

the regular 50 centimeter lens could not be produced of such 
quality that it would yield enlargements equal to contacts 
from a 120 centimeter lens. If so, lenses of larger aperture 
could be used, and a considerable saving in space require- 
ments effected. 



LENSES 61 

Focal lengths during the Great War were decided by the 
nature of the military detail which was to be revealed and 
by the altitudes to which flying was restricted in military 
operations. In the first three years of the war the develop- 
ment of defences against aircraft forced planes to mount 
steadily higher, so that the original three or four thousand 
feet were pushed to 15,000, 18,000, and even higher. Lenses 
of long focus were in demand, leading ultimately to the use 
of some of as much as 120 centimeters (Fig. 41). In the last 
months of the war the resumption of open fighting made 
minute recording of trench details of less weight, while the 
preponderance of allied air strength permitted lower flying. 
In consequence, lenses of shorter focus and wider angle came 
to the fore, suitable for quick reconnaissance of the main 
features of new country. At the close of the war the follow- 
ing focal lengths were standard in the U. S. Air Service, and 
may be considered as well-suited for military needs. Peace 
may develop quite different requirements, 



)cal length 


Aperture 


Plate size 


10 inch 


F/4.5 


4X5 inch 


26 cm. 


F/6 


13X18 cm. 


12 inch 


F/5.6 


18X24 cm. 


20 inch 


F/6.3 to F/4.5 


18X24 cm. 


48 inch 


F/10 to F/8 


18X24 cm, 



The question of the use of ielephoto lenses in place cf 
lenses of long focus is frequently raised. Lenses of this type 
combine a diverging (concave) element with the normal 
converging system, whereby the effect of a long focus is 
secured without an equivalent lens-to-plate distance. This 
reduction in "back focus" may be from a quarter to a half. 
Were it possible to obtain the same definition with telephoto 
lenses as with lenses of the same equivalent focus, they 
would indeed be eminently suitable for aerial work because 



m AIRPLANE PHOTOGRAPHY 

of their economy of length. But experience thus far has 
shown that the performance of telephoto lenses, as to defini- 
tion and freedom from distortion, is distinctly inferior, so that 
it is best to hold to the long focus lens of the ordinary type. 

Lenses Suitable for Aerial Photography. — ^Among the 
very large number of modern anastigmat lenses many were 
found suitable for airplane cameras and were used exten- 
sively in the war. A partial list follows : The Cooke Aviar, 
The Carl Zeiss Tessar, the Goerz Dogmar, the Hawkeye 
Aerial, the Bausch and Lomb Series Ic and Hb Tessars, the 
Aldis Triplet, the Berthiot Olor. 

The Question of Plate Size and Shape. — ^Plate size is 
determined by a number of considerations, scientific and 
practical. If the type of lens is fixed by requirements as to 
definition, then the dimensions of the plate are limited by 
the covering power. From the standpoint of economy of 
flights and of ease of recognizing the locality represented in a 
negative, by its inclusion of known points, lenses of as wide 
angle as possible should be used. If the focus is long, this 
means large plates, which are bulky and heavy. If the 
finest rendering of detail is not required a smaller scale 
may be employed, utilizing short focus lenses and correspond- 
ingly smaller plates. Thus a six inch focus lens on a 4 X5 inch 
plate would be as good from the standpoint of angular field 
as a 12 inch on an 8X10 inch plate. This is apt to be the 
condition with respect to most peace-time aerial photography, 
which may be expected to free itself quickly from the huge 
plates and cameras of war origin. 

For work in which great freedom from distortion of any 
sort is imperative, small plates will be necessary, for two 
reasons. One is that the characteristic lens distortions are 
largely confined to the outlying portions of the field. The 
other is that a wide angle of view inevitably means that all 



LENSES 63 

objects of any elevation at the edge of the picture are shown 
partly in face as well as in plan, which prevents satisfactory 
joining of successive views (Fig. 128). In making a mosaic 
map of a city, if a wide angle lens is employed with large 
plates, the buildings lying along the junctions of the prints 
can be matched up only for one level. If this is the ground 
level, as it would be to keep the scale of the map correct, 
the roofs will have to be sacrificed. In extreme cases a 
house at the edge of a junction may even show merely as a 
front and rear, with no roof, while in any case the abrupt 
change at these edges from seeing one side of all objects to 
seeing the opposite side is not pleasing. 

The table in a preceding section gives the relation of 
plate size to focal length found best on the whole for military 
needs. Deviations from these proportions in both directions 
are met with. In the English service the LB camera, which 
uses 4X5 inch plates, is equipped with lenses of various focal 
lengths, up to 20 inches. The German practice, as well as 
the Italian, was almost uniform use of 13X18 centimeter 
plates for all focal lengths. Toward the end of the war, 
however, some German cameras of 50 centimeter focal 
length were in use employing plates 24X30 centimeters. 

It will be recognized that these plate sizes are chosen 
from those in common use before the war. A similar obser- 
vation holds with even greater force on the question of 
plate shape. Current plate shapes have been chosen chiefly 
with reference to securing pleasing or artistic effects with the 
common types of pictures taken on the ground. These 
shapes are not necessarily the best for aerial photography. 
Indeed the whole question of plate shape should be taken up 
from the beginning, with direct reference to the problems of 
aerial photography and photographic apparatus. 

A few illustrations will make this clear, taking Fig. 17 



64 



AIRPLANE PHOTOGRAPHY 



as a basis. If it is desired to do spotting (the photography 
of single objectives), the best plate shape would be circular, 
for that shape utilizes the entire covering area of the lens. 
If it is desired to make successive overlapping pictures, 
either for mapping, or for the production of stereoscopic 
pairs, a rectangular shape is indicated. If the process of 
plate changing is difficult or slow, it is advisable, in order 

to give maximum time for 
this operation, to have the 
long side of the rectangle 
parallel to the line of flight 
(indicated by the arrow). If 
economy of flights is a con- 
sideration, as in making a mo- 
saic map of a large area, it is 
advantageous to have as wide 
a plate as the covering power 
of the lens will permit. Ref- 
erence to Fig. 17 shows that 
this means a plate of small di- 
mensions in the direction of 
flight. If the changing of 
plates or film is quick and 
easy, the maximum use of the lens's covering power is made 
by such a rectangle whose long side approximates the dimen- 
sions of the lens field diameter. This is in fact the choice 
made in the German film mapping camera (Figs. 61 and 63), 
whose picture is 6 X 24 centimeters. An objection to' this from 
the pictorial side, lies in the many junction lines cutting up 
the mosaic. Another objection, if the plane does not hold 
a steady course, is the failure to make overlaps on a turn. 
(Fig. 62.) Here as everywhere the problem is to decide on 
the most practical compromise between all requirements. 







) 


\ 
















/ 
/ 






\ 




/ 
/ 

/ 
/ 










\ 

\ 
\ 
\ 


























\ 

\ 

\ 

\ 










/ 

/ 

/ 
/ 




V ■ 

\ 

X 






/ 
/ 






>>» 


^ 








\ 


1 







Fig. 17. — Possible choices of plate shape. 



LENSES 65 

Focussing. — The process of focussing aerial cameras 
was at first deemed a mystery, though undeservedly so. 
A belief was long current that "ground" focus and "air" 
focus differ. In other words, that a camera focussed upon a 
distant object on the ground would not be in focus for an 
object the same distance below the camera when in the 
plane. Belief in this mysterious difference went so far that 
certain instruction books describe in detail the process of 
focussing a camera by trial exposures from the air. 

Careful laboratory tests performed for the IT. S. Air 
Service showed that neither low temperature nor low pres- 
sure, such as would be met at high altitudes, alter the focus 
of any ordinary lens by a significant amount, and that the 
possible contraction of the camera bod^ was of negligible 
effect on the focus (not more than 2^ per cent, per degree 
centigrade with a metal camera). In complete harmony 
with these tests has been the experience that if the ground 
focussing is done carefully, by accurate means, then the air 
focus is correct. The whole matter thus becomes one of 
precision focussing. 

The best method, applicable if the air is steady, is to 
focus by parallax. The ground glass focussing screen is 
marked in the center with a pencilled cross. Over this is 
mounted, with Canada balsam, a thin microscope cover- 
glass. The camera is directed on an object a mile or more 
away, and the image formed by the lens is examined by a 
magnifying glass through the virtual hole formed by the 
affixed cover-glass. With the pencil line in focus the head 
is moved from side to side. If the image and pencil mark 
coincide they will move together as the head is moved. If 
the image moves away from the pencil mark and in the 
same direction as the eye moves, the image is too near the 
lens. If the image moves away in the opposite direction to 
5 



66 AIRPLANE PHOTOGRAPHY 

the motion of the eye, it is too far from the lens. In either 
case the focus is to be corrected accordingly. 

In place of a distant object, which may waver with the 
motion of the air, we may use an image placed at infinity 
by optical means. The collimator, an instrument for doing 
this, consists of a test object (lines, circles, etc.) placed 
accurately at the focus of a telescope objective. The 
camera lens is placed against this and focussed by parallax, 
as with a distant object. Collimators are employed in camera 
factories, and should be part of the equipment of base labora- 
tories where repairing and overhauling of cameras is done. 

Lens Mounts. — ^All that is required for the mounting of 
an aerial camera lens is a rigid platform, with provision for 
enough motion of the lens to adjust its focus accurately. 
As already explained, the lens works at fixed, infinity, 
focus, and therefore needs no adjustment during use. It 
must be held far more rigidly than would be possible by the 
bellows, which is an almost invariable adjunct of focussing 
cameras. The use of ordinary types of hand cameras on a 
plane is rarely successful just because of the bellows, which 
is strained and rattled by the rush of wind. 

The lens mountings thus far used have been simple 
affairs. In the French cameras the lens is merely screwed 
into a flange which in turn is fastened by screws to a plat- 
form in the camera body. Adjustment for focussing is not 
provided; instead, the flange is raised on thin metal rings or 
washers, cut of such thickness by trial as to bring the lens 
to focus, once and for all. 

The U. S. Air Service method of mounting is to provide 
the lens barrel with a long thread, which screws into a 
flange that in turn is mounted on a platform in the camera 
cone, by means of thumb-screws. The lens is focussed by 
screwing in and out, and then clamped by a screw through 



LENSES 



67 



the side, bearing on the thread. The whole mount may be 
quickly removed by loosening the thumb-screws, and once 
focussed in one cone, can be transferred to another similar, 
machine-made cone without change of focus. Fig. 18 
shows a 20 inch lens mounted in this manner. The photo- 
graph shows as well the ring on the front of the lens by 
means of which circular color filters may be held in place. 




FxG. 18. — 50 centimeter F/6 lens in U. S. standard mount, showing color filter retaining ring and catch. 

This ring screws down on the filter, and the catch is dropped 
into the nearest vertical groove to the tight position. 

A somewhat different and better method of tightening 
the lens in the flange, when focussed, has been adopted in the 
English lens mount, which is in general similar to the 
American. The threaded part of the flange is split by a slot 
cut parallel to the flange base, and a screw is run into the flange 
from the front, through the split portion. By tightening 
this screw, which is always accessible, the split part of the 
flange is squeezed together, thus rigidly holding the lens barrel. 



CHAPTER V 
THE SHUTTER 

Permissible Exposure in Airplane Photography. — A 

definite limitation to the length of .exposure in airplane 
cameras is set by the motion of the plane. If we represent 
the speed of the plane by S, the altitude of the plane by A, 
and the focal length of the lens by F, we obtain at once 
from the diagram (Fig. 19), that s, the rate of movement of 
the image on the plate, is given by the relation, 

S A 
If we call the permissible movement d, then the permis- 
sible exposure time, t, is- given by the relation — 

~ s ~ FS 

x\s a representative numerical case, expressing all 

quantities in centimeters and in centimeters per second, 

^ _ 20,000,000, 

let r =50, S= — — (200 kilometers per hour), and 

^ = 300,000, then 

50 X 20,000,000 
' = 300,000X3600 = '^ ^^timeters 

If we take for the permissible undetectable movement, 
.01 centimeter, which is, as has been shown, a reasonable 
figure for lens defining power, we have, then, that the longest 
permissible exposure is .011 second — in round numbers, 
one-hundredth. 

In flying with a slow plane, or in flying against the wind, 
the exposure can sometimes be increased to as much as 
double this length. Diminishing F would similarly extend 
the allowable exposure, but the ratio of F to ^ approximates 

68 



THE SHUTTER 



69 



to a constant in actual practice; in other words, a certain 
resolution and size of image have been found desirable. 
If flying is forced higher, a longer focus lens is used; if lower 
flying is possible, a lens of shorter focus. This relationship 

^m> — >s 




Fig. 19. — Relative motion of plane and photographic image. 

has, of course, been derived from war-time experience. 
Probably much of the prospective peace-time mapping work 
will impose substantially easier requirements as to definition 
and will thus allow longer exposures. 

For low oblique views the longest exposure is much less. 
Taking 45 degrees as a representative angle for the fore- 
ground, and 500 meters as a representative height, the value 
of t becomes -^jfQ. , 



70 AIRPLANE PHOTOGRAPHY 

These figures will illustrate two important points: 
they show how severe is the limitation as to exposure, with 
the consequent heavy demand on lens and sensitive material 
speed; and they show how important it is to secure a 
shutter with the maximum light-giving power for a specified 
length of exposure. This leads to a study of the character- 
istics as to efficiency of the two common types of shutter, 
namely, shutters at or between the lens, and focal-plane shutters. 

Characteristics of Shutters Located at the Lens. — Of the 
various shutters located at the lens the most common is the 
type that is clumsily but descriptively termed the "between- 
the-lens" shutter. This is composed of thin hard rubber or 
metal leaves or sectors which overlap and which are pulled 
open to make the exposure. It may require two operations, 
one for setting and one for exposing, or it may, as in some 
makes, set and expose by a single motion. Clock escape- 
ments, or some form of frictional resistance, are depended on 
to control the interval between opening and closing. This 
shutter is the one almost universally employed on small 
hand cameras and on all lenses up to about two inches 
diameter. It gives speeds sometimes marked as high as 
3-J-0 second, although usually not over xw ^^ actual test. 

Between-the-lens shutters have been used to some extent 
on the shorter focus (up to 25 centimeter) aerial cameras, 
notably in the Italian service. They suffer, however, from 
two limitations. In the first place we have not yet solved the 
mechanical problems met with in trying to make the shutter 
of large size (as for 50 centimeter F/6 lenses) at the same 
time to give high speeds. In the second place the efficiency 
of the type is low because a large part of the exposure time is 
occupied by the opening and closing of the sectors. 

If we define the efficiency of a shutter as the ratio of the 
amount of light it transmits during the exposure to the 



THE SHUTTER 



71 



amount of light it would transmit were it wide open during 
the whole period, then the efficiency of the ordinary between- 
the-lens shutter is of the order of 60 per cent. This means 
1.6 times the motion of the image for the same photographic 
action that we should have with a perfect shutter. The 
accompanying photographic record (Fig. 20) of the opening 
and closing process of this type of shutter clearly illustrates 
its deficiencies. 

Characteristics of the Focal=Plane Shutter. — Long before 
the days of aerial photography the problem of a high- 




FiG. 20. — Effective lens opening at equal intervals of time: (a) during focal plane shutter exposure; 
(6) during between-the-lens shutter exposure. 

efficiency high-speed shutter for photographing moving 
objects on the ground — railway trains or racing automo- 
biles — ^had already led to the development of the focal-plane 
shutter. This is a type peculiarly adapted to the problems 
of the airplane camera. It consists essentially of a curtain, 
running at high speed close to the photographic plate, the 
exposure being given by a narrow rectangular slot. 

If the focal-plane shutter is in virtual contact with the 
sensitive surface the efficiency, as defined above, is 100 per 
cent., since the whole cone of rays from the lens illuminates 
the plate during the whole time of exposure. But if the 
curtain is not carried close to the plate the efficiency falls 



72 



AIRPLANE PHOTOGRAPHY 



off rapidly with distance, especially so for small apertures 
of the slot. 

The efficiency of the focal-plane shutter may be calculated 
as follows : Let the focal length of the lens be F, its diameter 




Fig. 21. — Calculation of focal plane shutter efficiency. 

be F/N, the width of the slot be a, and the distance from 
plate to curtain d (Fig. 21). Now if the curtain is moving 
at a uniform speed, the time taken for the slot to traverse the 
whole cone of rays, from the instant it enters till the instant 
it leaves, will be directly proportional to 



THE SHUTTER 



73 



If the curtain were in contact with the plate the time 
taken for the same amount of light to reach the sensitive 
surface would be proportional to a. Again defining shutter 
efficiency as the ratio of the light transmitted to what would 
have been transmitted were the shutter fully open for the 
total time of exposure, the efficiency, E, is given at once by 
the expression — 

As an example let the lens aperture be F/6, so that N= 6; 
let d=\, and a=l, then E=^. In the French de Maria 



Ur—IClh- 



Percentaoe of tota/ 5* 
lightfromlens reochipff ^0 . 
plate at any time during 
exposure 



UN5 APERTURE 
F/5 

SLIT APERTURE 
(a)=/cm. 




d= distance, curtiain top/of^. 

'"""' (r/sj 



V.^ 


_ _ 












\^ 


V, 


■ T - 










> 


^ 






~- ~ 


— 






V 

•^ 


^■••^ 














V. 















^ 


■-^^ 




— — 













■~^, 
















^ — 


-—^ 































^ a='3cm 



a' /cm. 
a= •5'cm 



I*- total duration of exposure - 

Fig. 22. — Characteristics of focal plane shutter. 

cameras, where (i=4 centimeters, E=60 per cent, for the 
aperture assumed, which is representative. Fig. 22 exhibits 
diagrammatically the chief characteristics of the focal 
plane shutter. 

In view of the necessity for some distance between shutter 



74 AIRPLANE PHOTOGRAPHY 

and plate it is obviously important to keep a as large as 
possible, depending for the requisite shutter speed on the 
velocity of the curtain. Large aperture and high curtain 
speed are also found to be desirable when we consider the 
distortion produced by the focal-plane shutter. 

Distortions Produced by the Focal=plane Shutter. — 
While the time of exposure of any point on the plate can, 
with the focal-plane shutter, easily be made y^ second or 
less, the whole period during which the shutter is moving is 
much greater than this. For instance, a 1 centimeter opening 
which gives y^ second exposure takes -^q second to move 
across a 10 centimeter plate, or nearly \ second for an 18 
centimeter plate. With a moving airplane this means that 
the point of view at the end of the exposure has moved 
forward compared to that at the beginning, by the amount 
of motion of the plane in the interval. If the shutter moves 
in the direction of motion of the plane the image will be 
magnified; if in the opposite direction, it will be compressed 
along the axis of motion. The amount of this distortion is 
calculated as follows: 

Let the velocity of the plane be F, and that of the shutter 
be V. Let the focal length of the camera be F, and the 
altitude A, If the camera were stationary, a plate of length 
I would receive on its surface an image corresponding to a 

distance ^X/ on the ground. Due to the motion of the 
shutter the end of the exposure occurs at a time — after the 

start. In this time the plane has moved a distance F X^j 

hence the point photographed at the end of the shutter 

. VI . . 
travel is — within or beyond the original space covered 

by the plate, depending on the direction of motion of the 
curtain. The distortion, D, is given by the ratio of this dis- 



THE SHUTTER 75 

tance to the length corresponding to the normal stationary 
field of view: 

When F=200 kilometers per hour, «; = 100 centimeters 
per second, F = 50 centimeters, ^=3000 meters, we have — 

20,000,000X50 . , 1 

^ = 3600X100X300,000 = ^PP^^^^^^^ely — 

Or if the actual distance error on the ground is desired, 

— = 10.8 meters 

V 

As a percentage error this one per cent, is small compared 
with other uncertainties, such as film shrinkage or the error 
of level of the camera. As an absolute error in surveying, 
thirty feet is, of course, excessive. 

The distortion is diminished for any specified shutter 
speed by making the speed of travel of the curtain as large 
as possible and by correspondingly increasing the aperture. 
In connection with film cameras, another solution which 
has been suggested is to move the film continuously during 
the exposure in the direction of the plane's motion. The 
requisite speed of the film v' to eliminate distortion is given 
by the relation : 

V A 

For the values of V, F, and A used above, z;' = .92 centi- 
meters per second. This speed is clearly that which holds 
the image stationary on the film — a fact which suggests 
another object for such movement, namely, to permit of 
longer exposures. 



76 AIRPLANE PHOTOGRAPHY 

The effect of focal plane distortion may be averaged out 
in the making of strip maps, if the shutter is constructed 
so as to move in opposite directions on successive exposures. 
The first picture will be magnified, the second compressed, 
and so on, but a strip formed of accurately juxtaposed 
pictures will be substantially accurate in over-all length. 
Such a shutter is embodied in one of the German film 
cameras (Fig. 61). 

Distortion of the kind above discussed is absent with 
between-the-lens shutters, which may conceivably be im- 
proved in efficiency and in feasible size. If so they would 
merit serious consideration for aerial mapping. 

Methods and Apparatus for Testing Shutter Perform= 
ance. — ^With a focal-plane shutter the desirable qualities in 
performance are three in number: (1) Adequate speed range, 
which may be taken as from ^ to 5^ second for aerial work, 

(2) good efficiency, which has already been treated, and 

(3) uniformity of speed during its travel across the plate. 
Before the advent of aerial photography little attention 
was paid to speed uniformity, differences of 50 per cent, in 
initial and final speed being common in focal-plane shutters, 
and but little noticed in ordinary landscape work because 
of the natural variation of brightness from sky to ground. 
In the making of aerial mosaic maps the non-uniformity of 
density across the plate results in a most offensive series of 
abrupt changes of tone at the junction points of the succes- 
sive prints (Fig. 140), an effect which must be minimized 
by manipulation of the printing light. 

Instruments for testing the speed and uniformity of 
action of focal-plane shutters are an essential part of any 
laboratory for developing or testing photographic apparatus 
and some simple device for setting and checking shutter 
speed should be available in the field. Every such speed 



THE SHUTTER 



77 



tester must contain some form of time counting element — 
pendulum, tuning fork or clockwork. Elaborate shutter 
testers, suitable for determining all the characteristics of 
all types of shutter, have been developed and used in certain 
of the photographic research laboratories. For the study and 
setting of focal-plane shutters (whose efficiency need not be 
measured, as it can be simply calculated from linear dimen- 
sions), the following simple kinds of apparatus are adequate: 
Clock dial type of shutter tester. This consists essentially 
of a black clock dial carrying a white pointer which makes 
its complete revolution in one second or less. If this dial 
is photographed by the camera under test, the width of the 




Fig. 23, — Apparatus for testing focal plane shutter speed throughout the travel of the curtain. 

sector traced during the exposure by the moving pointer 
shows the time interval. If the dial is photographed at 
several points on the plate — beginning, middle and end of 
the shutter travel — the complete characteristics of the 
shutter can be determined. 

Interrupted light type of shutter tester. For the study of 
uniformity of shutter action alone the apparatus shown in 
Fig. 23 may be employed. ^ is a high intensity light source, 
such as an arc or a gas filled tungsten lamp. Z is a convex 
lens, focussing an image of the light source on a small aper- 
ture in the screen E. Z) is a sector disc which, driven by the 
motor M, interrupts the transmitted light with a frequency 
determined by the number of openings of the sector and by 
the speed of rotation, which must be measured by a tacho- 



78 



AIRPLANE PHOTOGRAPHY 



meter. The light diverging from the aperture in E falls 
upon the shutter S, which for this test is reduced to a narrow 
slit of one millimeter or less. Passing through the shutter 
opening the light falls upon the photographic plate P. 
The principle is simple: If the light is uninterrupted, the 
plate P is exposed at all points; due to the interruptions, a 
series of parallel lines of photographic action result, and 



not' 


! 














5 lieu 
%i 

•^ 1800 

'1 - 

', ^ 500 
^^ 


1 .. 








■■"■"■"""-*■' 










1 






i 
1 






j 








/ ( 






r "'*'*" 




















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100 



V 


1 












j 




L. J 












i i i i 1 i i i i i i 


1 Z 8 4 5 J 7 8 . t W 11 tl IS, 14 B H 17 U 
Start Po3/t/o/7 of j//t m centtmeters Finis 


1 

h ; 



Fig. 24. — Performance of Klopcic shutter. 

their distance apart gives a measure of the speed of the 
shutter at any chosen point in its travel. A performance 
curve of the French Klopcic shutter is shown in Fig. 24. 
The variation in speed lies over a range of two to one. So 
serious is this defect in these shutters that diaframs are 
sometimes inserted in the French cameras to cut off part of 
the light from the lens on the most exposed end of the plate. 
This expedient produces uniformity of photographic action, 



THE SHUTTER 



79 



but does not overcome the movement of the image, which is 
one of the chief faults of excessive exposure. 

A more complete apparatus, adapted both to absolute 
speed determinations and to the study of uniformity of 
action, is that worked out and used in the United States 
Air Service (Fig. 25). At ^ is a high intensity light source, 
an image of which is focussed by the lens L^ upon a slit E, 
in front of which stands a tuning fork T, of period 1024 
or 2048 per second. The light diverging from the slit is 
received by a second lens, L^ which is arranged either to focus 
the slit image upon the shutter curtain or to render the rays 




UMP 



CONDENSERS FORrt'SLiT LENS CAMER/l SHUTTER LENSES DRUM 

Fig. 25. — Optical system of shutter tester for Air Service, U. S. Army. 



parallel, so that an entire camera may be inserted. In the 
latter case the camera lens L^ serves to focus the slit image 
on the curtain C, After passing through the curtain aperture 
the light is focussed by the lens X^^ on the rotatable drum D, 
which carries a strip of sensitive film. 

The operation of testing a shutter consists in focussing 
the slit image on the portion of the shutter whose perform- 
ance is required, striking the tuning fork to set it vibrating, 
rotating the drum rapidly and setting off the shutter. 
There is thus obtained on the sensitive film an exposed strip 
resembling in appearance the edge of a saw, the number of 
teeth showing the time interval in vibrations of the tuning 
fork. Three exposures usually give all the points necessary 



80 AIRPLANE PHOTOGRAPHY 

for a practical knowledge of the shutter's uniformity of 
action. A point of some importance, learned from numerous 
shutter tests, is that a focal-plane shutter should be tested 
in the position in which it is to be used. Aerial camera 
shutters should be tested in the horizontal position. 

Types of Focal=plane Shutters. — ^A variety of means 
have been utilized for securing the necessary variation in 
speed in focal-plane shutters. Their success is to be meas- 
ured by the actual speed range and by the uniformity of 
speed attained. In aerial cameras at present in use we find 
variable tension of the curtain spring, the aperture being 
fixed; variable opening with fixed tension; multiple curtain 
openings with fixed spring tension; and combinations of two 
or all of these methods of speed control. The problem of 
covering the aperture during the operation of winding up or 
setting the shutter has led to further elaborations of shutter 
mechanism. These take the form of lens or shutter flaps^ auxil- 
iary curtains, and shutters of the self -capping type. Shutters 
embodying all these features are briefly described below. 

Representative Shutters. — The Folmer variable tension 
shutter is used on the United States Air Service hand-held 
and hand-operated plate camera and on some of the film 
cameras. It consists of a fixed aperture curtain wound on 
a curtain roller in which the spring can be set to various 
tensions, numbered 1 to 10. The range of speeds attainable 
is at best about three to one, or from i-^-q to 3^ second, 
considerably shorter than the range indicated as desirable. 
Its uniformity of travel is variable with the .tension, as shown 
by representative performance curves in Fig. 30. Lacking 
any self -capping feature the shutter is provided either with 
an auxiliary curtain, or in the hand-held camera with flaps 
in front of the lens, opened by the exposing lever before the 
curtain is released (Fig. 39). This shutter is made a re- 



THE SHUTTER 



81 



movable unit in the 18X24 centimeter hand-operated cam- 
era, but is built into the hand-held and film cameras. 

The lea shutter used on the standard German aerial 
cameras is a good example of the multiple slit curtain 
(Fig. 26). Four fixed aperture. slits are provided, with a single 




Fig. 26. — Removable four-slit shutter of German (lea) camera, showing flaps. 

tension, the openings roughly in the ratio 1, 2^, i, "s, which 
when the spring tension is properly adjusted give exposures % 
of Wj T8o"> ■37T> t^o second. To pass from one exposure 
time to another the setting milled head is wound up to suc- 
cessively higher steps or else exposed one or more times 
without resetting, depending on the direction it is desired 
to go. Capping during setting, or during exposure, in order 
6 



82 AIRPLANE PHOTOGRAPHY 

to change the opening, Is provided- for by a pair of flaps on 
the shutter unit, which open into the camera body. The 
mechanical work on these shutters is of excellent quality, 
the curtain running with exceptional smoothness. Provision 
is made for adjusting the tension until the marked speeds 
are attained; this is presumably done in a repair laboratory 
to which the shutter only need be sent, as it is a removable 
unit. Tests made on one of these shutters wound to its 
highest tension are shown in Fig. 30. The marked speeds 
are not attained, and there is considerable lack of uniform- 
ity from start to finish of the travel. 

L camera variable-aperture shutter. The shutter of the L 
type camera (Fig. 27) is representative of one of the most 
primitive methods of varying aperture. The two jaws of 
the slit are held together by a long cord passing completely 
around the aperture, fastened permanently at one end and 
attached at its other end by a sliding clasp or saddle. As 
this saddle is forced in one direction the slit is closed, in the 
opposite direction the cord becomes slack, and after the shut- 
ter is released once or twice the slit assumes a wider opening. 
A chronic trouble is the breaking of the cords. Its opening 
can be changed only after the plate magazine is removed. 

U. S. Air Service variable-aperture shutter. This shutter 
is incorporated in the American deRam and in other late 
American cameras (Fig. 28). Its characteristic feature is 
the introduction of an idler, whose distance from the main 
curtain roller can be varied. Tapes whereby the following 
curtain is attached to the spring roller pass over this idler, 
and by changing its position the aperture or distance between 
the two curtain elements is altered over a large range. 
Tests of this shutter are shown in Fig. 30. A speed of -^ 
second is provided for by a slit width of five centimeters, 
and the highest speed is fixed only by the practical limit of 



THE SHUTTER 




Fig. 27. — "L" type camera showing open negative magazines and shutter mechanism. 



84 



AIRPLANE PHOTOGRAPHY 



approach of the jaws. Experiment shows great uniformity 
of rate of travel to be attainable by combining careful choice 
of spring length and tension with good workmanship in the 
mechanical features. Variable-aperture fixed-tension shut- 
ters have a definite advantage over the variable-tension 
type in that they can utilize for all speeds that tension which 
gives uniform action. The capping feature of this shutter is 
provided in the American deRam by flaps, in the automatic 




IDLER 



Fig. 28. — ^Variable aperture curtain developed in U. S. Air Service, and used in American deRam, 

and "K" type automatic film cameras. 

film camera by an auxiliary curtain. The shutter is remov- 
able in the deRam, but built into the other camera. 

The Klopcic variable-tension, variable-aperture, self- 
capping shutter is an example of an attempt to meet all 
shutter requirements with an entirely self-contained mechan- 
ism. It is shown diagrammatically in Fig. 29. Tapes 
G^, Gg are used to connect the following curtain B 
directly to the spring roller T, at a fixed distance, while the 
leading curtain. A, may be slid along the tapes by small fric- 
tion buckles, C-^^C^i auxiliary springs R^ , R^ serving to keep 
it taut in any position. When the shutter is being set the 



THE SHUTTER 



85 



buckles are arrested against stops while the winding-up 
continues for what is to be the following half of the curtain 
in exposing. When released the curtain moves across 
with an aperture fixed by the point of setting of the buckle 
stops. At the end of the travel the buckles are arrested by 
other stops, while the following portion of the curtain con- 




FiG. 29. — MecJianisiu of Klopcic variable aperture self-eapping shutter. 

tinues its travel to the end. On re- winding, therefore, the 
aperture is closed. Variable tension as well as variable 
aperture is provided, although little used. In the French 
cameras a lens flap is also inserted behind the lens, but this 
is not needed if the self-capping feature functions properly. 
On the hand cameras this flap is said to be necessary in 
order to prevent a curious kind of accident: if the camera 
is held on the knee, pointing upward, an image of the sun 
may be formed on the curtain and burn a hole through it. 



^6 



AIRPLANE PHOTOGRAPHY 



The performance of the French shutter in respect to 
uniformity has already been shown in Fig. 24. It leaves 
very much to be desired. Besides non-uniformity of action 
during its travel it exhibits another common defect of 
variable-tension shutters, namely, the curtain must be re- 
leased several times after a change of tension before the new 
speed is established (Fig. 30, tensions 5 and 5'). 



JUU 




















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200 








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TENSIOfHZ ZOO 



TENSION O 
TENSION 4 



too 



STAJBT MIDDU FINISH 

Vauiable Tension, Fixed Ape/sture, 
Used in Hand Operated ISx24 cm.PlateCameea ■ 



600 



500 
400 

300 

200 

100 



500 



S APCRTIXE 



400 



200 



Z "APei^TVUC 



100 





















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START MIDDLE FINISH 

Klopcic SHurrER. Variable Aperture, 
Variable Tension Type. 
















































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/ 


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— 1 


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START MIDDLE FINISH 

Fixed Tension, Vabiable Aperture, Idler 
Pattern Used in American DeRam. 



TENSION S 
TENSION 
TENSION t 



SLIT J^ 



SL/r J^ 



SLIT iio 
SLIT 5^ 



STAjer MIDDLE FINISH 

Fixed Tension, Multiple Slit Curtain, Used^ 
IN German Ica 13x18 ch Camera. 



Fig. 30. — Performances of various shutters used on aerial cameras. Speeds expressed in reciprocals 

of fractional parts of one second. 

The French shutter as made for the de Maria cameras 
is a removable unit. The small size (13X18 cm.) sets by 
the straight pull of a projecting pin, the larger (18 X24 cm.) 
by winding up a milled head. The former is the more con- 
venient motion for an aerial camera. Care must be taken 
with either type that the motion of setting is not stopped 
when the first resistance is encountered; this occurs when 
the tape buckles strike their stop and the slit begins to open. 



CHAPTER VI 
PLATE-HOLDERS AND MAGAZINES 

In the earlier days of airplane photography the ordinary 
plate-holder or double dark slide was used to some extent, 
but it is ill-suited to the purpose because of the considerable 
time and attention required for its operation. It has never- 
theless the merit of adding little to the length of the camera, 
and it works in any position. For these reasons it has re- 
mained in occasional use for the taking of oblique views 
with long focus cameras in a cramped fuselage. 

Next in order of progress rank the simple box magazines, 
for holding a dozen, eighteen or twenty-four plates, as used 
in the English C, E, and L type cameras. These are little 
more than boxes with sliding lids which when open permit 
the introduction or removal of the plates. Figs. 45 and 46 
illustrate the magazine of this type as made for the English 
C and E cameras. It is constructed of wood, grooved to fit 
tracks on the camera, and is furnished with a sliding door or 
lid hinged in the middle to fold down out of the way when 
open. The eighteen plates are carried in metal sheaths, 
both to provide opaque screens between them, and to protect 
them from injury in the mechanism of the camera. Fig. 27 
shows the all-metal magazine made for the American model 
L camera. This differs from the English in material of con- 
struction, plate capacity (24 instead of 18) and manner of 
operating the slide, which is built up of three thicknesses of 
phosphor bronze and draws out through metal guides bent 
into semicircular form.' A snap catch holds this slide at 
either end of its travel. The leather strap introduced in the 
American model for carrying and handling is a distinct 

87 



/ 



88 



AIRPLANE PHOTOGRAPHY 



improvement. These magazines contain no springs or other 
mechanism, as the cameras with which they are used depend 
upon the action of gravity for emptying the upper (feeding) 
magazine, and filHng the lower (receiving) one. 

Next in order of complexity may be ranked the bag 
magazine (Figs. 31 and 44). In this the exposed plate is 
pulled out of the magazine proper by a metal slide or rod 
into a leather bag. The rod is then pushed back, the plate 
in its metal sheath is grasped through the leather bag, lifted 




Fig. 31. — Aerial hand camera (U. S. type A-2). 

to the back of the magazine, and forced in behind the other 
plates. The number of plates exposed is indicated either by 
numbers on the backs of the sheaths, visible through a red 
glazed opening in the back, or else by a counter actuated 
by the metal slide rod. Usually twelve are carried in a 
magazine. For aerial work the common design of this 
magazine as used for ground work must be modified by 
providing extra large easily grasped hooks both on the draw 
rod and on the dark slide, which must be drawn before 



PLATE-HOLDERS 89 

making the first exposure and replaced after the last. The 
small rings and grips of the standard commercial magazine 
are almost impossible to handle through heavy gloves. 

The next type of magazine is represented by three 
designs, the Gaumont and de Maria, used very generally by 
the French during the war, and the Ernemann, used almost 
universally in the German air service (Figs. 32, 40 and 42). 
Li all of these the operation of plate changing is the same: 
the end of the magazine is pulled out and thrust back, a more 
simple operation than the bag manipulation just described. 
The internal workings are different according to size. In 
the smaller French magazines (13X18 cm.) the camera is 
first pointed upward, all the plates are drawn out except 
the one to be changed, and this, with the aid of springs, drops 
to the bottom, after which the other plates push back over 
it. The plates pull out in the direction of their long dimen- 
sion. In the larger French magazine (18X24 cm.) only the 
exposed plate pulls out. The pull is in the direction of the 
shorter dimension of the plate, which is lifted up by heavy 
springs and slides back over the top of the pile. In the 
Ernemann magazine only six plates are carried, which there 
is good reason to believe represent the maximum feasible 
number, judging by the reports of jambs and breakages in 
the twelve-plate French magazines. In all of these magazines 
laminated wood slides pull out and in at each operation, 
and while satisfactory if made and operated in one climate, 
experience indicates that if made in America and sent abroad 
swelling of the wood may be expected to prevent their 
successful operation. 

Alternative forms of magazine, somewhat more practical 
from the standpoint of manufacture and export, are several 
designs embodying two compartments (Fig. 32). In the most 
simple of these the plates are moved, immediately before 



90 



AIRPLANE PHOTOGRAPHY 



or after exposing, from the unexposed to the exposed side. 
Illustrative of this type are the Folmer designs, in which the 
to-and-fro motion is imparted by a rack geared to a pinion 



a 




Cm ^ 




deAior/a 



Qaumcnf 







CJ ^^ 



.Fiserint ^ Mondini 



Chaise/ 



Button 



Unejtposgd. 




W 


Exposed 







Unexposed 




Expe9*d 


\ ' 


guposcd 




\ 









Bou fanner Be Hi en i 

'lVLB:'BM' 

Fig. 32. — Various plate magazines used on aerial cameras. 

actuated either by a lever, in the hand camera, or by the 
power drive, in the automatic design (Figs. 33 and 53). An- 
other illustration is afforded by the Piserini and Mondini 



PLATE-HOLDERS 



91 




Fig. 33. — U. S. Air Service hand camera, with two-compartment magazine. 




Fig. 34. — Film type hand camera. 



magazine, in which the operation of changing is performed 
by a back-and-forth motion of a hand-grip, which also sets 
the camera shutter (Fig. 47). 



n AIRPLANE PHOTOGRAPHY 

In these magazines the center of gravity changes as the 
exposed plates are moved over, and only half the inside 
space is occupied with plates. These objections are over- 
come in the Chassel form, where both compartments are 
always full. Transfer of the bottom exposed plate from one 
compartment to the other is compensated for by the simul- 
taneous shift of the top plate in the receiving compartment, 
to the feeding side. In a modification of this idea by Ruttan 
the exposing position is when the plates are half-way through 
the shifting process, whereby the magazine may be symmet- 
rically mounted on the camera body. 




""^^^^^Hgg 



Fig. 35. — Apparatus for straightening plate sheaths. 

Other more complicated magazines have been designed, 
some of which are shown in the diagrammatic ensembles of 
Figs. 32 and 48. In the Jacquelin, the main body of plates 
is raised while the bottom (exposed) plate is folded against 
the side. The main body of plates then drops back to place, 
the exposed plate is carried on upward and folds down on 
the back of the pile. The Bellieni magazine system uses 
three, a central feeding one and two below for receiving, 
one on each side of the camera body. These were made 
solely for attachment to captured German cameras. In 
the Fournieux magazine the plates are carried in an interior 



PLATE-HOLDERS 



93 



rotating box. The plate to be exposed is dropped off the 
front of the pile, down to the focal plane, and after exposure 
is picked up and placed at the back of the pile, which has 
turned over in the meanwhile. The deRam rotating maga- 
zine is described in connection with the camera of which it 
is an essential part (Fig. 52). 




Fig. 36. — Training plane equipped for photography, showing "L" camera in floor mount and 
magazine rack forward of the observer. 

For the protection of the plates during their manipulation, 
and in the camera, all plate magazines thus far developed 
carry them in thin metal sheaths. These add greatly both 
to the weight and to the time necessary to handle the plates, 
but no means have as yet been found for dispensing with 



94 AIRPLANE PHOTOGRAPHY 

them. Fig. 35 shows a representative sheath or septum, as 
used in the L camera. On three sides the edge is bent up 
and turned over, forming a ledge for the plate to press 
against. The fourth side is left open for inserting the plate, 
which is then held in by a small upward projecting lip, and 
kept close against the ledges by narrow springs at the sides. 
To insert or remove the plate the projecting lip is depressed, 
either by springing the sheath by pressure from the sides 
or by using an appropriate tool. 

Care of sheaths. Unless systematically taken care of, 
plate sheaths become bent or dented They are then a 
menace to camera operation, catching or jamming in the 
plate changing process, breaking plates and damaging 
camera mechanisms. In order to maintain them flat and 
true, steel forms are necessary on which the sheaths may be 
hammered to shape with a mallet (Fig. 35). 

Magazine racks. Reconnaissance and mapping call for 
a number of exposures much greater than the capacity of 
one 12, 18, or 24 plate magazine. Additional magazines 
must therefore be carried. These should be in racks con- 
venient to the observer (Fig. 36), securely held yet capable 
of quick removal and insertion. In the rack designed to 
carry two of the metal magazines for the American L Camera, 
the magazines slide into loose grooves formed by a metal lip. 
They are prevented from slipping out by a spring catch, 
past which they slide when inserted but which is instantly 
thrown aside by pressure of the thumb as the hand grasps 
the magazine handle for removal. 



CHAPTER VII 
HAND-HELD CAMERAS FOR AERIAL WORK 

Field of Use. — The first cameras to be used for aerial 
photography were hand-held ones of ordinary commercial 
types. Indeed the idea is still prevalent that to obtain 
aerial photographs the aviator merely leans over the side 
with the folding pocket camera of the department store 
show window and presses the button. But the Great War 
had not lasted long before the ordinary bellows focussing 
hand camera was replaced by the rigid-body fixed-focus 
form, equipped with handles or pistol grip for better holding 
in the high wind made by the plane's progress through the 
air. Even this phase of aerial photography was compara- 
tively short-lived. The need for cameras of great focal 
length, and the need for apparatus demanding the minimum 
of the pilot's or observer's attention, both tended to relegate 
hand-held cameras to second place, so that they were com- 
paratively little used in the later periods of the war. 

Yet for certain purposes they have great value. They 
can be used in any plane for taking oblique views, and for 
taking verticals, in any plane in which an opening for unob- 
structed view can be made in the floor of the observer's 
cockpit. They can be quickly pointed in any desired 
direction, thus reducing to a minimum the necessary ma- 
neuvering of the plane, a real advantage when under attack 
by " Archies " or in working under adverse weather conditions. 

For peace-time mapping work the hand-held camera, 
when equipped with spirit-levels on top, and when worked 
by a skilful operator, possesses some advantages over any- 
thing short of an automatically stabilized camera. For 

95 



96 AIRPLANE PHOTOGRAPHY 

experimental testing of plates, filters and various acces- 
sories, the ready accessibility of all its parts makes the hand- 
held camera the easiest and most satisfactory of instruments. 

The limitations of the hand-held camera lie in its necessary 
restriction to small plate sizes and short focal lengths, and 
in the fact that it must occupy the entire attention of the 
observer while pictures are being taken — the latter a serious 
objection only in war-time. 

Essential Characteristics. — In addition to the general 
requirements as to lens, shutter and magazine, common to 
all aerial cameras, the hand camera must meet the special 
problems introduced by holding in the hands, especially over 
the top of the plane's cockpit. An exceptionally good system 
of handles or grips must be provided whereby the camera 
can be pointed when pictures are taken, and held while 
plates are being changed and the shutter set. The weight 
and balance of the camera must be correct within narrow 
limits; the wind resistance must be as small as possible; the 
shutter release must be arranged so as to give no jerk or tilt 
to the camera in exposing. 

As to the method of holding the camera, a favorite at 
first among military men was the pistol grip, with a trigger 
shutter release (Fig. 37). Because of the size and weight 
of the camera the pistol grip alone was an inadequate means 
of support and additional handles on the side or bottom had 
to be provided for the left hand. Small (8X12 cm.) pistol 
grip cameras were used to some extent by the Germans 
(Fig. 42), and a number of 4X5 inch experimental cameras 
of this type were built for the American Air Service (Fig. 37) . 
But the grasp obtained with such a design is not so good as is 
obtained with handles on each side or with flat straps to go 
over the hands. The camera balances best with the handles 
in the plane of the center of gravity. As to weight, no set 



HAND-HELD CAMERAS 



97 



rules are laid down, but experience has shown that a fairly 
heavy camera — as heavy as is convenient to handle — will 
hold steadier than a light one. The American 4X5 inch 
cameras described below weigh with their magazines in the 
neighborhood of twelve pounds. 




Fig. 37. — Pistol-grip aerial hand camera. 

Representative Types of Hand=held Cameras. — French 
and German hand-held cameras are essentially smaller 
editions of their standard long-focus cameras, and a descrip- 
tion of them will apply to a considerable extent to the large 
7 



98 



AIRPLANE PHOTOGRAPHY 



cameras to be discussed in a later chapter. The English and 
American hand-held cameras are generally quite different in 
type from the large ones, which are used attached to the plane. 
The French hand-held camera uses 13X18 centimeter 
plates, carried in a deMaria magazine, and has a lens of 26 
centimeters focus. The shutter is the Klopcic self-capping 








>:>; ., i 



Fig. 38.' — Diagram of French (deMaria) 26 cm. focus hand camera, using 13x18 cm. plates. 

type already described, and is removable. The camera body, 
built of sheet aluminum, takes a pyramidal shape. In Fig. 
38, A is the shutter release and B the rectangular sight, of 
which C is the rear or eye sight. The complete sight may be 
placed either on the top or on the bottom of the camera. 
At D are the handles, sloping forward from top to bottom; 
^ is a catch for holding the magazine; Fis a door for reaching 
the back of the lens and the lens flap; is a snap clasp for 



HAND-HELD CAMERAS 99 

holding the front door of the camera closed; fl^ is a ring for 
attaching a strap to go around the observer's neck; I is the 
lever which opens the flap behind the lens and releases the 
focal-plane shutter; J is a snap catch for holding the front 
door of the camera open. 

The operations with this camera are three in number. 
Starting immediately after the exposure, the camera is 
pointed lens upward and the plate changed by pulling the 
inner body of the magazine out and then in; next the shutter 
is set; then the camera is pointed, and finally exposed by a 
gentle pull on the exposing lever. 

The English hand-held camera (Fig. 186). This differs 
from the French in the size of plate (4 X5 inch), in the shape 
of the camera body, which is circular, and in the type of 
shutter, which is fixed-tension variable-opening. In the 
longer focus camera (10 to 12 inch) the shutter is self -capping, 
and the aperture is controlled by a thumb-screw at the side. 
In the smaller (6 inch) a lens flap is provided in front of the 
lens and the shutter aperture is varied by a sliding saddle 
and cord. The handles of the camera are placed vertical, 
instead of sloping as in the French. The shutter is released 
by a thumb-actuated lever. Double dark slides are used, as 
the multiple plate magazine has not found favor in the 
English service. 

The German hand-held camera (Fig. 42). The German 
hand-held camera is, like their whole series, built of canvas- 
covered wood, the body having an octagonal cross-section. 
It is equipped with the lea shutter and uses the Ernemann 
six plate (13X18 cm.) magazine. The excellent system of 
grips by which the camera is held and pointed is an especially 
commendable feature. On the right-hand side is a handle 
similar to the French type, but carefully shaped to fit the 
hand. The left-hand grip consists of a long, rounded block of 



100 



AIRPLANE PHOTOGRAPHY 



wood running diagonally from top to bottom of the side, 
with a deep groove on the forward side for the finger tips, 
while over the hand is stretched a leather strap, the whole 
aim being to provide an absolutely sure and comfortable 
hold on the camera during the plate changing and shutter 
setting operations. 




Fig. 39. — Front view of U. S. aerial hand camera, showing lens flaps partly open, and details of 

tube sight. 

United States Air Service hand cameras. The hand camera 
developed for the United States Air Service and manufac- 
tured by the Eastman Kodak Co. is made in three models, 
using the bag magazine, a two-compartment magazine, and 
roll film, respectively. The shutter is of the fixed (one or two) 
aperture variable tension type, built into the camera. A 



HAND-HELD CAMERAS 101 

distinctive feature is the double lens flap, in front of the lens 
actuated by the thumb pressure shutter release (Fig. 39). 
In the bag magazine camera the shutter is set, as a separate 
operation, by a wing handle, and a similar handle controls 
the tension adjustment. In the two-compartment type 
(Fig. 33) the shutter wind-up is geared to the plate changing 
lever, so that but one operation is necessary to prepare the 
camera for exposure. In the film type (Fig. 34) a single lever 
motion sets the shutter and winds up the film ready for 
the next exposure. After the last exposure of all the film is 
wound backward on its own (feeding) roller before remov- 
ing from the camera. The film is held flat by a closely fit- 
ting metal plate behind, and by guides at the edges in front, 
an arrangement which with small sizes works fairly well 
although the exquisite sharpness of focus attainable with 
plates is not to be expected. The saving in weight made 
possible by the use of film in place of plates in metal 
sheaths is about three pounds per dozen exposures. 

In all these cameras the sight — a tube with front and back 
cross wires — is placed at the bottom. This position has been 
found the most convenient for airplane work, as it neces- 
sitates the observer raising himself but little above the cock- 
pit, a matter of prime importance when the tremendous 
drive of the wind is taken into account. 



CHAPTER VIII 
NON-AUTOMATIC AERIAL PLATE CAMERAS 

The ideal of every military photographic service has been 
an automatic or at least a semi-automatic camera, in order 
to reduce the observer's work to a minimum. Yet as a 
matter of fact almost all the aerial photography of the Great 
War was done with entirely hand-operated cameras. The 
primary reason for this was that no entirely satisfactory 
automatic cameras were developed, cameras at once simple 
to install and reliable when operated. Even the propeller- 
drive semi-automatic L type of the British Air Service was 
very commonly operated by hand, for many of the pilots 
and observers regarded the propeller merely as another part 
to go wrong. 

Any automatic mechanism in the airplane must work 
well in spite of vibration, three dimensional movements, 
and great range of temperature. The requirements were 
well recognized when the war closed, but had not yet been 
met. Careful study of the conditions and needs by compe- 
tent designers of automatic machinery may be expected to 
result at an early date in reliable cameras of the automatic 
type, but the description below of hand-operated cameras 
really covers practically all the cameras found satisfactory 
in actual warfare. 

General Characteristics of Hand=operated Cameras. — 
As distinguished from the hand-held cameras the larger 
hand-operated cameras are characterized by the greater 
focal length of their lenses, the size of plate employed, and 
the manner of holding — ^by some form of anti-vibration 
mounting attached directly to the fuselage. 

102 



NON-AUTOMATIC CAMERAS 103 

Except for the early English C and E type cameras which 
called for 10 inch lenses and 4X5 inch plates, the general 
practice at the close of the war by agreement between the 
French, English and American Air Services, was for the use 
of 18X24 centimeter plates and for lenses" with focal lengths 
of approximately 25, 50 and 120 centimeters. The English 
also made use of a 14 inch (35 centimeter) lens, and never 
made a regular practice of anything larger than 50 centi- 
meters. The Germans and Italians restricted themselves 
to the 13.x 18 centimeter size of plate, while a lens of 70 
centimeters focal length was standardized with the Germans, 
in addition to the 25, 50, and 120 centimeter. 

The particular focal length was determined by the nature 
of the photographic mission. Where large areas were to be 
covered at low altitudes or without the demand for exquisite 
detail, the shorter focus lenses suffice. The mos.t commonly 
used lens in the French Service was the 50 centimeter, while 
the 120 was employed when high flying was necessary or 
when minute detail was required. As already mentioned, 
the common practice was to keep cameras of all focal lengths 
available, but the ideal at the close of the war was to have 
the camera nose and lens a detachable unit, so that any focal 
length desired could be secured with the same camera body. 

The standard French camera. The hand-held form of 
French camera has already been described. The cameras 
for larger plate sizes and longer focus lenses differ only in 
the addition of a Bowden-wire distance release for the 
shutter and in the use of the Gaumont magazine which 
operates without the necessity of pointing the exposed side 
of the magazine upward. Fig. 40 illustrates the 50 centi- 
meter camera, and Fig. 41 the 120. 

The German lea cameras. These are larger editions of 
the light wood hand camera already described, but with the 



104 AIRPLANE PHOTOGRAPHY 




Fig. 40. — 50 ceniimeter deMaria hand operated camera on tennis ball mounting. 



NON-AUTOMATIC CAMERAS 105 

■ 1 




Fig. 41. — 120 centimeter deMaria camera. 



106 



AIRPLANE PHOTOGRAPHY 



addition of a Bowden-wire shutter release. The body of the 
larger cameras carries a distinctive feature in the distance 
control of the lens diafram, worked by means of a lever 
which actuates racks, pinions and connecting rods leading 
to the lens. On the side of the camera body a shallow box 




Fig. 42. — German aerial cameras. 



is provided for carrying the color filter in its bayonet joint 
mount to fit on the lens (Figs. 42 and 43). 

The hand-operated bag-magazine camera of the United 
States Air Service (Type M) is similar to the small hand- 
held camera, but differs in three respects: a removable 
shutter (of the variable-tension fixed-aperture type) embody- 
ing an auxiliary curtain for capping during the setting opera- 



NON-AUTOMATIC CAMERAS 107 




Fio. 43. — Diagram of Germau 50 centimeter camera* 



108 



AIRPLANE PHOTOGRAPHY 



tion; a Bowden-wire shutter release; and the employment 
of a set of standard interchangeable cones to hold lenses of 
several focal lengths. The 20 inch and 10 inch cones are 




Fig. 44. — U. S. hand-operated aerial camera (type M) with 10 and 20 inch cones. 

shown in Fig. 44. The operation of this camera is similar 
to the French standard cameras, but not so simple because of 
the number of motions required in manipulating the bag. 



NON-AUTOMATIC CAMERAS 109 



Its chief objection for war work lies in fact in the magazine, 
which should be superseded by a two-compartment or other 
satisfactory type of plate changing chamber. The camera 
alone, with 20 inch cone, weighs approximately 40 pounds; the 
loaded magazine, with its plates in metal sheaths, 15 pounds. 
The English C and E type cameras. The C and E type 
cameras have now chiefly an historic interest. They were 




Fig. 45. — English C type aerial camera. 

the first used in the English service, fixed to the fuselage, 
and were later used in training work in England and in the 
United States. They were never built for plates larger than 
4X5 inch nor for lenses of more than 12 inch focus, a limita- 
tion set by the lenses available at the time of their design. 
In several respects the mode of operation of the two 
types is the same. The unexposed plates are held in a maga- 
zine lying above the camera, in the axis of the lens (Fig. 32). 



110 



AIRPLANE PHOTOGRAPHY 



After exposure the bottom plate is carried to one side and 
allowed to fall by the action of gravity into the receiving 
magazine. In the C type (Fig. 45) an opaque slide is drawn 
between the lens and the (variable-opening) shutter during 
the setting operation. During the exposure period this slide 
projects into a compartment on the opposite side of the 




Fia. 46. — English type "E" hand-operated plate camera. 

camera from the receiving magazine, thus making the 
camera mechanism three plates wide. In the E type (Fig. 
46), a flap over the lens makes it possible to dispense with 
the sliding screen, and reduces the camera to about the 
width of two plates. In the € type the plates are changed 
by a handle on top of the camera; in the E type provision 
is made for distance control by cords, and for shutter release 



NON-AUTOMATIC CAMERAS 111 

by a Bowden wire. In both cameras the operation of plate 
changing also sets the shutter, a definite advance over the 
two preparatory motions in the French apparatus. The C 
type was constructed of wood, the E of metal. 




Fig. 47. — Italian (Piserini and Mondini) two compartment magazine hand-operated camera. 

Italian two-compartment magazine camera. A camera 
designed by Piserini and Mondini was used to some extent 
by the Italian service toward the close of the war (Fig. 47). 
This has the desirable feature just noted in the C and E 
cameras: the operations of plate changing and shutter 
setting are performed in a single motion. Unlike those 
cameras, however, the plates are changed from one compart- 



m AIRPLANE PHOTOGRAPHY 

ment to another of the magazine already described, without 
dependence on gravity, by an entirely positive shifting 
action. The setting of the self-capping focal-plane shutter 
is accomplished by a projecting finger engaging the shutter 
mechanism. Cameras of this general type, built for 18 by 
24 centimeter plates, with interchangeable lens cones, 
removable shutters, and preferably magazines in which the 
center of gravity does not shift as the plates are changed, 
represent the next step in advance of the French practice, 
and may indeed prove all that is necessary or desirable in 
camera complexity for peace-time photography from the air. 

The standard Italian camera and similar types. The 
camera (Lamperti) which the Italian Air Service used 
almost exclusively during the war exemplifies a type quite 
different from anything as yet described (Figs. 48 and 49). 
Plates to the number of twenty -four (13 X18 cm.) are loaded 
into a chamber at the top of the camera. Each plate is held 
in a septum furnished with projecting lugs at one end. A 
lever acting through a Bowden wire, exposes the bottom 
plate, which then swings downward about these lugs as 
pivots, and is forced by a pair of fingers into a compartment 
at the side. The between-the-lens shutter has a single speed 
of 1/150 second, and variation of exposure is achieved by 
altering the lens aperture. 

The great advantage of this camera is its simplicity, a 
single motion performing all the operations. Its disadvan- 
tages are its dependence on gravity for operation, its be- 
tween-the-lens shutter, the limitation set to the number of 
exposures, and the necessity for removing the whole camera 
to take out the plates for developing. In actual practice 
the camera has worked out well. The better light found in 
the Italian as contrasted with the northern theatre of war 
makes the between-the-lens shutter at high speed adequate, 



NON-AUTOMATIC CAMERAS 113 




n-r 


'k ~T 


v 'r; 


Shutter— "^ 


h 


1 fioset/ 




Exposed 
plates 



Aubry 




Unexposed 




K 


Exposed 


\ / 





Larnperti 



5hvHtr- 





it 



Jacque/in 



Fr em aero 





Fourrjieux 

Fig. 48. — Various plate changing devices. 





114 AIRPLANE PHOTOGRAPHY 

while the limitation to the number of exposures has been 
met by carrying several complete cameras in each plane. 
Because of the Bowden-wire operation these cameras need 
not be accessible to the observer or pilot, so that the practice 




Fig. 49. — Italian (Lamperli) single-motion plate camera, on anti-vibration tray. 



of carrying them in single-seaters was common. Attempts 
at standardization of Allied practice through the adoption 
of standard lens cones were, of course, out of the question 
with this camera. With its limitations of shutter efficiency 



NON-AUTOMATIC CAMERAS 115 

and plate size it is doubtful whether it would have been 
s^atisfaetory outside the service for which it was developed. 
The limitations set by the between-the-lens shutter in 
this type have been overcome in an experimental camera 
along similar lines made by the Premo Works of the Eastman 
Kodak Company, and in the French Aubry model (Fig. 48). 
These employ focal-plane shutters which swing out of the 
way and are set as the exposed plate swings or drops to the 
receiving chamber. The dependence on gravity in this type 
could doubtless be avoided by positive finger" mechanisms. 
If so, the resultant cameras, set and exposed by a single 
motion, would acquire a highly desirable simplicity of opera- 
tion. They would have peculiar merit because of the very 
short interval required between exposures — a characteristic 
needed for making low stereo-oblique views. The cameras 
just mentioned have, however, departed far in form from the 
lines of standardized practice and have not been followed up. 



CHAPTER IX 
SEMI-AUTOMATIC AERIAL PLATE CAMERAS 

In the hand-operated camera the limit to progress is set 
when the number of operations is reduced to a minimum. 
In cameras using the larger sizes of plates a reduction in the 
number of operations almost inevitably results in inflicting 
considerable muscular labor upon the operator. Furthermore, 
distance operation becomes difficult to arrange for, because 
the common reliance — the Bowden wire — is unfitted far 
heavy loads. Consequently, for setting the shutter and 
changing the plates we must resort to some other source of 
power than the observer's arm. Air-driven turbines or 
propellers have been used on aerial cameras, as well as clock- 
work, and also electric power, the latter derived either from 
a generator or from storage batteries. The relative merits 
of these sources of power form the subject of a separate 
chapter. Mention only is here made of the form of drive 
actually employed in connection with the various cameras. 

The term semi-automatic camera is best used to designate 
that type in which the observer (or pilot) has merely to 
release the shutter, after which the mechanism performs all 
the operations necessary to prepare for the next exposure. 
There has been some difference of opinion as to whether it 
is ever advisable to go further than this with plate cameras. 
The English Service holds that completely automatic expos- 
ing, in addition to plate changing, is apt to encourage the 
making of many more pictures than necessary, involving 
carrying an excessive weight of plates. The French Service 
has rather generally favored entirely automatic cameras in 

116 



SEMI-AUTOMATIC CAMERAS 117 

theory, although during the war practically all the work 
of the French army was done by the hand-operated cameras 
already described. 

The English L Type Camera. — The L, a modification of 
the earlier C and E models, differs from its predecessors 
chiefly in the addition of a mechanism which when connected 




Fig. 50. — American model, English "L" type semi-automatic camera. 

with a suitable source of power can be used whenever desired 
for changing the plates and setting the shutter. As in the 
C and E types, all unexposed plates are carried in a magazine 
above the camera, while the exposed plates are shifted in a 
horizontal direction to one side and fall thence to a re- 
ceiving magazine. 

Fig. 50 shows the American model, which is a copy, 
with modifications, of the original English design. Its weight 



118 AIRPLANE PHOTOGRAPHY 

with one loaded magazine is about 35 pounds. Its manner 
of functioning may be studied from the picture of the 
mechanism (Fig. 51). The part of the mechanism to the 
left is inoperative during hand operation, and the large 
toothed wheel is locked by the removable pin shown hanging 
on its chain in Fig. 50. To change a plate and set the shutter 



Fig. 51. — Mechanism of "L" camera. 



the projecting lever (Fig. 50) is thrown over and back. 
This causes a sliding tray, in which the exposed plate rests, 
to travel to the right, over the receiving magazine, where 
the plate is dropped. After this the tray returns to the left 
exposing position. Simultaneously the shutter is wound up. 
Exposure is made either by pressing down upon the plunger, 
or better, by using a Bowden wire. Provision for both 
methods of exposing, one for the pilot and one for the obser- 



SEMI-AUTOMATIC CAMERAS 119 

ver, is shown in Fig. 81. The shutter is th variable- 
aperture type already described, provided in addition with 
a tension adjustment on the back of the camera. A flap 
behind the lens does the capping during the setting operation. 

For power operation the camera is connected through a 
flexible shaft with a wind driven propeller (Figs. 50, 83 and 
84). The locking pin is now moved over from the toothed 
wheel to the lever arm, so that the rotation of the worm 
driving the large toothed wheel forces the lever through its 
plate changing motion. To prevent repetition, a part of the 
periphery of the toothed wheel is cut out, so that it stops when 
its cycle is run. When the Bowden wire actuates the shutter 
release it forces the toothed wheel around into engagement 
(aided by oije spring tooth) and so starts the cycle once more. 

When connected with the air propeller the worm is ro- 
tated continuously. Other sources of power — an electric 
motor, for instance — can be attached through the same kind 
of flexible shaft. If an electric motor is employed it may be 
run continuously or it may be operated with an insulated 
sector introduced into the large toothed wheel so that the 
electric circuit is broken and the motor stops until the wheel 
is once more forced around by the exposing lever. 

Faults of the L camera. The L camera was the mainstay 
of the English Air Service. In fact for the last two years of 
the war it was practically the only camera the English used, 
and they thought highly of it. It is, of course, subject to the 
limitation of small plate size and short focus lens. It is in 
many ways an inconvenient camera to handle. For instance, 
the upper magazine cannot be closed or removed until all 
the plates are passed through. Its dependence upon gravity 
for the plate changing operation is a fundamental weakness, 
responsible for its frequent tendency to jam in the air. 
Experience made the English observers very expert in 



120 AIRPLANE PHOTOGRAPHY 

relieving these jams. Sometimes they would turn the 
propeller backward (mounting it in an accessible position 
to provide for this contingency), sometimes they would 
shake or thump the camera. But while these makeshifts 
would serve to secure pictures — the chief object, of course, of 
the photographic service — they can scarcely be said to render 
the camera satisfactory. 

Moreover, the propeller drive has not been universally 
approved, as it furnishes an additional mechanism to make 
trouble. Since it is not feasible to change from power to 
hand operation while in the air, the camera is put out of 
commission whenever the propeller or shaft is disabled. 
Bowden-wire controls for both plate changing lever and 
shutter release were common in the British service, which 
considered the extra operation or the extra muscular exer- 
tion unimporant when compared with the greater assurance 
of reliable action. 

The English LB and BM Cameras. — ^During the closing 
months of the war an improved L type camera was con- 
structed, the LB. This differs from the L in a number of 
detail changes, dictated by experience. The shutter is now 
made removable and self -capping. Pivoted lugs are pro- 
vided to hold the exposed plate horizontal until the very 
instant it drops, in an effort to prevent jams caused by 
the plates piling up at an angle in the receiving magazine. 
The chief addition, however, is the provision of several 
interchangeable cones and cylinders, for carrying lenses of 
focal lengths from 4 to 20 inches. Fig. 95 shows the LB with 
20 inch lens cylinder mounted on a bell crank support in 
the camera bay of an English plane. 

The BM camera is but a larger edition of the LB, for 
18X24 centimeter plates. It also carries several inter- 
changeable lens cones. 



SEMI-AUTOMATIC CAMERAS 121 

The American model deRam camera. — The rotary chang- 
ing box devised by Lieutenant deRam of the French army 
and incorporated in his entirely automatic plate camera, 
has been adapted by the American Air Service to a very 
successful semi-automatic camera. Fig. 52 shows the prin- 
ciple of this changing box. The pile of fifty plates, each in 
its sheath, is carried in a rectangular box open at top and 
bottom. The lower plate next the focal-plane shutter is 
first exposed; the pile then rotates about a horizontal axis 
through a complete turn. When the exposed plate arrives 
in a vertical position it is allowed to drop off, by the opening 
of cam actuated fingers, and lodges against the side of the 
enclosing camera box proper. Still further along in the cycle 
the plate is thrown off from its lodging place into a "scoop" 
on the top of the rotating container and laid on the top of 
the plate pile. Meanwhile the curtain of the focal-plane 
shutter winds up, at the same time that it is depressed out of 
the way of the revolving plate container. Although the 
plate changing operation depends on gravity, it nevertheless 
functions satisfactorily up to 30 degrees from the vertical. 

The shutter in this model is the variable-aperture fixed- 
tension type, adjusted by pivoted idlers (Fig. 28). In 
the exposing position it runs within three millimeters of the 
plate surface, and is therefore of high efficiency for all open- 
ings. Capping during the operation of setting is performed 
by flaps at the bottom of the camera body. Interchange- 
able cones are supplied for lenses of various focal lengths. 

For hand operation the changing box is turned over by 
means of a handle, which rotates four times for the complete 
cycle (Fig. 90) . For semi-automatic operation an additional 
mechanism is provided on the side of the rectangular 
camera body, copied with some necessary modifications 
after the L camera power drive. From the observer's stand- 



122 AIRPLANE PHOTOGRAPHY 



=• ^^ ■=--\« 






3K0CK 



rOLMER 



1 


'""1 






1 / 

\ / 


s 
\ 

\ 



DcRAK 



1 







Fig. 52, — Diagram of automatic plate camera movements. 



SEMI-AUTOMATIC CAMERAS ns 



point the operation of the whole camera is the same as in the 
L camera, with the important exception that power operation 
in no way interferes with hand operation. Indeed, the hand 
can help out if the power flags or fails. 

This camera is most satisfactorily driven by a 12 volt 
1/10 HP electric motor working through a flexible shaft 
attached to a swivel connection at the front of the semi- 
automatic drive box. A change once every four to five 
seconds is possible, but greater speed is apt to throw the 
changing plate too violently for safety. 

The chief practical objection to this camera is its bulk. 
Its great height makes it impractical for many planes. Its 
weight of nearly a hundred pounds is a formidable load for 
a plane to carry, but this is no more and probably less than 
that of any other camera when taken up with the same num- 
ber of plates in magazines. The price paid for economizing in 
magazine weight is that the whole camera body, excluding 
the lens cone, must be carried to and from the plane for both 
loading and unloading. 



CHAPTER X 
AUTOMATIC AERIAL PLATE CAMERAS 

General Characteristics. — The ideal in the automatic 
plate camera is to provide a mechanism which will not only 
change the plates and set the shutter, as does the semi- 
automatic, but make the exposures as well, at regular inter- 
vals under the control of the operator. Such a wholly auto- 
matic camera would leave the observer entirely free for 
other activities than photography and it is to meet this 
tactically desirable aim that the war-time striving for auto- 
matic cameras was due. 

It is obvious that the one essential difference between the 
automatic and semi-automatic types lies in the self-contained 
exposing mechanism with its device for the timing of the 
exposures. There is no difficulty in arranging for the driving 
power to trip the shutter, but it is no easy matter to design 
apparatus which will space the exposures equally, and at 
the same time permit of a variation of the interval. It is 
indeed the crux of the problem of automatic camera design 
to provide for the easy and certain variation of the interval 
from the two or three seconds demanded for low stereo- 
scopic views to the minute or more that high altitude wide 
angle mapping may permit. This problem is one intimately 
bound up with the question of means of power drive and its 
regulation, and will be treated in part in that connection. 
It is to be noted, however, that there are in general two modes 
of exposure interval regulation. One is by variation in the 
speed at which the whole camera mechanism is driven. The 
other is by the mere addition to a semi-automatic camera of 
a time controlled release which affects in no way the speed 

124 



1 



AUTOMATIC PLATE CAMERAS U5 

of the plate changing operation. In many respects the latter 
is the best way to make an automatic camera. 

While the advantages of automatic cameras are great it 
must not be overlooked that a camera which can only be 
operated automatically is of limited usefulness. It is not 
suited for "spotting" at any definite instant, as, for illustra- 
tion, at the moment of explosion of a bomb. It should, there- 
fore, be the aim of the automatic camera designer to so 
build the apparatus that it can, at will, be used semi-auto- 
matically. In addition, to meet the contingency of any 
break-down in the source of power, the camera should be 
capable of hand operation, as in the case of the American 
semi-automatic deRam. In short, the automatic camera 
should not be a separate and different type; it should merely 
have an additional method of operation. 

Certain desirable mechanical features of all aerial cameras 
have already been enumerated. Some of these may be 
repeated here with the addition of others peculiar to auto- 
matic cameras. As a general caution, mechanical motions 
depending on gravity or on springs should be avoided. 
Movements adversely affected by low temperatures (20 to 
30 degrees below zero. Centigrade), are unsuitable. All 
adjustments called for in the air must be operable by dis- 
tance controls whose parts are large, rugged, and not depend- 
ent on sound or delicate touch for their correct setting. 
The center of gravity of the camera should not change 
during operation (important in connection with the problem 
of suspension). The camera should work in the oblique as 
well as in the vertical position. The power required for 
operation must not exceed that available on the plane. 
Electrical apparatus, for instance, should not demand more 
than 100 watts. 

Any devices which diminish the weight of the camera are 



126 AIRPLANE PHOTOGRAPHY 

particularly desirable in automatic plate cameras, because of 
the large number of exposures which such cameras encourage. 
For instance, if the plates could be handled without placing 
them in metal sheaths we should gain a substantial reduction 
in weight (the sheaths weigh nearly as much as the plates) 
as well as in the time necessary for handling. 

The Brock Automatic Plate Camera. — This camera is 
somewhat similar to the same designer's film camera, both 
in shape, in size, and in its employment of a heavy spring 
motor for the driving power. It uses 4X5 inch plates, and 
carries a 10 to 12 inch lens. 

The plate-changing operation is unique. As shown 
diagrammatically in Fig. 52, the unexposed plates are carried 
in a magazine on top of the camera, the exposed ones in a 
magazine inserted in the body of the camera, directly below 
the unexposed magazine. The bottom plate of the exposed 
pile drops into a sliding frame and is carried along the top 
of the camera to the exposing position. After exposure, the 
plate is carried back and drops into the receiving magazine. 
In order for the plate to fall only the proper distance at each 
stage of the cycle, special plate sheaths are necessary. These 
are cut away to form edge patterns which clear or engage 
control fingers so as to ride or fall through the sliding frame 
as required. 

The camera is entirely automatic in operation. Regula- 
tion of the exposure interval is by a special form of variable 
length escapement controlled through a Bowden wire, in a 
manner parallel to that in the Brock film camera, described 
elsewhere. These plate cameras were never produced 
in quantity. 

Folmer 13X18 Centimeter Automatic Camera. — This 
camera, also never manufactured in quantity, is shown in 
Fig. 53, and a sketch of its manner of operation is included in 



AUTOMATIC PLATE CAMERAS 1£7 

the ensemble of automatic camera diagrams (Fig. 52). Its 
most distinctive feature is perhaps the use of a two compart- 
ment magazine. This is similar in form to the one already 
described in connection with the hand-held cameras, but 
larger, to hold eighteen 13X18 centimeter plates. The 
unexposed plates are placed in one compartment, and after 
exposure are shifted to the other. The transfer is effected 




Fig. 53. — Folmer 13x18 centimeter automatic and semi-automatic plate camera. 

by the motion of a rack, which is part of the magazine and 
which is driven by a toothed pinion, also part of the magazine, 
which in turn engages in a toothed wheel projecting upward 
from the camera body. This toothed wheel is turned first 
in one direction and then in the other by an arrangement of 
gears and levers driven by the source of power, which as 
shown in Fig. 53 is a wind turbine connected through a 
flexible shaft. Operation is either automatic or semi- 
automatic as desired, and the camera can be put through 
its cycle by hand if necessary. 



ns AIRPLANE PHOTOGRAPHY 




Fig. 54. — French model deRam automatic plate camera. 



AUTOMATIC PLATE CAMERAS 129 

As with several other designs, the completion of the work- 
ing model of this camera occurred after agreements had 
been reached by the Allies, as to plate size, standard lens 
cones, and other features, not easily incorporated in it, thus 
making manufacture inadvisable. The validity of the design 
for peace-time work is, of course, not affected by this fact. 

The deRam Camera. — The only completely automatic 
plate camera actually produced commercially before the 
end of hostilities was the French model deRam (Fig. 54). 
Its plate-changing action has already been described in 
connection with the American semi-automatic model (Figs. 
52, 90 and 91). It differs from the American model in the 
shutter, which is of the self-capping variety, carried on a 
rising and falling frame; and in the exposing mechanism. 
The latter embodies a clutch whose point of attachment to 
a uniformly rotating disc in the camera is governed through a 
Bowden wire, whereby the interval between the plate-chang- 
ing operation and the shutter release is varied. The inter- 
vals are indicated by figures on the dial to which the observ- 
er's end of the Bowden wire is attached. The source of 
power for the camera is a constant speed propeller. Complete 
semi-automatic operation is not possible, as an interval of 1 
to 2 seconds elapses between the time a single exposure is 
called for and its occurrence. No arrangement is provided 
for hand operation. 

It will be noted that while this camera is a true automatic 
apparatus it does not meet even a majority of the require- 
ments listed above as found desirable by experience. There 
exists a great opportunity for designing and developing an 
entirely satisfactory automatic plate camera — provided it is 
agreed that anything more than semi-automatic operation 
is ever advisable for plates. 



CHAPTER XI 
AERIAL FILM CAMERAS 

The weight of the glass and the sheaths in the plate 
camera forms its most serious drawback. This weight must 
be reckoned at least three quarters of a pound for each 
18 X24 centimeter plate. Consequently, with the use of these 
large plates, and with the demands for ever increasing num- 
bers of pictures to be taken on long reconnaissance flights, a 
serious conflict arises between the weight of the photographic 
equipment and the carrying capacity of the plane. Among 
plate cameras probably the most economical in weight is 
the deRam. It carries fifty 18X24 centimeter plates, and 
has a total weight of approximately 100 pounds. An ad- 
vance to 100 or 200 plates — not feasible in the deRam con- 
struction — even if we assume the lightest possible magazines, 
would bring the weight of camera and plates to 150 or 200 
pounds, which would be detrimental to the balance and 
would seriously infringe on the fuel carrying capacity and 
ceiling of any ordinary reconnaissance plane. 

Early and persistent attention was therefore paid to the 
possibilities of celluloid film in rolls, as used so widely in 
hand cameras and in moving picture work. The two great 
advantages of film would be its practically negligible weight 
(approximately one-tenth that of plates, not including 
sheaths) and its small bulk, which would permit the greatest 
freedom in the development of entirely automatic cameras 
to make exposures by the hundreds instead of by the dozens. 
Certain disadvantages were foreseen at the outset: the 
diflSculty of holding the film flat and immune from vibration 
in the larger sizes; the difficulty of quickly developing and 

130 



AERIAL FILM CAMERAS 131 

drying large rolls; the question whether as good speed or 
color sensitiveness could be obtained in sensitive emulsions 
when flowed on a celluloid base as on glass. Early trials 
revealed a further problem to solve: how to eliminate the 
discharge of static electricity occurring at high altitudes, 
especially when the weather is cold. 

As far as camera construction is concerned the chief 
problems are to hold the film flat, and to eliminate static. 

Methods of Holding Film Flat. — Several means have been 
proposed and used for holding the film flat. Disregarding 
mere pressure guides at the side, which are suitable only for 
small area films (up to 4X5 inch), the successful means 
have taken three forms: pressure of a glass plate, pressure 
of the shutter curtain, and suction. A glass pressure plate 
can be used in either of two ways; the film may be in con- 
tinuous contact with it or may be pressed against its surface 
only at the moment of exposure. The advantage of this 
first method lies solely in its mechanical simplicity; its dis- 
advantage in the likelihood of scratches or pressure markings 
on the film. Where a glass plate is used there is always the 
chance of a dust or dirt film accumulating, or of the conden- 
sation of moisture, to impair the quality of the negative. 
There is, moreover, an inevitable loss of light (about 10%), 
together with some slight distortion, due to the bending of 
the marginal oblique rays through the thickness of the glass. 
In cases where a filter would normally be employed, the loss 
of light is minimized by using yellow glass for the plate, so 
that it serves for filter and film holder as well. 

Pressure of the shutter curtain is utilized in the Duch- 
atellier film camera by furnishing the edges of the curtain 
aperture with heavy velvet strips, whose light and gentle 
pressure during the passage of the shutter holds the film 
against a metal back. In many ways this is the simplest 



132 AIRPLANE PHOTOGRAPHY 

film-holding device; it occasions no loss of light, and needs 
no mechanical movements or external accessories, such as are 
called for in the suction devices next described. There is 
always danger of marldngs on the film, if the velvet is not of 
the right thickness and softness, and the operation and speed 
control of the shutter are necessarily complicated by the 
additional frictional load. 

Suction of the film against a perforated back plate has 
been found a very successful means of securing flatness. 
Suction at the moment of exposure may be produced by the 
action of a bellows, which has been compressed beforehand 
by the camera-driving mechanism. Continuous suction can 
be produced either by a continuously driven pump, or by a 
Venturi tube placed outside the fuselage. The Venturi tube 
(Fig. 55) consists of a pipe built up of two cones, placed 
vertex to vertex, to form a constriction. When air is forced 
through this at high velocity suction is produced in a small 
diameter tube taken off at the constriction. A suction of two 
centimeters of mercury, acting through holes about one centi- 
meter equidistant from each other in the back plate, has been 
found adequate to hold flat a film 18X24 centimeters. 

One merit of suction applied only at the moment of 
exposure is that the film-driving mechanism does not have 
to work against the drag of the suction. Continuous suction, 
on the other hand, gives a longer opportunity for flattening 
out kinks in the celluloid, and easily permits movement of 
the film during the exposure, either for the purpose of per- 
mitting a longer exposure or for the purpose of preventing 
distortion due to the focal -plane shutter. A disadvantage 
of continuous suction is the production of minute scratches 
on the celluloid surface as it drags over the suction plate. 
These are ordinarily too small to cause trouble, but may show 
up when printing is done in an enlarging camera. 



AERIAL FILM CAMERAS 



133 



Static discharges are produced by the friction of the 
celluloid against the pressure back or other surfaces with 
which it comes into contact. They show in the developed 
film as branching tree-like streaks (Fig. 56) and in cold dry 
weather may be numerous enough to ruin a picture. The dis- 





FiQ. 55. — Venturi tube on side of plane. 

charges are noticeably less frequent with film coated on the back 
with gelatine ("N.C."), but the extra gelatine surface is ex- 
tremely undesirable. When handled by developing machines, 
as large rolls must be, this back gelatine surface becomes 
scratched and bruised in a serious manner. Plain unbacked 
film is much to be preferred if the static can be obviated. 
To avoid static, it is necessary to provide for the imme- 
diate dissipation of all acquired electrical charges. Experi- 



1S4 



AIRPLANE PHOTOGRAPHY 



ments made by the United States Air Service have shown 
that nothing is so good as rather rough cloth, thoroughly 
impregnated with graphite, held in close contact with the 
celluloid during as great a portion of its travel as possible. 
In the United States Air Service film camera which uses 




Fig. 56. — Print from film camera negative, showing static discharge, and (upper left-hand comer) 
record of altitude and compass direction made by Williamson film cameraauxiliary lens (Fig. 58). 

suction through a perforated back plate, the plate has been 
covered with thin graphited cloth, and similar cloth sheets 
are pressed against the film rolls by sheets of spring metal 
(Fig. 65). In cameras with this equipment no trouble has 
been experienced with static. . 

Representative Film Cameras. — The English F type 
(Williamson). This is one of the earliest cameras designed 



AERIAL FILM CAMERAS 



135 



for film, as is indicated by the nature of the power drive, 
which presupposes that the camera is to be carried on the 
outside of the fuselage. Its essential features are shown in 
Figs. 57 and 58. It consists of a rectangular box with a cone 
at the front on which is mounted a propeller, intended to be 
rotated by the wind made by the motion of the plane. This 
drives, through a governor controlled friction clutch, a train 
of gears which draws the (5X4 inch) film across the focal 
plane, sets and exposes the shutter at regular intervals. 




jM 



Fig. 57. — English type "F" (Williamson) automatic film camera. 



Above the camera, supported on a tripod, are a compass 
and altimeter, both recording on a single dial, illuminated 
from below by the light reflected from a circular white disc 
painted on top of the camera. An image of the dial is throw^n 
on a corner of the film by a lens, whose shutter is actuated 
in synchronism with the main focal-plane shutter. No 
special means are provided for holding the film flat. Special 
film with perforated edges is used. 

The camera was designed for mapping work on the 
Mesopotamian and other fronts where no maps at all existed. 



136 



AIRPLANE PHOTOGRAPHY 



The Duchaiellier camera is essentially a film magazine to 
fit on the standard French deMaria camera bodies, of the 
18X24 centimeter size. In its simplest form it embodies a 
shutter (the regular focal-plane shutter of the camera being 
removed) and a film-moving mechanism, both actuated by 
a single motion of the hand. Automatic and semi-automatic 
operation are accomplished by an auxiliary mechanism to 




Fig. 58.— Interior of type "F" camera,' showing lens for photographing compass and altitude readings. 

which Bowden wires from the hand lever are attached. The 
motive power is an air propeller. Variation of speed is ob- 
tained by changing the point of contact of a roller on a friction 
disc, the disc being directly connected to the propeller shaft, 
the roller to the camera drive shaft. 

The most distinctive features of the Duchatellier camera 
is its use of the focal-plane shutter to hold the film flat during 
the exposure. As already explained, this is accomplished by 
pressure, velvet strips on the shutter edges keeping the film 
close against the back plate. The return of the shutter 



AERIAL FILM CAMERAS 137 

curtain to the "set" position is accomplished by locking it 
to the film by perforating points, so that it is pulled across 
as the film is wound. This introduces between each pair of 
pictures a strip of tremendous over exposure, as wide as the 




Fig. 59. — G. E, M. automatic film camera. 



curtain opening. A fixed-aperture variable-tension shutter 
is used. The magazine carries a roll of film long enough for 
200 exposures, feeding the long way of the picture. When 
film needs to be changed in the air, this is done by changing 
the entire magazine, including its shutter. 



138 



AIRPLANE PHOTOGRAPHY 



The G. E. M. camera (Fig. 59) is a very light self-contained 
clockwork-drive camera taking 36 pictures six inches square. 
The film is unrolled from a small-diameter feeding roller on 
to a large-diameter receiving roller to which the driving 
mechanism is attached. By this means approximately equal 
spacing of pictures on the film is assured. The film is held 
flat by continuous contact with a glass plate, which is made of 
yellow glass, so that it serves at the same time as a color filter. 




Fia. 60. — Brock automatic film camera. 



The lens — of 8 to 12 inch focus — is equipped with a single 
speed between-the-lens shutter. The operation of the 
camera is entirely automatic. The interval between pictures 
is controlled by varying the clockwork speed, through a lever 
on the outside of the camera box. Protection of the camera 
from vibration is sought by supporting it on four spring 
cushions mounted on a solid frame, to which the camera is 
held by spiral springs attached to its sides. 

The Brock Film camera (Fig. 60) is an entirely automatic, 



AERIAL FILM CAMERAS 139 

very compact self-contained camera, taking one hundred 
4X5 inch pictures. The motive power is clockwork, regu- 
lated in speed by an escapement controlled by a flexible shaft 
carried to a dial which may be fastened to the instrument board 
or to some other convenient part of the plane. The lens is 
6, 12, or 18 inch focus. The shutter is of the fixed-aperture 
variable-tension type, of long travel, and with a flap behind 
the lens for covering during the setting period. None of the 
special means above described for holding the film flat are 
provided. A metal plate resting on the back, and a flat 
metal frame in front with a 4 X5 inch aperture, are considered 
sufficient check on the excursions of the small-sized film. 
A ball bearing double pivoted frame serves to support the 
camera in a pendulous manner, permitting it to assume a 
vertical position after tilting. Damping of oscillations and 
vibration is arranged for by two pneumatic dash pots. 

The German film mapping camera, shown in Fig. 61, is 
distinguished by a number of special features. The size of 
the pictures, 6X24 centimeters, is unusual. It has its ad- 
vantages, however. Since the short dimension is in the line 
of flight, the maximum width of field covered by the lens is 
utilized (Fig. 17). This of course necessitates a larger num- 
ber of exposures to complete a strip, which is perhaps an 
added advantage, since the narrower the individual pictures 
the better the junctions will be, especially if large overlaps 
are made. This proved to be the case with captured German 
mosaics. Difficulty is experienced in making overlaps on a 
turn (Fig. 62), but this is not a vital objection. The shutter 
has a fixed aperture, narrower at the center than at the ends, 
to compensate for the falling off in illumination away from 
the center of the lens. No safety flap is needed because the 
curtain moves in opposite directions on successive exposures, 
thereby also compensating for shutter distortion, as has 



140 



AIRPLANE PHOTOGRAPHY 



already been discusjsed. Shutter speed is controlled by vary- 
ing the tension of the actuating spring. 




Fig. 61. — German automatic film camera. 



The camera is driven by an electric motor, connected to a 
set of gears, whose shifting provides for speed variation. 
The film is moved by rubber rollers which are cut away for 



AERIAL FILM CAMERAS 



141 




llG. 62. — ^Method of joining and printing film from German camera. 

part of the circumference, allowing the film to stand still 
until they bite again. A yellow glass pressure plate holds the 
film during the exposure and serves as color filter also 



14^ AIRPLANE PHOTOGRAPHY 

(Fig. 63). An electric heater is provided near the shutter, as 
in all the later German cameras. 

United States Air Service automatic film camera — Type K 
(Figs. 64, 65, 92, 93, 98, 99). This is an entirely automatic 
camera, manufactured by the Folmer and Schwing Division 
of the Eastman Kodak Co., taking 100 pictures of 18X24 
centimeter size at one loading. As with all the American 
cameras of this size, it uses the standard lens cones of any 
desired focal length. The camera proper consists of a com- 
pact chamber in which the film rollers are carried at each end 
forward of the focal plane, the shutter lying between. In 
consequence of this arrangment the vertical depth of the 
camera is the absolute minimum — short of decreasing the 
length of the optical path by mirror arrangements — making 
it possible to suspend the camera diagonally in the American 
and British planes, for taking oblique pictures. 

Flatness of the film is secured by a suction plate covered 
with graphited cloth and connected with a Venturi tube. 
The top cover is removed for re-loading. The shutters on 
the first cameras of this type are of the variable-tension 
fixed-aperture design, though later ones have the variable- 
aperture curtain controlled by an idler, as used in the Ameri- 
can deRam. An auxiliary curtain shutter serves to cap the 
true shutter during setting. 

The operation of the film driving mechanism is compara- 
tively simple. It consists of a train of gears, driven by a 
flexible revolving shaft attached to some separate source of 
power capable of speed variation. The action of the gears 
is to move the film, set the shutter and then expose it; in the 
earlier cameras with the film continuously moving. In the 
first cameras constructed the space between the pictures 
varies as the film rolls up, due to the increasing diameter of 
the roll. In later cameras the film roller is disengaged from 



AERIAL FILM CAMERAS 



143 




spoo' 



RtuDber ' ySced 







/^lnr\ spool 




gr 






Pr. 



cs^s^uoi, o 










51 

tj •'* c 



|^c.vi_ plA.r->e, pl^J*-,,- 

!i:> V<<tpt" /°W" ^soAcI irv H-\e 

3cp<?CiScrv . 






Q 



Fio. 63. — Film winding and exposing mechanism in German film camera. 



144 AIRPLANE PHOTOGRAPHY 

the gears just before the shutter is tripped, so that the film 
stands still during the exposure, and is then re-engaged at a 
new point on a ratchet wheel governed by the diameter of 
the receiving roll, whereby the pictures are equally spaced. 
In all the cameras, punch marks made at the time of exposure 
enable the limits of the picture to be detected in the dark 
room by touch. 

Variable speed is arranged for in any one of several ways. 
For peace-time uses a turbine attached to the side of the plane 
is simple and positive, and, provided it is made of sufficient 
size — which is not the case with the one shown in the Figure 
— will give adequate speed regulation upon varying the 
aperture through which the air enters. The Venturi tube 
may be carried upon the same mount, or a small rotary pump 
can be attached on the same shaft. Where the high wind 
resistance of the turbine is an objection the camera is driven 
electrically, by a motor acting through the intermediary of 
a variable speed control described in the next chapter 
(Fig. 68). 

The camera weighs complete about forty pounds, and 
the film rolls about four pounds. The latter can be changed 
in the air without great difficulty provided the camera is 
mounted accessibly and so that the top may be opened. 



CHAPTER XII 
MOTIVE POWER FOR AERIAL CAMERAS 

As long as circumstances permit, hand operation still 
remains the most reliable and satisfactory method of driving a 
camera. It is always available, can be applied to just the 
amount desired, and at the time and place needed. For 
instance, in a magazine of the Gaumont type (Fig. 40), what is 
needed is the periodic application of a very considerable 
force rather quickly, and while this can be done quite simply 
byhand,no mechanism has even been attempted to go through 
this same operation automatically. Instead, the fundamental 
design of automatic magazines has been made along other 
lines calculated to utilize smaller forces more steadily applied. 

It must be granted, however, that for war planes, and 
particularly for single seaters, cameras should be available 
which are capable of operating semi-automatically or auto- 
matically. This necessarily means the employment of arti- 
ficial power, whose generation, transmission to the camera 
and control as to speed present a mechanical problem of no 
small difficulty. 

Available Sources of Power. — The sources from which 
power may be drawn on the plane are four, although the 
various combinations of these present a large number of 
alternative approaches to the problem. These sources are: 

1. The engine of the plane. 

2. Wind motors. 

3. Spring motors. 

4. Electric motors. 

10 145 



146 AIRPLANE PHOTOGRAPHY 

These may first be considered largely from the descrip- 
tive standpoint, leaving questions of performance and effi- 
ciency for separate treatment. 

Power may be derived directly from the engine through a 
flexible shaft, similar to that used for the revolution counter. 
This source of power is inherently the most direct and effi- 
cient, since the engine is the seat of all the lifting and driving 
energy of the plane. There is no loss through transformation 
into other forms of energy, such as electrical; or by the use of 
more or less inefficient intermediary apparatus, such as wind 
propellers. Against the direct drive of the camera from 
the eng;ine may, however, be urged that the usual distance 
between engine and camera is too great for reliable mechani- 
cal connection, as by flexible shafting. Objections also arise 
from the standpoint of speed. This cannot be controlled by 
the camera operator; and varies over too wide a range, as the 
engine changes from idling to full speed, to fit it for automatic 
camera operation. The first objection may be met by that 
combination of methods of power drive which consists in 
transmitting the power electrically; that is, by letting the 
engine operate a generator from which cables run to a motor 
close to the camera. This method, of course, sacrifices 
efficiency, and it breaks down when the engine speed drops 
below the speed necessary to generate the requisite voltage. 
This defect may in turn be met by floating in storage batter- 
ies, which brings up the whole question of electrical drive, 
to be treated presently. While use of the engine for direct 
drive or for generating electric current has not been adopted 
in the American service, it is known that some German 
planes were supplied with electric current in this way. 

Coming next to the wind motors, these possess one very 
great merit : they utilize a motive power that is always 
present as long as the plane is in motion through the air. 



MOTIVEPOWER 147 

On the other hand, the process of using the main propeller 
of the plane to pull another smaller propeller through the 
air appears a roundabout way to utilize the driving power 
of the airplane engine. Yet on the whole it is probable that 
some form of propeller or wind turbine is the simplest and 
most convenient device we have for the operation of airplane 
auxiliaries. As long as the amount of power required is 
small, such inefficiency as is inherent in its use is offset by 
its convenience and reliability. An advantage of the pro- 
peller is that its speed is almost directly proportional to that 
of the plane through the air, a desirable feature in auto- 
matic cameras provided the proportionality is under control. 
Yet it is just in this matter of varying the speed at will that 
the propeller presents difficulties, to be met only by additional 
mechanisms for gearing down or governing. Propellers 
have the practical disadvantage that they present an easily 
bent or broken projection to the body of the plane (Figs. 83 
and 84). The strength of small propellers for operating 
auxiliaries is never so much in question with reference to 
their resistance to whirling and thrust of air as it is to their 
ability to withstand the inevitable knocks and careless hand- 
ling that will fall to their lot. The propeller bracket is just 
what the pilot is looking for to scrape the mud off his boots 
before climbing in. 

The wind turbine has the advantage over the propeller 
that its speed can be varied rather simply by exposing more 
or less of its face to the wind. A turbine fitted with an 
adjustable aperture for admitting the wind is shown in Fig. 
64, in connection with the type K automatic film camera. 
The turbine has the advantage of being compact and lying 
close against the body of the plane. In the form figured, 
altogether too much head resistance is offered — just as much 
for low as for high speeds — but with proper design this need 



148 AIRPLANE PHOTOGRAPHY 




MOTIVE POWER 



14d 



not be the case. It is, moreover, quite too small to give the 
needed speed regulation, as it only begins to operate near its 
full opening. 

Spring motors have the very real advantage that by their 
use the camera can be made entirely self contained. The 
simplest application of the spring motor would be to the semi- 
automatic camera, where no close regulation of speed is 
required. In such a camera the operation of exposing the 
shutter would release the spring, which would then change 
the plate or film and re-set the shutter, repeating this opera- 
tion as long as the spring retained sufficient tension. Small 
film hand -cameras of this type, using self -setting between- 
the-lens shutters, have been designed, though not for aerial 
work. The possibilities of using springs as motive power 
in semi-automatic cameras have not apparently been seri- 
ously considered. 

When a spring motor is used for automatic camera opera- 
tion it at once becomes necessary to add to the motor an 
elaborate clock mechanism for controlling and regulating its 
speed of action. Springs are much better fitted for giving 
power by quick release of their tension than by slow release, 
and the necessary clock mechanisms for their regulation 
become very heavy, as well as complicated and delicate, 
when they are made large enough to do any real work. For 
their repair they require the services of clock makers rather 
than the usual more available kind of mechanic. 

Coming next to electric motors, we meet with a source of 
power of very great flexibility both in its derivation and in 
its application. If a source of electric current is already pro- 
vided for heating and lighting as it is on the fully equipped 
military plane, and if it has sufficient capacity to handle the 
camera, its use is rather clearly indicated, irrespective of 
how efficiently or by what method it is produced. Especially 



150 AIRPLANEPHOTOGRAPHY 

is this the case, from the standpoint of economy and simpHc- 
ity, if a propeller-driven generator is the source of current, 
and the alternative power drive is an additional propeller 
for the camera. If, on the other hand, the camera must have 
its own source of electric power, the advantages and dis- 
advantages must be closely scrutinized. In this case either 
a generator must be provided, or resort be made to storage 
batteries, or a combination of the two. 

Ruling out a special propeller-driven generator, we are 
left with either the generator driven from the engine or the 
storage battery. Inasmuch as storage batteries are prac- 
tically indispensable with generators, in order to maintain the 
voltage constant at all speeds, it is on the whole advisable 
to rely upon batteries alone. An advantage of their use is 
that the power plant is entirely within the plane: All pro- 
jections such as propellers are avoided. Another merit is that 
the power is drawn upon only as needed. Against storage 
batteries is their weight, the need for frequent charging, 
and their loss of eflficiency at low temperatures — a loss so 
serious with those of the Edison form as to preclude their use. 

When once the source of electrical energy is decided upon, 
its method of application needs to be considered. Here we 
meet at once the peculiar merit of electrical energy, namely, 
the ease and convenience with which it may be transmitted. 
All we need is a pair of wires, led to any part of the plane by 
any convenient route and connected by simple binding posts. 
It may with equal ease be turned on or off by merely making 
or breaking a contact with a switch. For operating semi- 
automatic cameras this feature may be utilized in the interest 
of economy, if the power is automatically turned off as soon 
as the plate-changing operation is finished. Exceptionally 
reliable make and break contacts are necessary to insure the 
success of this latter scheme. 



MOTIVE POWER 151 

Two methods of transforming the supply of electrical 
energy into mechanical motion are available. The first is 
by the use of a solenoid and plunger. This is a device prac- 
tically restricted to semi-automatic cameras, in which the 
operation consists of a straight to-and-fro motion, initiated 
at the will of the operator. It has been used little if at all. 
The second motion is the continuous rotary one secured by 
the use of an electric motor. This motion is the most prac- 
tical one for the continuous operation of any mechanism, but 
on the other hand requires that the imposed load be reason- 
ably uniform at all times through the cycle of operations. 
Assuming that the camera mechanism is of this character, 
the motor may be attached directly to the camera, or if it 
must be so large as to cause danger by vibration, it may be 
connected through a flexible shaft. This use of an electric 
motor is very practical for semi-automatic cameras such as 
the "L" or the American deRam, in planes supplied with a 
suitable source of current. 

When it comes to entirely automatic cameras, where 
uniform and regulatable speed is required, as in making 
overlapping pictures for mapping, the electrical drive is not 
so conveneint. The shunt- wound motor runs at nearly 
constant speed, while the series-wound motor in which the 
speed can be regulated by the interposition of resistance, 
has nothing like a sufiicient range of variation for the pur- 
pose (at least five to one is imperative) before it fails to carry 
the load. Hence we must either incorporate in the camera 
some mechanism for varying the interval between exposures 
while the speed of the motor remains constant, or introduce 
an auxiliary device to effect the required transformation in 
speed. If we do use an auxiliary device the train of appara- 
tus, consisting of battery (or generator), motor, speed con- 
trol and camera, is altogether too long; it is apt to cause 



152 AIRPLANE PHOTOGRAPHY 

annoying delays in connecting up in an emergency, and it 
offers an excessive number of chances for break-down. 

Performance and Efficiency Data. — The first step in 
deciding upon methods of power drive, and indeed in decid- 
ing whether power drive is feasible at all, is to assemble 
definite data as to the power required to drive representative 
cameras. Approximate figures for some of the cameras 
described in previous chapters are: 

L camera, 26 watts, 

deRam, 60 watts, 

"K" film, 30 watts. 

These requirements — not exceeding 1/10 horse power — are 
insignificant in comparison with the total of 100 to 400 
horse power available for all purposes from the plane's engine. 

Propeller characteristics. Data on the performance of 
small propellers are somewhat meagre. However, the results 
of the rather extensive researches on large ones, suitable for 
driving planes, may be applied, with proper reservations, to 
give a fair guide to the study of the application of small 
propellers for driving plane auxiliaries. 

The first factor to be considered is the thrust or head 
resistance offered by a propeller to motion through the air. 
This varies as the square of the velocity, as the density of the 
medium, and as the area of the body projected normally to the 
wind, the formula being 

T=cdaV^ 

where r= thrust, c?= density, a=area, F= velocity. Data 
on the L camera propeller are shown in Fig. 66, where its 
thrust both when free and when loaded with the camera is 
given, as well as that of a solid disc of the same diameter as 
the propeller. For this propeller, which is double-bladed, 
and six inches in diameter, C(ia = . 000275 with the load on. 



MOTIVE POWER 



153 



The total thrust amounts to only about three pounds when 
the plane velocity is 100 miles per hour. The head resistance 
of the whole plane is a matter of hundreds of pounds, so that 
the propeller resistance is quite negligible. 

The next factor is the speed of revolution of the propeller, 
expressed in revolutions per minute. This varies with the 



THRUST ON L PROPELLER 




a 3 -1- 

THRU3T m Pounds 

Fig, 66. — Wind propeller data. 



design — the number of blades, their area, and pitch. For a 
given design the speed of revolution is directly proportional 
to the speed of motion through the air, and to the density of the 
Representative data for the L camera propeller are 



air 



shown in Fig. 67. It will be noted that the speed goes up to 
8000 for 120 miles per hour air speed. This illustrates the 
necessity for great strength to withstand centrifugal force. 
Propellers should be constructed of tough material, and 



154 



AIRPLANE PHOTOGRAPHY 



subjected to whirling tests up to speeds considerably in 
excess of any the plane will attain in any maneuver. At 
low speeds the linear relationship fails, as a critical velocity 
is reached — about 3500 r. p. m. for this propeller — where it 
refuses to turn. 

The fact that the speed of the propeller depends on the 



t^not 






1 


. CAm 


RA PR 


OPELLI 


;r 
















Rl 


IVOLU 


IONS 






^ 




^000 


















y^ 




! 

E 














y 


y 






9 














y 








.r 












/ 


/^ 








- 5*000 












^^ 










i. 












r^ 










t 

o: 








y 














GC 








y^ 














•^00 






^ 
















3000 























so 60 70 80 30 

WIND VELOCITY Mi|ea per Hour 

Fig. 67. — Relation between air speed and propeller revolutions. 



100 



density of the air has an interesting corollary, which is that 
a propeller adequate at low altitudes will fail at high ones. 
The density of the air varies with altitude according to the 
following figures : 

At 3000 meters, 72 per cent, of sea level 

5000 meters, 59 per cent, of sea level 

6000 meters, 52 per cent, of sea level 

If we take the r. p. m. at 90 miles per hour at sea level as 
6000, then at the above altitudes the speeds will be 4300, 



MOTIVE POWER 155 

3500, and 3000, respectively. The last figure is below that 
for which this size of propeller stalls with its normal load, 
as noted in the last paragraph. Consequently, if flying is to 
be done at these altitudes a larger propeller must be carried, 
which will still deliver enough power at the lower density. 

The next factor to be considered is the power furnished 
hy the propeller. As a representative figure may be quoted 
the performance of the L propeller. This gives 27 watts at 
3600 revolutions per minute {5Q miles per hour). From this 
figure the performance of other propellers may be deduced 
from the basic laws, which are : that the power varies as the 
density of the medium and as the cube of the velocity (assuming 
constant efficiency). Since the power delivered by the six 
inch diameter L propeller is already adequate at 60 miles per 
hour, the necessary dimension to function satisfactorily at 
100 miles per hour would need to be only a little more than 
three inches, except for the desirability of a safety factor for 
high altitudes and low air densities. 

The efficiency of the propeller is defined by the relation — 

^ff, . _ power delivered by the propeller 
power supplied to the propeller 

The denominator of this fraction is the thrust times the 
velocity, for which the curves of Fig. 66 supply us data for 
the L propeller. Using the figures 3600 r. p. m., 56 miles 
per hour, and 27 watts, we find the efficiency to be about 50 
per cent. This increases with the velocity, with a possible 
upper limit of 70 to 80 per cent. Since the main propeller 
of the plane is not over 80 per cent, efficient we have at most 
an efficiency of 64 per cent, in using a propeller drive, as 
compared with taking the power directly off the engine. 

In considering the use of spring and clock-work motors we 
meet at once with the problem of comparing the effect on the 
performance of a plane of a carried weight, as against a 



156 AIRPLANE PHOTOGRAPHY 

head resistance. The efficiency of a spring motor is measured 
in terms of its weight, that of a propeller in terms of its head 
resistance. The general answer to this question is given by 
the relation that a 'pound of dead weight is equivalent to }/^ 
pound head resistance. 

In order to apply this relation to the study of spring 
motors for driving cameras, data are necessary on the power 
delivery per pound weight of such mechanisms. Such data 
are not easily accessible, largely because clock-work has not 
generally been seriously considered as a motive power for 
large apparatus. To arrive at an approximate figure we 
may take the fact that in an 8 X 10 inch film camera designed 
by one of the manufacturers v/ho have utilized clock-work, 
the motor weighed 30 pounds. This is equivalent to six 
pounds head resistance. Now the type K, 18 X24 centimeter 
film camera is operated, even with the addition of a friction 
drive speed control, by means of the L camera propeller. As 
shown in Fig. 66, at 100 miles per hour the head resistance 
of this propeller is still less than three pounds. Consequently, 
it appears that from the efficiency standpoint the clock 
mechanism is quite outclassed by the wind propeller. 

Coming next to the electric motors, the L camera and the 
K are both operated satisfactorily with a 3^o horse power 
motor, weighing 6 pounds. For the deRam a Jf q horse power 
motor has been adopted. 

Taking up efficiency considerations, we have, if the cur- 
rent is supplied by a generator from the engine, a transfor- 
mation factor of 70 to 80 per cent, from mechanical to elec- 
trical energy and a similar factor in using a motor for the 
camera. When batteries are employed the matter of weight 
versus head resistance again arises. The batteries found 
most satisfactory for operating the K and deRam cameras 
are of the six-cell 12 volt lead type. Their capacity is 40 



MOTIVE POWER 157 

ampere hours at three amperes or 36 at five amperes — more 
than is necessary for a single reconnaissance, but a practical 
figure when economy of charging and replacement are con- 
sidered. The weight of this unit is 27 pounds. To this must 
be added the weight of the motor — 6 lbs. — making a total 
of 33 pounds, equivalent to a head resistance of nearly 7 
pounds. This is more than twice the propeller head resist- 
ance invoked to do the same work. 

These considerations of efficiency have been gone into 
because they are usual in studying any engineering problem 
and because of the insistent demand from the plane designer 
that every ounce of weight and head resistance be saved. 
Actually, as already stated, the load imposed by any method 
of power drive is trivial in comparison with the whole load 
of the plane. There is, however, an important reservation to 
be made, which applies against clock-work and batteries: 
This is, that while the equivalent head resistance of any 
camera motive power carried as dead weight is small, its 
effect on balance may not be so. While the use of a propeller 
need not disturb the plane's balance, the weight of the camera 
alone, without any driving apparatus, is already seriously 
objected to on this score. The merely mechanical superiority 
of the propeller as a source of motive power is on the whole 
rather marked. 

Control of Camera Speed. — In the semi-automatic 
camera the only control required on the speed of the operat- 
ing motor is at the upper and lower limits. It must not go 
so fast as to anticipate the completion of any steps in the 
cycle of camera operation, such as the fall of plates or pawls 
into position, which would jam the camera. On the other 
hand, it must not be so slow that pictures cannot be obtained 
with the requisite overlap for maps or stereoscopic views. 
In the American deRam camera the cycle of operations can- 



158 AIRPLANEPHOTOGRAPHY 

not safely be put through in less than four seconds, a short 
enough interval for most purposes. It is also highly desirable 
in the semi-automatic camera to have the motive power 
capable of stopping completely. This saves wear and tear 
on both motor and camera mechanism. 

In the automatic camera an extreme range of speed is 
called for by the several problems of mapping, oblique 
photography, and the making of stereoscopic views. For 
mapping alone, the shortest likely interval may be taken as 
that required for work at approximately 1000 meters alti- 
tude, for a plane speed of 150 kilometers per hour, which 
demands an interval of six seconds with a ten inch lens on a 
4X5 inch plate. For vertical stereos at the same altitude 
and speed this interval is divided by three, and low oblique 
stereos need even quicker operation. Hence a range of from 
1 to 30 pictures per minute should be provided for. This 
requirement is difficult to meet with any simple mechanism. 

From the standpoint of simplicity in speed regulation the 
wind turbine of adequate vane surface has much to recom- 
mend it. It is only necessary to present more or less of its 
vane area to the wind in order to secure a considerable range 
of speed. The method of doing this by a shutter interposed 
in front is uneconomical, but it is probable that the design 
can be so altered that more or less of the turbine is exposed 
beyond the side of the plane, possibly by varying the angle, 
to secure the same result without introducing useless head 
resistance, A serious practical objection to the turbine lies 
in the large vane surface necessary to give adequate power 
combined with proper speed variation. In the automatic 
film camera (Type K) this area should be as much as 40 to 
50 square inches. 

The wind propeller does not lend itself at all well to speed 
variation. It cannot be partially covered from the air stream. 



MOTIVE POWER 159 

as can the turbine, because of the resulting strain on its 
mount. A possible form of variable speed propeller, one 
which, however, has not yet been practically developed, is a 
propeller with controllable variable pitch. If this could be 
made mechanically sound it would be well-suited for camera 
operation. That such a propeller could be worked out is 
indicated by the good performance of a constant speed 
propeller developed for radio generators and used on the 
French deRam camera (Fig. 54) . Parenthetically, it may be 
questioned whether a constant speed propeller is really de- 
sirable with an airplane camera. What is required is not 
exposures at a definite time interval — although most of the 
data are in that form — ^but exposures at definite intervals 
with respect to the motion of the plane, which practically 
means with reference to its air speed. Rather than build a 
camera calculated to give exposures at intervals of so many 
seconds when it is attached to a constant speed propeller, 
we would do better to use a propeller which responds to the 
speed of the plane, in conjunction with some form of tacho- 
meter to show the rate at which exposures are being made. 
This in turn should be coordinated with the indications of a 
proper camera-field indicating sight. 

One solution of the problem of speed control with a pro- 
peller of practically fixed speed, is to use a governor and slip 
clutch as in the English Type F film camera (Fig. 57). 
Here the propeller shaft and the camera driving axle are 
connected by two friction discs. That on the camera mechan- 
ism is forced against the other by a spiral spring, whose tension 
is controlled by a ball governor. If the camera speed becomes 
too high the governor reduces the tension on the spiral spring 
and the discs slip over each other. The point where this shp- 
ping occurs is determined by the position of the governor as a 
whole, and this is controlled by a lever on top of the camera. 



160 



AIRPLANE PHOTOGRAPHY 



Another speed control device, perhaps more positive but 
certainly more complicated and wasteful of power, consists 
of a large flat disc, driven by the propeller or electric motor, 
and from which the camera is driven by a shaft from a smaller 
friction disc which may be pressed against any point from 




Fig. 68. — Friction disc speed control. 

the center to the periphery of the larger disc. The speed 
range attainable in this way is limited only by the size of 
the large disc. An application of this idea is shown in the 
speed control (Fig. 68), designed for the American Type K 
camera when operated on an electric motor or on a simple 
propeller. The same idea is utilized in the Duchatellier film 
camera, in connection with the constant speed propeller 
already described. 



MOTIVE POWER 161 

On the whole it is eminently desirable from the stand- 
point of power operation that the automatic camera should 
embody its own means for altering the interval between 
exposures, so that all the external attachment needed is a 
single connection to a source of power either of constant 
speed, as an electric motor, or of speed proportional to that 
of the plane, as with a simple wind propeller. This makes the 
camera largely independent of the nature of the power supply, 
whereas a camera designed for a special variable speed device 
is of little use on a plane where this is not available. 

Transmission of Power to the Camera — It has already 
been pointed out that the ease of transmission of electrical 
energy makes it particularly convenient for use in a plane. 
All other sources of power, except clock-work incorporated 
in the camera, require flexible shafting, so that the question 
of bearings and connections becomes a serious one, especially 
when the shaft runs continuously for long periods at very 
high speeds. 

The shafting found most suitable is the spirally wound 
form commonly known as dental shafting. This must be 
encased in a smoothly fitting sheath, flexible enough to per- 
mit of easy bends. The ends of the shaft should be equipped 
with square or rectangular pins to fit into corresponding slots 
in the motor and camera shafts. The ends of the shaft casing 
may be fitted either to attach by bayonet joints or by 
smoothly fitting screw collars. At the point of attachment 
to the camera it is desirable to have some form of junction 
adjustable as to the direction from which the shaft may be 
connected, so that it need be bent as little as possible. A 
right angle bevel gear offers one means of doing this. Bear- 
ings, such as those of the propeller, should be of the ball 
variety, while heavy lubrication, such as vaseline, should be 
freely used, both in the bearings and in the shaft casing. 
11 



162 AIRPLANE PHOTOGRAPHY 

An important feature of any power drive system should 
be a safety device, so that the power will race in case of any 
jam or stoppage in the camera. This will often prevent 
serious damage through the breakage of some relatively 
weak part of the camera mechanism on which the whole force 
of the driving apparatus is suddenly thrown. The "L" 
camera propeller is fitted with a spring friction clutch with 
the idea that if the camera refuses to operate the propeller 
will slip instead of wrenching the shaft to pieces. 



CHAPTER XIII 

CAMERA AUXILIARIES 

Distance Controls and Indicators. — All operations con- 
nected with the exposing and changing of plates (except the 
changing of whole magazines) should be arranged for accom- 
plishment at a distance. Other operations, such as changing 
the shutter speed or the interval between exposures in an 
automatic camera, which are usually done on the ground, 
may sometimes be satisfactorily left for performance at the 
camera. Conditions of extreme inaccessibility may, however, 
make it necessary to carry even these controls to a distance. 
Indicators of the number of exposures already made, and of 
the readiness of the camera for the next exposure, may be 
attached to the camera, but often are more profitably placed 
at a distance. Distance control and indication are especially 
necessary if the pilot makes the exposures — a common 
English practice in two seaters, and the only recourse in 
single seaters. 

When electric power is available, electrical distance con- 
trol devices are perhaps the simplest kind, as they transmit 
motive power without displacing or jarring the camera. 
Solenoids suffice for the simple pressing of releases or for 
counting mechanisms, while small service motors may be 
utilized for operations involving more work. A standing 
practical objection to electrical control lies in the necessity 
for using contacts, which are apt to be uncertain under 
conditions that involve vibration. 

The Bowden wire — a wire cable carried inside a heavy non- 
extensible but flexible sheath — constitutes the most satis- 
factory mechanical means for transmitting straight pulls. 

163 



164 AIRPLANE PHOTOGRAPHY 

By means of "the Bowden" a pull may be transmitted so as 
to be made entirely relative to two parts of the same body, 
calling forth no tendency of the body as a whole to move. 
Thus in the L camera shutter release (Fig. 50), the releasing 
lever with its attached counter is several feet distant from 
the camera. If the plate bearing the lever and sheath end 
is rigidly fastened down, the pressure exerted on moving the 
lever acts between the lever and the end of the sheath. This 
pressure passes immediately to the other end of the sheath, 
while the pull on the wire is transmitted to its farther end on 
the camera. In this way the conditions at the lever are 
reproduced, but with the advantage that, due to the flexible 
cable and sheath, any vibration of the lever support is 
damped out. 

Due to its stretching, there is a pretty definite limitation 
to the feasible length of the Bowden wire. This length is 
about four feet. Where according to English practice the 
pilot makes the exposure, a considerably longer wire and 
sheath are called for. In this case the effective length of the 
release is increased by giving the pilot a second releasing 
lever, connected to the first by a rigid rod (Fig. 69). The 
releasing lever, wire, and all mechanical parts of the Bowden 
release should be made much stronger than would be indi- 
cated by bench tests of the camera. In the air it is impos- 
sible to decide either by sound or by delicacy of touch 
whether the mechanism has acted, so that the observer is apt 
to pull much harder than necessary and to strain or break 
the release if it is weak. 

The Bowden wire is used in the American service only 
for shutter release. In the English service it has been used 
for plate changing with the L camera. 

Sights. — In airplane photography the need for a finder 
or sight is fully as great as in everyday work. A new condi- 



3 



t 



w 

o 

si 






-i o 




166 AIRPLANE PHOTOGRAPHY 

tion, however, prevails, for except with hand-held cameras, 
and even to some extent with them, the operation of pointing 
the camera involves pointing the whole vehicle that carries 
the camera. The pointing of airplane cameras is therefore 
akin to the sighting of great guns. "While the observer may 
perform the actual operation of taking the picture, the 
responsibility for covering the objective rests with the pilot. 
Teamwork counts equally with tools. Airplane camera sights 
may accordingly be divided into two classes: sights attached 
to the camera, for use principally with hand-held apparatus, 
and sights attached to the plane, for the use of pilot, of 
observer, or of both. 

Sights for Hand=held Cameras. — The simplest form of 
sight attached directly to the camera is modeled on the gun 
sight, consisting of a forward point or bead and a rear V. 
This sight of course serves merely to place the objective in 
the center of the plate and gives no indication of the size 
of field covered. Another simple sight of rather better type 
is the tube sight — a metal tube of approximately one Inch 
diameter and three inches length, carrying at each end pairs 
of wires crossed at right angles. The camera is in alignment 
when the front and back cross wires both exactly match on 
the object to be photographed. The best way to mount the 
cross-wires is with one pair turned through 45 degrees with 
respect to the other, so that it is at once apparent which is 
the front and which the rear pair (Figs. 31 and 39). 

Sights to indicate the size of the field are usually less needed 
on hand cameras than on fixed vertical cameras. Yet certain 
circumstances make them most desirable, for instance in 
naval work where a complete convoy must be included on 
the plate. A sight of this kind can be made up of two wire 
or stamped metal rectangles, a large one in front and a 
smaller one behind, of such relative sizes and separations 



CAMERA AUXILIARIES 167 

that the true camera field is outHned when the eye is placed 
in position to see the two rectangles just cover each other. 
The dimensions should be so chosen that the correct position 
of the eye is approximately its natural location with respect 
to the camera when this is held in the hands in the plane. 
It is usual to provide the rectangular sights with cross-wires 
to indicate the center of the field. Alternative rear sights 
are simple beads or peep-holes — the use of the bead assuming 
that the camera is held at about the right distance from the 
eye for the rectangle to indicate the field. The peep-sight 
is not a desirable form, as it is hard to hold the camera as 
near the face as is necessary. These various types of rec- 
tangle sights are well illustrated in the cameras shown in 
Figs. 38, 40 and 186. They are all made so as to fold down 
flat on the camera and to snap quickly open when needed. 
The springs to support the sights must be fairly strong, and 
the surface presented to the wind as small as possible. Wire 
frames give very little from the pressure of the wind, but 
flat metal frames are apt to be bent back. 

The position of the sight on the camera is important. If 
the observer can stand, or if he sits up well above the edge 
of the cockpit, the conventional position of the sight on a 
pistol, namely, on top, is unobjectionable. But if the ob- 
server sits very low, as he usually does, then the sight should 
be on the bottom of the camera, thereby avoiding any need 
for the observer to raise his head unduly into the slip stream. 
Similarly, if the camera is used over the side for verticals, 
as it is in flying boats, a sight on the top is impractical, since 
it requires the observer to lean out dangerously far (Fig. 185) . 

Sights Attached to the Plane. — Any of the sights just 
described can be attached to cameras fixed in the plane, but 
they would be useless in the positions ordinarily occupied 
by the camera. It has therefore become common practice 



168 



AIRPLANE PHOTOGRAPHY 



to attach the camera sight to some accessible part of the 
plane. The most primitive method of sighting is merely to 
look downward over the side — a method in general use to 
the very end of the Great War. One step in advance of this 

is to mark a large inverted V 
on the side, with its vertex at a 
point where the observer can 
place his eye and so see the 
fore and aft extension of the 
field of view covered by the 
camera. This kind of sight 
was common on the French 
"photo" planes. On some of 
the English planes the tube 
sight was carried on the out- 
side of the cockpit. Any of 
the sights described can be 
carried on the inside of the 
fuselage, provided a hole is 
cut in the floor. For satisfac- 
tory sighting a hole in the 
floor is really necessary, as it 
enables the terrain on both 
sides of the vertical to be 
seen. One drawback to the 
simple hole, however, is that it 
cannot be made large enough 
to show the whole field from 
the ordinary height of the observer's eye, thus forcing him 
to bring his head down near the floor. This difiiculty is 
gotton over in a very beautiful way by the use of the negative 
lens sight shown diagrammatically in Fig. 71. 

Let Fi be the distance at which the edge of the hole (or a 




Fig. 71. — Diagram of negative lens sight. 



CAMERA AUXILIARIES 169 

rectangle marked on the lens) appears the size of the camera 
field (if the hole is the size of the plate, Fi is the focal length 
of the camera lens). Let F2 be the distance from the floor 
to the observer's eye. What is desired is a concave lens 
which will diverge the rays from their normal meeting point 
at Fi to a new meeting point, F2. The focal length of lens 
required is given at once by the simple lens formula — 

Fi Ft F 

Thus if Fi is 12 inches, and F2 is 36 inches, F will be 18 inches. 
The lens is to be marked with a rectangle showing the shape 
and size of the camera field, and a central mark such as a 
cross. An upper rectangle, or a bead, or a pair of cross wires 
a few inches below the lens, may be used for the other sight. 
For precision work the sight above or below the lens should 
be adjustable in position, especially where the camera sus- 
pension permits the camera to be adjusted for the angle of 
incidence of the plane. 

A negative lens sight should be placed in the observer's 
cockpit, if he takes the pictures, and also in the. forward cock- 
pit, so that the pilot may be accurately guided in his part of 
the task. In addition, it is advisable to place a negative lens 
well forward in the pilot's cockpit, to enable him to see the 
country some distance ahead. The lenses should be plano- 
concave with the flat side upward; otherwise, all the loose 
dirt in the airplane settles in the middle of the concave de- 
pression. A negative lens sight in a metal frame forming a 
completely self-contained unit ready for mounting in the 
plane is shown in Figs. 72 and 73. 

Devices for Recording Data on Plsiies.— Numbering 
devices. The number of the camera is impressed on negatives 
taken with the American L camera through the agency of a 



170 



AIRPLANE PHOTOGRAPHY 



small transparent corner of celluloid. It would be entirely 
possible to incorporate a rotating disc which should turn by 
the operation of plate changing and carry a series of numbers, 
so that each exposure could be numbered serially. Number- 
ing of individual plates is more commonly done by holes, 
notches, or even numerals, in the turned over portion of the 
sheaths, which are then recorded photographically when a 
picture is taken (Fig. 75). The chief objection to this method 




Fig. 72. — Negative lens and mount, viewed from above. 

is the difficulty of keeping the sheaths together in sets, 
especially as individual ones become damaged or lost. In 
practice there is also danger of the sheaths being carelessly 
loaded in wrong order. 

The more ambitious idea of recording on the plate all the 
information given by the instrument board of the plane 
occurs independently and spontaneously to all aerial photo- 
graphic map makers. These ideas vary from attempts to 
photograph the actual instrument board on every plate — a 
difficult task indeed with the instruments and camera placed 



CAMERA AUXILIARIES 



171 



as they are in the ordinary plane — to the incorporation of 
compass, altimeter, and inclinometer in the camera itself. 

Figure 58 shows the plan adopted in the English F type 
film mapping camera already described, for photographing a 
compass and an altimeter on the film. Here the combined 
compass and altimeter dial is above the camera, and is 




Fig. 73. — Negative lens and mount, side view. 

mounted in a cell with a glass bottom. Below it is a lens 
focussing the needles and compass points on the plane of the 
film. The light for photography is furnished by a diffusely 
reflecting white surface on top of the camera, illuminated 
by the sky. (The camera was carried outboard.) In Fig. 56 
is shown a picture with the compass image impressed upon it. 
Figure 74 shows a type of inclination indicator found in 
some captured German cameras. It consists essentially of 



172 



AIRPLANE PHOTOGRAPHY 



Section 





PJon 



•Jensitive P/o te 

* I- ■ 3 




i^/evation 



/ncf/nometer 




Sensitive Ptaie 

Assemblu 



Fig. 74. — Diagram of inclinometer used in some German cameras. 



CAMERA AUXILIARIES 



173 



two small pendulums or plumb-bobs; one to indicate lateral, 
the other longitudinal inclination, arranged to be photo- 
graphed in silhouette on the plate, as shown in the lower 
part of the diagram and in the print from a captured nega- 
tive (Fig. 75). 




Fig. 75. — Photograph made with German camera, showing inch'nometer record, four points for 
locating diameters and center of plate, and (upper right-hand corner) number of the plate sheath. 

Both these devices suffer from the deficiencies of the 
instruments they photograph. The compass and the incli- 
nometer, as already mentioned in the discussion of airplane 
instruments, only behave normally in straight-away flying, 
failing to indicate correctly when the plane is subject to 
accelerations in any direction. In general all attempts to 
record directional data in the camera are of little promise, 
unless either the instruments or the camera are automatically 



174 AIRPLANE PHOTOGRAPHY 

held level by some gyroscopic device. If the instruments are 
so controlled, rather elaborate means for photographing them 
are necessary. If the camera is stabilized, the inclinometers 
are unnecessary, and the compass behaves rationally. 

Another scheme for indicating inclinations, which is not 
subject to the above objections, is to photograph the horizon 
either on a separate film or on the same sensitive surface, 
simultaneously with the principal exposure. The diflBculty 
here is the practical one that it is only feasible in localities 
of great atmospheric clearness. Ordinarily, especially any- 
where near the sea-coast, the horizon is too rarely seen to be 
a reliable mark (Fig. 4). It is possible, however, that this 
objection could be overcome by the use of specially red 
sensitive plates and suitable color filters, as discussed in the 
chapter on "Filters." The method would in any case be 
useless in mountainous country. 

The difficulties discussed with reference to direction indi- 
cating instruments of course do not hold with the altimeter. 
Ordinarily, though, the altitude changes slowly enough to 
permit of sufficiently accurate records being made by pencil 
and pad. For high precision map making a photographic 
record of altimeter readings has a legitimate claim. As we 
have seen, a small altimeter is incorporated in the English F 
camera, but the bulk which a really precision altimeter 
would assume would be a bar to its use in this way. A time 
or serial number record on the plate or film, synchronized 
with a similar record on the film of an auxiliary camera 
which photographs the altimeter and other instruments, 
may be the simplest way to preserve the majority of the 
desired data. 

Devices for Heating the Camera. — ^Parts of the camera 
mechanism which depend on the uniformity of action of 
springs or upon adequate lubrication are susceptible to change 



CAMERA AUXILIARIES 



175 



with variation of temperature. At high altitudes low tem- 
peratures are met which may freeze ordinary machine oils 
or may cause springs to seriously alter their tension, even to 
break. To meet this difficulty, and probably also to dispel 
the occasional condensation of moisture on the optical parts, 
the German cameras are equipped with an electrical heating 
coil placed just below the shutter, and arranged to connect 
with the general heating and lighting current of the plane. 
Two contacts are ordinarily provided, for offsetting the 
effects of temperatures of —15 and —30 degrees centigrade. 
An additional function of this heating coil is perhaps to 
maintain the sensitiveness of the plates or film. 



Ill 

THE SUSPENSION AND INSTALLATION 
OF AIRPLANE CAMERAS 



I 



CHAPTER XIV 

THEORY AND EXPERIMENTAL STUDY OF 
METHODS OF CAMERA SUSPENSION 

General Theory. — ^In addition to the limitation of ex- 
posure set by the ground speed of the plane another limita- 
tion is set by the vibration of the camera. This may be 
caused either by the motor, or by the elastic reactions of the 
plane members to the strains of flight. Unlike the move- 
ment of the image due to the simple motion of the plane, 
movements due to vibration may be eliminated by proper 
anti- vibrational mounting of the camera. 

The effect of vibration may show as an indistinctness of 
the whole image — this is its only effect with a between-the- 
lens shutter — or as a band or bands of indistinctness parallel 
to the curtain opening (Fig. 76). These are due to shocks 
or short period vibrations during the passage of the focal- 
plane shutter. 

The obvious remedy for vibration troubles is to mount 
the camera on some elastic, heavily damping support, like 
sponge rubber or metal springs. Such a mounting should, 
however, be designed on sound principles derived from a 
proper analysis of the nature and effect of the possible 
motions of the camera. Otherwise, the vibrational disturb- 
ances may be increased rather than diminished by the camera 
mount. Such an analysis, based merely on general mechan- 
ical principles, shows that all motions of the camera are 
resolvable into six. These are three translational motions, 
namely, two at right angles in one plane such as the hori- 
zontal, and one in the plane at right angles to this (vertical) ; 

179 



180 



AIRPLANE PHOTOGRAPHY 




00 

s 
•c 






a 

4 



o 



O 



CAMERA SUSPENSION 181 

and three rotational motions, one about each of the above 
directions. of translational motion as an axis (Fig. 77). 

Brief consideration will show that only the latter — the 
rotational motions — are of any importance when the small 
displacements due to vibration are in question. To illustrate 
the negligible effect of vibrations which merely move the 
camera parallel to itself in any direction it is only necessary 
to imagine the camera moved parallel to the ground through a 
large distance, such as 10 centimeters. Now 10 centimeters 
motion of the camera at 3000 meters elevation means, with 
a 25 centimeter camera lens, 

.25 1 . 

X 10 = -,^„ centimeter 



I 



8000 '^ 1200 

motion on the plate, which would be only a tenth the distance 
separable by a good lens. If we reduce this motion to the 
small fraction of a centimeter which vibration would actually 
produce, it is evident that such vibration is of absolutely 
no importance. Similarly, if we imagine the camera, under 
the same conditions, moved vertically with reference to the 
ground by ten centimeters, the scale of the picture would 
merely be changed by j .^ ^ ^ ^ or by yoVo centimeter on a 
12 centimeter plate, again quite negligible. 

When we consider motions of rotation, however, the case 
is quite different. If the camera is mounted so that the 
effect of any vibration is to rotate it around a horizontal 
axis, this is exactly equivalent to rotating the beam of light 
from the lens so that it sweeps across the plate. Thus a 
millimeter displacement of the lens of the camera with the 
plate remaining fixed gives approximately a millimeter mo- 
tion of the image. Consequently, a rotation producing only 
a fraction of a millimeter's relative motion of lens and plate 
during the period the curtain aperture is over a given point 



18^ 



AIRPLANE PHOTOGRAPHY 



would cause fatal blurring — and the visible vibration of 
plane longerons and cross members is easily of half milli- 
meter amplitude or more. Reduced to angular units it is 
easily shown that a rotation of one degree per second — which 
is quite slow as plane oscillations go — is beyond the limits 

of toleration. Translational mo- 
tions of large amplitude may be 
allowed, but the mounting of the 
camera must not permit these 
translations to be at all different 
for different parts of the camera. 
The proper way to eliminate 
vibrational effects is to devise a 
mounting that will transmit only 
the translational shocks or that 
will transform the rotational ones 
into translations. Platforms 
pivoted and cross-linked so as to 
be free to move only parallel to 
themselves (described in the next 
chapter) represent one attempt 
to reach this result. Quite the 
simplest and most scientific form 

Fig. 77.— Diagram showing possible ^f mOUntiufiJ tO achlcVC this CUd Is 

tions of the airplane camera: three of *-^ 

combkatio^""* *^'** °' rotation, and their |-q g^pport the camcra solcly in the 

plane of the center of gravity. The 
principle here involved is easily grasped if we note that when 
we jar a camera supported above or below its center of 
gravity, the effect is to start the camera vibrating with the 
center of gravity oscillating pendulum-like about the point 
of support. The closer the center of gravity to the center 
of support, the smaller the moment of the body about the 
latter point. 




motions 



CAMERA SUSPENSION 183 

Experimental Study of Methods of Camera Support. — 

Conclusive evidence as to the merits of any system of camera 
momiting can be obtained only mider conditions that elimi- 
nate the effect of other variables which may be equally 
efficacious in diminishing the effects of vibration, but which 
have only limited application. Very brief exposures — ^-^ 
second and less — will, for instance, result in good pictures 
with almost any condition of vibration. Hence a sharp 
picture offers no proof of the merits of a camera mounting 
unless it is known that the exposure was no shorter than the 
limit set by the ground speed of the plane. In fact it may be 
said that the chief object of studying methods of camera 
suspension is to increase the allowable exposure to a maxi- 
mum, thus lengthening the working hours and multiplying 
the useful working days for aerial photography. 

The most satisfactory method of test yet developed is to 
fly over a light or a group of lights on the ground with the 
camera shutter open. In the first use of this method, which 
originated in the English Service, such flights were made at 
night, but later it was found that good results could be got 
by placing the lights in a forest and making the tests when 
the sun was fairly low. One of the group of lights must be 
periodically interrupted, at a known rate, to furnish the 
time intervals. 

Some characteristic "trails" obtained by this method of 
test are shown in Fig. 78. The first trail is that given by a 
camera rigidly fastened to the fuselage. The second and 
third show hand camera trails, made by an inexperienced 
and by an experienced observer, respectively. They show 
by comparison with the other figures that the human body 
is an excellent block to vibration, but in unskilled hands a 
poor check to rapid erratic (probably rotational) motions 
of the camera. The fourth is the trail given by a camera 



184 AIRPLANE PHOTOGRAPHY 




} i-v^ch } 









HtLY^i He(c( 




Fig. 78. — Tests of camera mounting, made by flying over a bright light against a dark background. 
(a) Rigid fastening on side of plane; (6) held in the hand, inexperienced observer; (c) held in the hand, 
experienced observer; (d) camera mounted at center of gravity on gimbals bedded in sponge rubber. 



supported by gimbals bedded in sponge rubber accurately 
in the plane of the camera's center of gravity. Other trails 
are shown in the next chapter in connection with the de- 
scription of practical camera mountings. Clearly the best 



CAMERA SUSPENSION 185 

suspension is that giving the smallest amplitude of displace- 
ment during the interval of time covered by an average 
exposure. It is, in fact, possible to determine from these 
trails the permissible exposure for any assumed permissible 
blurring of the image. The rigid mounting trail indicates 
very bad conditions, calling for literally instantaneous 
exposures. The center of gravity trail, at the other extreme, 
shows practically no limitation of exposure in so far as vibra- 
tion is concerned, thus bearing out the theoretical conditions 
above discussed. An interesting conclusion from these 
experiments is that a rapidly running motor gives less harm- 
ful vibration than a slow one, although in the war it was a 
common practice to throttle the motor before exposing. As 
might be expected, the greater the number of cylinders, 
the shorter the period and the smaller the amplitude of 
the vibration. 

Pendular Camera Supports. — The design of the camera 
support may be approached from a different standpoint, 
namely, with the aim of carrying the camera so that it will 
tend to hang always vertical. In mapping this is of funda- 
mental importance. It is, indeed, a question whether aerial 
mapping will ever be worthy of ranking as a precision method 
unless the camera can be mounted so that its pictures are 
taken in the horizontal, undistorted position. 

The simplest way to hold the camera vertical is to mount 
it on gimbals, with its center of gravity below the point of 
support. When so mounted the camera swings as a pendu- 
lum. Delicacy of response to variation of level is obtained 
by leaving a considerable distance between the center of 
gravity and the center of support. Oscillation about the 
vertical position is to be prevented by some system of dash 
pots or other damping. A suspension of this kind is furnished 
with the Brock film camera (Fig. 60). 



186 AIRPLANE PHOTOGRAPHY 

It will be seen at once that the relation of center of 
gravity to center of support called for here is in direct con- 
tradiction to the requirements for eliminating vibration. 
Either one requirement or the other must be sacrificed, or 
else a compromise made in which neither delicate response to 
inclination of the plane nor fully satisfactory freedom from 
vibration is attained. This is a very serious objection to the 
pendular support. But the really vital objection to the 
pendular support is that it performs its function only very 
partially. It is entirely satisfactory only under conditions 
of steady flying, as in a uniform climb or glide, with the plane 
tail or nose heavy, or in flying with one wing down. As 
soon as we introduce any acceleration, as in making a turn, 
the camera follows the plane and not the earth. 

It is true that mapping photography is done from a plane 
flying as level as possible, and that except under bad air con- 
ditions it holds its course with very little turning, if handled 
by a skilled pilot. Nevertheless, a surprisingly small devia- 
tion from straight flying causes quite serious variations from 
the vertical. It is of interest to calculate how large may be 
the horizontal accelerations that accompany swervings 
from a straight course which one might think insignificant. 
For instance, consider the horizontal acceleration due to 
a turn having a radius of a kilometer when the plane is 
moving at 100 kilometers per hour. If a is the accel- 
eration, V the velocity of the plane, and r the radius, we 
have from elementary dynamics that 

a = — 

r 



Substituting the values chosen, we have- 



_ 100,000^ ^ meters 
° ~ 36002 X 1000 * sec2 



CAMERA SUSPENSION 187 

. meters 
The acceleration of gravity being 9.80 ^ — we nave that 

the ratio of the horizontal acceleration to the vertical is 

.77 

This is the tangent of the angle of deviation from the ver- 
tical, from which the angle turns out to be about 4>^ degrees, 
a very considerable error, rapidly multiplied as the speed of 
the plane is increased. It is, indeed, open to question whether 
the average deviations from the vertical are not apt to be 
less with the camera rigidly fixed to the plane, if guided by 
a skilled pilot who will hold the ship level at the expense 
of "skidding" the slight turns he] must make to hold 
his direction. 

Gyroscopic Mountings. — The ideal support for the 
aerial camera will undoubtedly be one embodying gyroscopic 
control of the camera's direction. By proper utilization of 
the principles of the gyroscope it is to be expected that not 
only can the camera be maintained vertical, but it may be 
supported anti-vibrationally as well. At the present time 
the problem of gyroscopic control is in the experimental 
stage, so that only the elements of the problem and the pos- 
sible modes of solution can be laid out. 

The gyroscope consists essentially of a heavy ring or 
disc rotating at a high speed on an axis free to point in any 
direction (Fig. 79). If mounted so that the axes of the sup- 
porting gimbals pass through the center of gravity of the 
rotating disc, the result is a neutral gyroscope. Its charac- 
teristic is that its axis maintains its direction fixed, but this 
fixity is with respect to space and not with respect to the 
gravitational vertical. Consequently, as the earth revolves 
the inclination of the gyroscopic axis changes with respect 



188 



AIRPLANE PHOTOGRAPHY 



to the earth. In latitude 45° this change is approximately a 
degree in five minutes. Furthermore, the action of friction in 
the supports, which can never be entirely eliminated, also 
acts to slowly alter the direction of the gyroscopic axis. 
Therefore, unless some erector is applied even the gyroscope 
will not perform the task required of it. 



r' 







-X 



Fig. 79. — Diagram of simple gyroscope. 

Before discussing possible forms of erectors it may be 
noted in general, first, that these must depend upon gravity; 
second, that such being the case, they must respond to the 
resultant of gravity and any acceleration, that is, to the 
apparent or pseudo-gravity. As already seen, this pseudo- 
gravity, during a turn, is exactly what limits the usefulness 



t 



CAMERA SUSPENSION 189 

of the pendular support, and necessitates recourse to the 
gyroscope. The problem thus becomes one of making an 
erector-gyroscope combination which will respond to true 
gravity and not to pseudo-gravity. 

In general this problem would be insoluble, since there is 
no difference in the nature of the acceleration of gravity and 
that due to centrifugal force. A way out is offered, however, 
by the fact that true gravity acts continuously and at a 
small angle to the axis of the gyro, while the components 
which cause the pseudo-gravity are of short duration, liable 
to rapid changes of direction, and, on a turn, act at a large 
angle. What we require, therefore, is an erector which will 
respond slowly but surely to the average acceleration, which 
is downward, but too sluggishly to be affected by the shorter 
period accelerations due to turns or rolls. Slowness of 
response is a matter of the erecting forces being small and of 
the mass and angular velocity of the gyro disc being large. 
The success of the compromise called for depends on the 
relative times taken for the gyroscope to tilt seriously from 
the true vertical, due to the causes above mentioned, and 
for the average turn or roll. Fortunately the former is a 
matter of minutes, the latter of seconds or at the worst of 
fractions of a minute. More than this, since the roll or turn 
is apt to be of much greater angle than any normal deviation 
of the gyroscopic axis from the vertical in the same time, we 
are offered the possibility of some device for filtering out the 
deviations which alone are to effect the erector. Forinstance, 
by shunting the restoring force whenever it is called upon to 
act through more than a predetermined small angle. 

As to the method of erecting the gyroscope, its charac- 
teristic property must be kept in mind. This is that the 
axis does not tilt under an applied force in the direction it 
would if the gyro were not rotating, but around an axis at 



190 AIRPLANE PHOTOGRAPHY 

right angles to that of the apphed couple. Thus in Fig. 79, 
if a weight is attached as shown, the disc does not incline 
downward toward the weight, around the axis Y, Y\ but 
precesses about the vertical axis Z, Z'. Some means is 
therefore needed to translate the pull which any gravita- 
tional control, such as a freely swinging pendulum, would 
give, into a pull with at least a component at a finite angle 
to this. 

In the Gray stabilizer several metal balls are slowly 
rotated in a tray above the center of gravity of the gyroscope. 
Specially shaped grooves or compartments limit the freedom 
of motion of these balls so that when the gyro is inclined the 
balls travel at different distances from the center on the 
ascending and descending sides. By this scheme a couple is 
produced about the axis through the center and the low point 
of the disc, which tilts the apparatus to the gravitational 
vertical. In an alternative form the balls are carried past 
the low point by their momentum and are prevented from 
returning by the walls of the containing compartment, which 
have meanwhile been advanced by the rotation of the erector 
as a whole. The net result is to shift the center of gravity 
of the system of balls in the proper direction to erect the 
gyro. The rectifying action is purposely made quite slow 
so that the displacements of the balls due to pseudo-gravity 
will be averaged out. 

In a design due to Lucian, small pendulums work through 
electric contacts to actuate solenoids which in turn move 
small weights in the appropriate directions to give the 
desired tilt. Response is made fairly quick and delicate, 
and pseudo-gravity, due to turns and rolls, is rendered inop- 
erative by the contacts breaking whenever the pendulums 
swing more than three or four degrees. This can only 
happen if they move too quickly for the erecting forces to 



CAMERA SUSPENSION 



191 



act, reliance being here placed on the characteristic differ- 
ences of action in respect to time of real and pseudo-gravi- 
tational forces. 

Besides the neutral gyroscope as just considered there is 
the pendular or top type, in which the center of gravity is 
not in the plane of the supports. In general this type depends 
on a couple resulting from the gravitational pull and the 
inevitable friction of the supports to slowly tilt the axis to 
the gravitational vertical. This type is slower to respond 





Fig. 80. — Diagram of camera linked to gyroscopic stabilizer. 

than the designs in which a definite couple in the proper 
direction is provided and it reaches the true vertical only 
through a circuitous path. 

Three methods of controlling a camera by a gyroscope 
are suggested. One is to fasten the gyroscope rigidly to the 
camera and mount the whole system on gimbals. A second 
is to mount both camera and gyro side by side on gimbals, 
linking the two so that the camera is moved parallel to the 
gyro (Fig. 80). A third method is to utilize the gyro to 
make electric contacts to operate motors which in turn move 
the camera. 

Considerable weight and space are required for a gyro- 



192 AIRPLANE PHOTOGRAPHY 

scope capable of stabilizing a camera. The rotating disc 
should be about half the weight of the camera, and with its 
mounting may be expected to double the room required for 
the camera alone. Motive power for maintainng the gyro 
in continuous rotation may be supplied by an air blast, or 
the gyro may be made up as an induction motor — ^the latter 
necessitating an alternating current supply. 

In view of the space and weight limitations in a plane it 
is a question still to be decided whether it is more economical 
to stabilize the camera or to stabilize an inclinometer and 
photograph its indications simultaneously with the release 
of the shutter which takes the aerial picture. 



CHAPTER XV 
PRACTICAL CAMERA MOUNTINGS 

General Considerations. — Camera mountings as used 
during the war were far from being developed on the basis 
of scientific study or test. At first the need for special sup- 
porting apparatus was not realized, and the suspensions 
later in use were largely field-made affairs, often dependent 
on adjustments made accordmg to individual taste. Through 
lack of accurate methods of test and of conclusive evidence 
on the subject, it was quite common to find extremists who, 
on the one hand, denied the efficacy of suspensions in general, 
and on the other ardently supported crazily conceived sup- 
porting arrangements which accurate comparative test show 
to be even worse than useless. 

In the French service, despite numerous types of sus- 
pension available, the very general practice was to lift 
the camera from its support and hold it between the knees. 
Or else the hand was pressed on the top of the camera 
during exposure, more reliance being placed on the damp- 
ing qualities of the body than on any of the rubber or 
spring mechanisms. 

As is clearly shown by the experimental data described 
in the last chapter, a correctly designed supporting device, 
carrying the camera accurately in the plane of its center of 
gravity, accomplishes practically perfect elimination of 
vibrational troubles. So important is the use of a mount and 
so important is it that the mount should be correctly dimen- 
sioned and adjusted for the camera, that an entirely different 
attitude should be adopted from the prevalent one which 
focuses attention on the camera and regards the mounting 

13 193 



194 AIRPLANE PHOTOGRAPHY 

as a mere auxiliary to be left more or less to chance. The 
mounting should he considered an integral fart of the camera. 
The man in the field should receive camera and mount 
together, leaving as his only problem the attachment of the 
complete camera — and — mount unit to the plane. This may 
be arranged, by proper designing, to be a simple matter 




Fig. 81. — "L" camera mounted outside the fuselage. Observer using exposure plunger, pilot using 

Bowden wire release. 

of rigid bolting or strapping, requiring ingenuity perhaps 
but not the scientific knowledge which is required for 
mounting design. 

Outboard Mountings.^In the English service the 
camera was first attached to the plane outside the fuselage 
by a rigid frame, to which the camera was strapped or 



CAMERA MOUNTINGS 195 

bolted (Fig. 81). Obvious objections exist to placing the 
camera in this position, such as the resistance of the wind 
and the difficulty of changing magazines. However, in the 
earlier English planes with their fuselages of small cross 
section no other accessible place for the camera was to be 
found. Vibrational disturbances with the rigid outboard 
mounting are quite serious, as is so clearly indicated by the 
trace shown in Fig. 78. Extremely short exposures are 
alone possible, and a very large proportion of the pictures 
are apt to be indistinct. 

Floor Mountings.— A step in advance of the outboard 
mounting is to support the camera snout in a padded conical 
frame on the floor of the plane (Fig. 82). This mounting 
avoids the objection on the ground of wind resistance that 
holds with the outboard, and has possibilities of being worked 
out as an entirely satisfactory support. Yet to be satis- 
factory, the point of support must lie in the plane of the 
center of gravity of the camera, and the camera must be of a 
type that preserves its center of gravity unchanged in posi- 
tion as the plates are exposed. Unless these conditions are 
fully met the floor mounting gives results little better than 
does the outboard. 

Cradles or Trays. — Floor space in the cockpit being 
unavailable in the battle-plane, due to duplicate controls, 
bomb sights, etc., the English service was driven to the prac- 
tice of carrying the camera in the compartment or bay 
behind the observer. Here it was attached either to the 
structural uprights or longerons, or to special uprights and 
cross-pieces built into the plane to serve photographic ends. 
As an intermediary between the camera and the supporting 
cross-pieces there was introduced the camera tray or cradle. 
This is essentially a frame carrying sponge rubber pads into 
which the camera is more or less deeply bedded. Figs. 83 and 



196 



AIRPLANE PHOTOGRAPHY 



84 show an American L camera cradle based on the design 
of the English L camera tray. Thick sponge rubber pads 
support the two ends of the camera top plate, and additional 
pads are provided to hold the nose of the camera. Careful 




Fig. 82. — "L" camera in floor mounting. 

tests show this cradle to be superior to the outboard mount- 
ing, but still leave much to be desired. Its performance is 
better with the nose of the camera left free. 

Tennis=ball Mounting. — A very simple mount used by 
the French consists of a frame enclosing the nose of the 



CAMERA MOUNTINGS 



197 



camera, and carrying four tennis balls, on which the whole 
weight rests (Fig. 40). If the center of support is in the plane 
of the center of gravity and if the four balls are of uniform 




Fig. 83. — "L" camera and cradle mount in skeleton DeHaviland 4 fuselage, side view. 

age and elasticity, this form of support is good. As provided 
by the camera manufacturer, the tennis ball frame fits much 
too far down on the camera. Another application of the 



198 



AIRPLANE PHOTOGRAPHY 



tennis ball idea was frequently made in the French service, 
in which the balls were close up under the shutter housing 
(Fig. 85). Additional support was, however, given to the 
camera nose by flexible rubber bands, the success of the 
whole being largely a matter of the adjustment of the tension 
on the bands. 




Fig. 84. — " L" camera and cradle mountin skeleton deHaviland 4 fuselage, front view. 

Parallel Motion Devices. — ^A form of suspension favored 
by the French consists of parallel bell cranks, rigidly linked 
together and held up by springs. Mountings of this sort 
are illustrated in Figs. 86, 87, 88 and 96. The guiding prin- 
ciple is that any sort of shock will be transformed into a 
straight up-and-down or side-wise motion of the camera, 
which is harmless. The mounting as adapted by the English 
surrounds the camera body, making the plane of support 



CAMERA MOUNTINGS 



199 



somewhere near the center of gravity. In certain of the 
French suspensions employing this principle the whole 
camera is hung below the bell cranks (Fig. 86) , and then the 




Fig. 85. — ^Tennis ball suspension, assisted by elastic bands attached to nose of camera, 

nose is restrained by heavy rubber bands. The net result 
is largely a matter of adjustment. 

Tests on the English design made in the United States 



200 AIRPLANE PHOTOGRAPHY 




Fig. 86. — French spring and bell crank suspension. 



CAMERA MOUNTINGS 



201 




202 AIRPLANE PHOTOGRAPHY 




1 




\ 

* 

1 


1 sec 1 


, 


- - — " 


_ _, -^-- 


- . -„_ _^ 


— - .™. _„ — ■ 






ft J% JiS, ^^ 


^■H 


1 


, 


tStc. 


^^ 


1 Unch i~^~~"~~~~~-~~-__ 1 




1 




\ 






^.^' ^ 


1^x31^ FVtti 


„^'^ 









Fig. 89. — ^Tests of camera mountings: (a) deRam camera on bell-crank-and-spring mount, 
below the center of gravity; (6) same, at center of gravity; (c) type "K" film camera on universal 
mounting (Fig. 88). 




CAMERA MOUNTINGS 20S 

Air Service appear to show that the chief virtue of the mount- 
ing lies in the approximation of the point of support to the 
center of gravity in the Enghsh cameras. A deRam camera 
supported by its cone, so that its center of gravity was con- 
siderably above the center of support gave rather poor results 
(Fig. 89a), but when the bell cranks were attached near the 
center of gravity, highly successful results were obtained 
(Fig. 896) . The French deRam camera as ordered for the 
American Expeditionary Force was fitted with a bell crank 
supported in this position. 

Figures 90 and 91 show a bell crank mounting furnished 
with a rotating turret. This was designed to facilitate the 
changing of magazines in the English B M camera, which 
is swung around through 90 ' degrees from the exposing 
position to bring the magazine near the observer. The 
camera shown in the mounting is the American hand-operated 
model (type M), in which there is the same necessity for 
turning in order to manipulate the bag magazine easily. 
The camera is shown in both exposing and plate changing 
positions. An important detail of these mounts is a safety 
catch, which must be fastened before the plane lands, in 
order to prevent the shocks of landing from producing 
oscillations sufficient to throw the camera out of the mount. 

Center of Gravity Rubber Pad Supports. — Given a camera 
whose center of gravity does not change during operation, a 
simple and entirely adequate anti-vibration support is fur- 
nished by a ring of sponge rubber in the plane of the center of 
gravity. But if provision has to be made for oblique views 
or for adjusting the camera to the vertical, something more 
elaborate is necessary. 

Mountings for the American deRam and for the Air 
Service film camera, embodying the results of complete 
study of the anti-vibration problem, are shown in Figs. 90, 



204 



AIRPLANE PHOTOGRAPHY 




CAMERA MOUNTINGS 



205 



92 and 93. Trusses carrying the cameras on pivots rest on 
four pads of sponge rubber which are mounted on frames 




Fig. 92. — U. S. type "K" film camera on universal mounting, vertical position. 




Fig. 93. — U. S. type "K" film camera on universal mounting, oblique position. 

correctly spaced ready for attachment to the cross-pieces 
of the airplane camera supports. In the deRam (Fig. 90) 
the pivots, attached to the camera body, permit it to be 



206 



AIRPLANE PHOTOGRAPHY 



leveled fore and aft, to compensate for the inclined position 
of the fuselage assumed at high altitudes or in some condi- 









I \ luck 






Fig. 94. — ^Tests on two types of camera mount: (a) Support at bottom of camera; (6) support 

above center of gravity. 

tions of loading. This will sometimes amount to as much 
as 11 or 12 degrees, which is very serious, since one degree 
causes (with an angular field of 20 degrees) about one per 



CAMERA MOUNTINGS ^07 

cent, difference of scale at the two sides of the plate. The 
film camera mounting carries the camera in a conical ring, 
and is pivoted not only for vertical adjustment, but for the 
taking of obliques as well (Fig. 93). These mounts transmit 
practically no vibration. 

A caution must be noted with regard to center of gravity 
mountings. Any change in the camera, in particular the 
substitution of a short for a long lens cone, must be made so 
as to cause no alteration of the relative positions of the center 
of support and the center of gravity. Either the short cone 
must be weighted, or additional supporting pivots must be 
provided in the plane of the new center of gravity. 

The Italian and Q. E. M. Mountings. — These mounts 
(Figs. 49 and 59) are similar in that the protection from 
vibration is furnished by an elastic support at the bottom of 
the camera. Tests show that these two cameras give very 
similar results, of the unsatisfactory sort to be expected from 
this kind of mounting in view of the lessons of the last 
chapter on the proper point of support. Fig. 94, a, shows a 
trace given by the Italian mount. The permissible exposure, 
on the criterion adopted, is very short with either mount, 
about -2^ second. 

The Brock Suspension. — This consists of a pair of frames 
into which the camera is fitted by ball bearing pivots, so 
that it is free to move in any direction (Fig. 60). In order to 
permit gravity to control the direction of the camera, the 
point of support is made considerably (ten inches) above the 
center of gravity. Air dash pots are pro.vided for damping the 
swings. As already explained, the pendular method of sup- 
port is in basic contradiction to the requirements for vibra- 
tion elimination. Tests of the Brock suspension,shownin Fig. 
94, b, indicate it to be of low efficiency in damping out the short 
period vibrations which are responsible for poor definition. 



CHAPTER XVI 

THE INSTALLATION OF CAMERAS AND MOUNT- 
INGS IN PLANES 

Conditions to Be Met. — The characteristic difficulty 
in installing the airplane camera is that there is no place for 
it. After the gasoline supply, the armament, the wireless, 
the oxygen tank, the bombs, and other necessities are taken 
care of there is neither space available nor weight allowable. 
Where space may be found it will be inaccessible, or acces- 
sible only through a maze of tension and control wires; or 
it will be in a position where any weight will endanger the 
balance of the plane. Plane design has in fact been more or 
less of a conflict between the aeronautical engineer, who is 
designing the airplane primarily as a machine to fly, and the 
armament and instrument men, who look upon it as a plat- 
form for their apparatus. Lack of appreciation of the 
extreme importance of aerial photography resulted, during a 
large part of the war, in the camera installation being neg- 
lected until the plane was supposedly entirely designed, and 
even in production. At that stage the installation could be 
but a makeshift. Only in the later stages of the \^ar, when 
plane design became a matter of cooperation between all 
concerned, were fairly convenient and satisfactory arrange- 
ments made for the camera. Always, however, the rapid suc- 
cession of new plane designs, with various shapes of fuselage 
and details of structure, made camera installation in the 
war plane a matter calling for the greatest ingenuity. 

The problem was met in part by constructing both 
cameras and mountings in sections, to be laboriously wormed 
in through inadequate apertures, in part by later structural 
208 



CAMERAS AND MOUNTINGS ^OD 

changes in the planes, such as the substitution of veneer 
rings or frames for the tension wires. In certain cases the 
rear cockpit controls were omitted, thereby freeing accessible 
and often adequate space for the larger cameras. Rear con- 
trols were never used in the German planes, so that their 
standard practice was to carry the camera forward of the 
observer. This, together with the general restriction to the 
13X18 centimeter size plate, made the installation problem 
less difficult in the German aircraft than in the Allied. 

Practical Solutions. — ^An important feature of camera 
installation has already been mentioned, but may well be 
repeated for emphasis. The camera and its anti- vibration 
mounting should always be considered as a unit, and should 
be so designed that simple bolts or straps will suffice to fasten 
it in its place in the plane. Even should the spacing of the 
structural parts of the plane not correspond to that antici- 
pated by the mounting design, the ingenuity of the man in 
the field may be depended upon to make the necessary 
alterations or additions to the plane. The design of the 
camera suspension itself cannot, however, be left to unedu- 
cated ingenuity. 

Assuming the camera and mounting supplied, the next 
step — a very difficult one — is to insure uniformity in the 
structures to be built into the planes for the purpose of sup- 
porting the camera mountings. With this uniformity must, 
however, be combined the greatest possible flexibility to 
provide for various designs of cameras. 

In the English service the standard camera installation 
consists of wooden uprights with cross bars athwart the 
plane, adjustable as to height (Fig. 95). A distance between 
the cross bars of 13^^ inches has been standardized, and all 
camera cradles and mountings are notched or otherwise 
spaced to fit this dimension. The installation adopted in 
14 



^10 



AIRPLANE PHOTOGRAPHY 



the American planes is similar, but with a distance of 16 
inches between cross bars. These uprights and cross bars 
are ordinarily situated in the bay behind the observer, but 
can be placed in any available space. Fig. 83 shows, in a 
model bay, the arrangement of uprights and cross bars in 




Fig. 95. — "L-B" camera with 20-inch lens, mounted on bell-crank suspension in skeleton 
fuselage. Stream-lined hood below to cover projecting end of lens cylinder. Propeller and Bowden 
release in place. 

the American DH 4, with the L camera in place in its cradle. 
It is just possible to introduce camera and cradle separately 
from the observer's cockpit through the tension wires, and, 
by uncomfortable reaching, magazines may be changed. 

A step in advance is made when the top tension wires 
and superstructure are replaced by a rigid frame with an 



CAMERAS AND MOUNTINGS 211 

opening large enough to admit the entire camera and mount- 
ing. When this is done considerably larger cameras may be 
accommodated in the same sized bay, as shown in Fig. 96. 
A further advance, from the standpoint of accessibility and 
convenience of installation, follows when the tension wires 
between observer's and camera bay are replaced by a ply- 
wood ring, as shown in Fig. 97. Here the only serious limi- 
tations are those due to the vertical height of the camera, 
and of course its weight. 

Openings for the lens to point through are simply provided 
in the fabric covered aircraft, by cutting through the canvas 
and stiffening the edge of the hole by wire. Tension wires 
are often in the way. They may either be disregarded, since 
they merely cut off a little light, or replaced in part -by metal 
rings, as shown in Fig. 96. In veneer covered fuselages the 
hole must of course go through the wood. This may be 
undesirable, since the veneer is depended on to furnish 
structural strength, a point which further emphasizes the 
importance of the photographic requirements being thor- 
oughly considered while the plane is being designed. 

Single seater or scout planes do not lend themselves to 
the insertion of such standardized uprights and cross-pieces, 
because of their small size and the common utiKzation of all 
space inside the fuselage for gasoline tanks and control 
wires. Some French scouts, whose fuselages are very wide, 
due to the rotary engines, have been fitted with compart- 
ments for contemplated automatic film cameras. The most 
commonly used camera in the single seater was, however, the 
Italian 24-plate single-motion apparatus (Fig. 49). This 
camera and its carrying tray occupy very little lateral space 
and have in actual practice been carried beneath the seat 
or pushed up through an opening in the bottom of the fusel- 
age under the gasoline tank. Whatever criticism may be 



212 



AIRPLANE PHOTOGRAPHY 




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CAMERAS AND MOUNTINGS ^15 

made of the adequacy of the mounting, it must be said that 
the camera, as used, is perhaps the most eminently practical 
of all developed in the war, as its use on scouts testifies. 

Special Photographic Planes. — ^As cameras grew in size, 
the difficulty of installing them in planes built without regard 
to photographic requirements greatly increased. Few planes 
could carry even the 50 centimeter focus camera obliquely 
without the necessity of poking its nose through the side 
where it would catch wind and oil; while the 120 centimeter 
camera could be carried obliquely only in the fore and aft 
position. Even vertical installation of the latter camera was 
really feasible in but few planes; sometimes the camera was 
carried to the exclusion of the observer — and, in fact, this 
size was never used by the English, whose fuselages were 
small in cross-section. 

This situation led, late in the war, to steps toward pro- 
ducing planes designed primarily for photographic recon- 
naissance. In these the camera would be entirely accessible, 
and cameras of any size could be carried in any desired 
position. One scheme which properly belongs under this 
heading was the provision of a special removable photo- 
graphic cockpit, for the front or nose of a twin-motored three 
seater. Other noses, for bombing and heavy machine guns, 
were also planned, all to be interchangeable. Since the 
regular photographic bay with uprights and cross-pieces was 
also provided to the rear, this special photographic ship 
could on occasion do two classes of work, such as long focus 
spotting and short focus mapping. 

The most completely worked out photographic plane was 
probably the model designated Pi by the United States Air 
Service. This is a modified de Haviland 4 in which the rear 
controls have been removed and the cowling raised and at the 
same time made squarer in cross-section. The space formerly 



^14 AIRPLANE PHOTOGRAPHY 

occupied by the rear controls provides ample room for all 
types of camera. These are carried on uprights at the 
standard distance apart, 16 inches, with cross-pieces adjust- 
able as to height. The camera space is accessible not only 
from the observer's cockpit, but from above, upon folding 
back the metal cover. Doors at the bottom and at each side 
permit not only vertical but oblique exposures. The latter 
are not interfered with by the wings, as they would be in 
some designs of plane if the camera occupied the same posi- 
tion relative to the cockpits. Fig. 91 shows the deRam 
caniera in place, as seen from the rear. Figs. 98 and 99 show 
the 18X24 centimeter film camera, set both for vertical and 
oblique views. 

Negative lenses are provided for both pilot and observer, 
the one for the pilot permitting him to see from a point far 
ahead to directly underneath, while the observer's is fur- 
nished with cross wires below and etched rectangles of the 
camera field sizes on the upper surface. Windows of non- 
breakable glass assist in sighting obliques. The accompany- 
ing picture (Fig. 100) of the plane showing an oblique camera 
in position gives an excellent idea of its appearance. Its 
special features are worthy of copying in peace-time photo- 
graphic aircraft. 

Installation of Auxiliaries. — ^It is quite necessary that 
the camera lens be protected from splashing mud and often 
from oil spray due to the motor. For this purpose an easily 
opened and closed door is essential, unless the camera is 
carried well up in the plane. An alternative, possessing 
certain advantages, is to incorporate into the camera pro- 
tecting flaps operating in front of the lens, which open only 
when the exposure is made. If the camera projects beyond 
the fuselage, stream lined hoods (Fig. 95) must be provided to 
protect the camera nose with the minimum of air resistance. 



CAMERAS AND MOUNTINGS 215 




216 



AIRPLANE PHOTOGRAPHY 




CAMERAS AND MOUNTINGS 217 

The mounting of the regular camera auxiliaries — releases, 
sights, propellers, speed controls, motors — is usually a great 
bother, due to lack of space and to the severe restrictions on 
methods of fastening. Screws in longerons or uprights are 
taboo. Metal straps to go around structural parts are the 
approved device, but with variations in the size of these 
members, the holes, straps, bolts and nuts provided are very 
apt not to fit. Changes of construction, such as that from 
skeletons covered with fabric to veneer bodies, also interfere 
with any standard means of attachment, and leave this, like 
many other problems in war-time aerial photography, to 
the resourcefulness of the man in the field. 

Magazine racks must be tucked away in any available 
space. Under the seat is a position frequently utilized. 
Especially with plates is it desirable to carry the extra 
magazines in a position to interfere as little as possible with 
the balance of the plane. In the DH 4 this means that they 
should be carried if possible forward of the observer, even 
though he must turn completely around to get and insert 
each magazine. 



^. 



IV 

SENSITIZED MATERIALS AND CHEMICALS 



CHAPTER XVII 

THE DISTRIBUTION OF LIGHT, SHADE AND 
COLOR IN THE AERIAL VIEW 

The general appearance of the earth as viewed from above 
has already been described and illustrated (Figs. 10 and 11). 
It remains to deal with the earth's appearance in a more 
analytic and quantitative manner, in order to decide upon 
the characteristics to be sought in our photographic sensi- 
tive materials. 

Range of Brightness. — The absence of great contrasts 
so apparent in the view of the earth from a plane is confirmed 
by photometric observations. These show that the average 
landscape, as seen from the air, rarely presents a range of 
brightness of more than seven to one, even when seen with- 
out the presence of veiling haze. It is to be remembered that 
shadows constitute no important part of the aerial landscape. 
Vertical walls in shadow, which form a substantial part of 
the surfaces seen by an observer on the ground, are invisible 
or greatly foreshortened from the air. Moreover, they are 
never contrasted against the sky, which is photographically 
often the brightest part of the ordinary picture. To the 
aviator's eye shadows on the ground are only of any length 
at early and late daylight hours. Even at these times they 
cover but a small area, since the number of high vertically 
projecting objects in a representative landscape is small. 
Lacking shadows, the brightness range is only that between 
various kinds of earth, water, and vegetation. Chalk (from 
freshly dug trenches), reflected sunlight from water, or 
marble buildings, furnish almost the only extensions to the 
brightness scale as above given. 

221 



222 



AIRPLANE PHOTOGRAPHY 



Diurnal and seasonal changes. During the winter 
months on the Western Front photography from the air 
was only possible for two or three hours around noon, on 
clear days. This calls attention to another factor of prime 
importance, namely, the large variation in the intensity of 



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Fig. 101. — Variation of average daylight intensity during the day. 

daylight during the course of the day and during the course 
of the year. 

Measurements showing typical variations from morning 
to night are exhibited in Fig. 101, from which it appears 
that there is an increase in illumination of four to five times 
from 8 o'clock — when it would be considered full daylight 
for purely visual observation — until noon, while there is a 
corresponding decrease by four o'clock. Fig. 102 shows sets 



LIGHT, SHADE AND COLOR 



223 



of measurements by two different authorities which give 
the average intensity of daylight for each month throughout 
the year. From December to July there is an increase of 
approximately ten times. From both sets of data it there- 
fore appears that — neglecting the frequent occurrence of 
clouds which reduce the illumination to a half or a quarter 
or even less — a variation in illumination of forty or fifty 



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Fig. 102. — Variation of intensity of daylight through the year; two different sets of measurements. 

times occurs between mid-day in summer and morning in 
winter. In the photography of stationary objects on the 
ground this range of intensities is easily taken care of by 
selection of lens stop and shutter speed. On the airplane 
it is quite otherwise, because the shutter speeds called for 
at the lower illuminations are much slower than the motion 
of the plane will allow. 

Haze. — At low altitudes the brightness range is sub- 
stantially that which would be obtained by photometric 



224 AIRPLANE PHOTOGRAPHY 

measurements of soil and vegetation made at the earth's 
surface. At higher altitudes, especially above 2000 meters, 
this brightness range is materially decreased by atmospheric 
haze. The significance of this lies in the fact that for safety 
from anti-aircraft guns, war-time aerial photography must 
be carried out at very great elevations. Toward the end of 
the Great War photographic missions traveling at from 5000 
to 7000 meters were the rule. At these heights, even in very 
clear weather, a veil of bluish- white haze reduces the already 
small contrasts still more. Some means for overcoming the 
effect of this haze becomes imperative, therefore, in order 
to secure in the picture even the normal contrast of the object. 

Haze is to be sharply distinguished from clouds or fog. 
Clouds and fog consist of globules of water vapor of large 
size, opaque to light. Haze, on the contrary, is more opaque 
to some colors than to others, or is selective in its veiling 
effect. Its scattering action on light is greatest in the violet 
and blue of the spectrum, decreasing rapidly through the 
green, yellow, and red, the exact relation being that the 
scattering is inversely as the fourth power of the wave- 
length. It is, consequently, possible to pierce or cut haze by 
using yellow, orange, or red color screens. It is this possi- 
bility which has led to the extensive use of yellow or orange 
goggles for shooting and for naval lookout work. In aerial 
photography the equivalent is to be found in color filters, 
used with color sensitive (orthochromatic or panchromatic) 
plates, which have been found essential for all high alti- 
tude work. 

Color. — ^Visual observation from the airplane is aided in 
no inconsiderable degree by the differences of color that exist 
between various objects of nearly the same brightness. This 
means of distinguishing differences of character fails in the 
photographic plate, which is color-blind; that is, it reproduces 



LIGHT, SHADE AND COLOR 225 

all objects as grays of varying brightness. It is color-blind 
in another sense as well, in that it evaluates colors as to 
brightness differently from the way the eye does, overrating 
blues and violets and underrating yellows and reds. This 
first kind of color-blindness is a positive disadvantage, for it 
leaves available for differentiating objects only their bright- 
ness differences. The second kind of color-blindness may on 
occasion actually be an advantage. For it may happen, by 
accident, or by design (through the skilful use of color filters), 
that objects appearing nearly the same to the eye appear 
different in the plate. More will be said about this in con- 
nection with the use of filters for the detection of camouflage. 

The range of hues seen in the aerial landscape is not 
large. Greens (grass and foliage) predominate, followed by 
browns (earth) , neither color being bright or saturated . Over 
towns or cities we find that grays (roads) and redder browns 
(brick) are conspicuous. Blues are practically never seen, 
although it is to be noted that a fair share of the illumination 
of the ground is by blue sky light and that the haze itself is 
bluish. Consequently, the general tone of a landscape is 
much bluer than one would be apt to imagine it from con- 
sideration of the general green and brown character of the 
constituent objects. A color photograph from the air would 
greatly resemble a pastel in its low range of tones and the 
absence of bright colors. 

The Photographic Requirements Dictated by Brightness 
and Color Considerations. — Considering only the demands 
made by the character of the view presented to the airplane 
camera, and leaving out of account other limitations to 
photographic operations in the plane, certain requirements 
as to sensitized materials may be outlined. First of all, the 
photographic process must not reduce, but should rather be 
capable of exaggerating, the range of brightness of the object. 
15 



226 AIRPLANE PHOTOGRAPHY 

Preferably the seven-to-one range of the object photographed 
should be lengthened out to the full range of the printing 
paper, which may be two to three times this. With such an 
increase of range, those minute differences of brightness are 
accentuated, on which the detection of many objects depends. 

Next, the plate or film must be sensitive to the portion of 
the spectrum transmitted by a yellow or orange filter which 
will cut out the effect of haze. This calls for orthochromatic 
or panchromatic plates, depending on the depth of filter 
required. Next, if the objects to be photographed differ 
little in brightness but are different in color composition, 
we may have to rely on color filters of peculiar transmissions, 
capable of translating these color differences into brightness 
differences. These will, in general, call for fully color sensi- 
tive, or panchromatic plates. 

In conclusion it may be pointed out that the endeavor in 
ordinary orthochromatic photography — to reproduce the 
visual brightness of colors in the photographic print — ^has no 
real justification in aerial work. Neither in respect to color 
values nor in respect to brightness range is it the object of 
aerial photography, especially for war purposes, to present a 
truthful tone reproduction. Its aim is rather the adequate 
differentiation of detail, by whatever means necessary. 



CHAPTER XVIII 

CHARACTERISTICS OF PHOTOGRAPHIC 
EMULSIONS 

The purely photographic problem in aerial photography, 
as distinct from the instrumental one, is the selection of 
photo sensitive materials which will yield useful results under 
the conditions peculiar to exposure from the air. After such 
materials have been found by extensive field tests, it is pre- 
eminently desirable to determine their characteristics in such 
terms that the kind of plate or film may thereafter be speci- 
fied and selected on the basis of purely laboratory tests. 
Specification must be made in terms of the ordinary sensi- 
tometric constants of the photographic emulsion — its speed, 
contrast, fog, development factor, its color sensitiveness, its 
ability to render fine detail, and its grosser physical properties 
such as hardness and shrinkage. 

Sensitometry. — The most generally used system of 
sensitometry is that of Hurter and Driffield, commonly 
referred to as the "H & D." By this system, in order to 
determine the characteristics of a given photographic plate, 
it is necessary to take a series of graduated exposures, a 
standard illumination of the plate being varied in known 
amount by a rapidly rotating disc cut to a series of different 
openings, or by some other suitable means. The negative 
thus obtained is developed in a standard developer for a 
definite time, at a fixed temperature, and is then measured 
for transmission on a photometer. The following terms are 
defined and used in plotting the results : 

_, _ rp _ intensity of light transmitted _ / 

intensity of incident light h . 

227 



228 AIRPLANE PHOTOGRAPHY 

. _ f\ _ intensity of incident light _ ^<» _ 1 
intensity of transmitted light / T 
Density = D = — logio T = logio 

Hurler and Driffield pointed out that a negative would 
give a true representation of the differences in the light and 
shade of the object if it reproduced these differences by 
equivalent differences in opacity. This is equivalent to 
stating that if the densities are plotted against the logarithms 
of the corresponding exposures, a straight line should be 
obtained at 45 degrees to the axis of exposure times. If the 
line is at another angle the opacities of the negative will be 
proportional to the brightness of the object photographed, 
but the contrast will be different. 

A typical H & D plot is shown in Fig. 103. It will be 
noted that two curves are shown. These are obtained with 
different developments, and illustrate the fact that the con- 
trast or proportionality between exposure differences and 
opacity differences is a matter of time of development. Each 
of these curves exhibits certain characteristics which are 
common to all made in this way. There is primarily a 
straight line portion, where opacities are proportional to 
illumination. This is commonly called the region of correct 
exposure. The slope of this straight line portion — the ratio of 

l ogXosure — ^ ^^^ development factor, commonly denoted 
by 'V/' a gamma of unity denoting exact tone rendering. 
Below the region of correct exposure is a '*toe," or region of 
smaller contrast, called the region of under exposure. Above 
the correct exposure region is another where the opacity 
approaches constancy (afterwards decreasing or "revers- 
ing"), called the region of over exposure. 

The speed of a plate on the H & D scale is ^ven by the 
intersection of the straight line portion of the characteristic 
curve when produced, with the exposure axis. This inter- 



PHOTOGRAPHIC EMULSIONS 229 



section point, called the inertia, is the same irrespective of 
the time of development, as is shown in Fig. 103. The numer- 
ical value of the speed is obtained by dividing 34 by the iner- 
tia, when the exposure is plotted in candle-meter-seconds. 

If a plate is developed until no more density and contrast 
can be obtained, its development factor is then y^:,, (gamma 



H. and D. Cbartioteristio Currs. 



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Fig. 103. — ^Typical characteristic curves of photographic plate. 

infinity), and the larger this is the more a plate can be forced 
in development. If the plate fogs in its unexposed portions 
this fog is measured and recorded in density units along with 
the other constants. The speed of development is repre- 
sented by the velocity constant, commonly symbolized by k. 
The length of the straight line portion determines the 
latitude of the plate, or the range of permissible exposures to 
secure a "perfect negative." Thus if we assume that an 



230 AIRPLANE PHOTOGRAPHY 

object has a range of brightness of 1 to 30, then a plate with 
a straight Kne characteristic extending over a range of 1 to 120 
would have a latitude of ^^ or 4. That is, the exposure could 
be as much as four times the necessary one, and still give the 
same result on a sufficiently exposed print. If thelatitudeof the 
plateis too small, theshadows will fall in the under exposure re- 
gion, the high-lights in the over exposure portion of the char- 
acteristic curve, with consequent poor rendering of contrasts. 

Criteria of Speed. — ^In airplane photography speed is of 
paramount importance, but great care must be exercised to 
insure that all the factors are considered which can contribute 
toward yielding the desirable pictorial quality in the brief 
exposure which alone is possible from the moving plane. A 
"fast" plate on the H & D scale is not necessarily suitable 
for aerial work, when we remember that accentuation of 
natural contrast is desirable, particularly under hazy condi- 
tions. For, as is shown in Fig. 104, it is a common character- 
istic of "fast" plates to have comparatively small latitude 
and low contrast at their maximum development. 

It is to be noted that the Hurter and Driffield measure of 
speed is bound up. with the idea of correct tone rendering and 
with the use of the straight line portion of the characteristic 
curve. Other criteria of speed exist. For instance, the expos- 
ure necessary to produce a just noticeable action (threshold 
value); and the exposure necessary to give a chosen useful 
density in the high-lights when development is pushed to the 
limit set by the growth of fog. 

As has already been pointed out, correct tone rendering 
is not necessary or even indicated as desirable in aerial views. 
It is, moreover, a matter of experience that the majority of 
aerial exposures with existing plates fall in the "under 
exposure " period, where contrasts with normal development 
are less than in the subject. This being the case, the problem 



PHOTOGRAPHIC EMULSIONS 231 



H.t D. Characteristic Curves 









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Log Exposure 
Fig. 104. — Characteristic curves of fast and slow plates, developed to maximum contrast. 

is to select not necessarily a fast plate, by the H & D criterion, 
but a plate which will develop up workable densities in the 
under exposure region. A plate of medium speed will some- 



232 AIRPLANE PHOTOGRAPHY 

times develop to greater densities in the short exposure 
region, if development is forced, than will a fast plate. The 
contrast in the normal exposure region will be excessive, but 
this is of no significance if no exposure falling in this region 
is present on the plate. 

In addition to its capacity for developing density, the 
plate should have as low a threshold as possible, thus meet- 
ing to some extent the requirements of both the alternative 
criteria of speed given above. At the same time it is true 
that low threshold and good density for short exposures 
are not to be found in really slow plates. Consequently, 
while high speed, as ordinarily understood, is undoubtedly 
the first requirement, we may expect the complete specifica- 
tion for the best aerial plate to be a rather complicated thing, 
describing the characteristics of a workable "toe" of the 
curve, in terms of which several (e.gr., contrast and speed) are 
derived from another and quite different exposure region. 

Effect of Temperature on Plate Speed. — It has been found 
by Abney and Dewar that very low temperatures materially 
decrease the speed of photographic emulsions. This decrease 
may amount to as much as 50 per cent, in the temperature 
range from 30 degrees Centigrade above zero to 30 degrees 
below zero, which is the range over which aerial photographic 
operations will have to be carried on in war-time. This 
effect has not been at all fully studied, and it is not known 
whether it is general or only found in certain kinds of plates. 
The remedy indicated is to provide means for heating the 
plates or films when low temperatures are encountered. This 
is fairly easy in film cameras, or in plate cameras like the 
deRam, where the entire load of plates is carried in the 
camera body. Plates carried in magazines present a more 
difficult problem. The heating coil incorporated in the 
German cameras is perhaps partly for this purpose. 



PHOTOGRAPHIC EMULSIONS 233 

Color Sensitiveness. — Complete specifications for an 
aerial plate cannot be made solely on the basis of its speed, 
contrast, latitude, threshold, and other sensitometric values 
which have to do only with the intensity of the light acting 
on it. These in general apply to photography from low alti- 
tudes, where the illumination and natural contrast of the 
subject are the only factors to consider. When higher alti- 
tudes are reached the interposition of haze decreases the 
already deficient contrast, calling either for the development 
of more contrast in the plate, or for the use of color filters 
to cut out the action of the blue and violet light predominant 
in haze. Along the lines discussed in the last section, it is 
not surprising to find that some plates are better than others 
for bringing out gradations masked by haze, even though no 
filters are used and though the plates are similar in color 
sensitiveness. But the limitations to securing contrast by 
manipulating the characteristic curve of the plate are soon 
reached, and it becomes necessary to resort to haze-piercing 
color filters, used with color sensitive plates. 

Roughly, two general types of color sensitive emulsions 
may be distinguished: first, those in which sensitiveness to 
green and yellow is added to the natural blue sensitiveness, 
and second, those sensitive in a useful degree to all colors of 
the spectrum. The former are called iso- or ortho-chromatic, 
the latter panchromatic emulsions. Spectrograms exhibiting 
the distribution of sensitiveness throughout the spectrum 
for several representative plates are shown in Fig. 105. 
Orthochromatic plates are adequate for use with light yellow 
filters and have the slight practical working advantage that 
they can be handled by red light. Panchromatic plates are 
necessary for use with dark orange or red filters. They must 
be handled in total darkness or in an exceedingly faint blue- 
green light, taking advantage of the common drop in sensi- 




Fig. 105. — Spectrograms of representative photographic plates: a, ordinary plate; 6, ortho- 
chromatic plate; c, specially green-sensitive plate; d, red sensitive plate, insensitve to green; e, pan- 
chromatic plate; /, specially red-sensitive panchromatic plate. 



PHOTOGRAPHIC EMULSIONS 235 

bility in that region of the spectrum. Plates can, indeed, be 
sensitized for the red alone, leaving a gap of almost complete 
insensibility in the green, as shown in the fourth spectrogram 
of Fig. 105. When used with a yellow filter these plates 
behave as do panchromatic plates with a red filter. 

A rougher idea of color sensitiveness than is given by 
spectrograms is furnished by the tri-color ratio, which is the 
ratio of exposure times necessary with white light to give 
equal photographic action through a certain set of red, green 
and blue filters, expressed in terms of the blue exposure as 
unity. In an excellent panchromatic plate the three expos- 
ures would be equal. In an orthochromatic plate the red 
exposure will be too large to be figured. In interpreting either 
spectrograms or tri-color ratios care must be taken that the 
absolute exposures necessary are known. Thus a relatively 
high red sensitiveness may mean merely low absolute 
blue sensitiveness. 

Two methods are used in imparting color sensitiveness. 
Either the sensitizing dye is incorporated in the plate emul- 
sion before it is fiowed; or the plate is bathed in a dye 
solution not long before using. The latter method gives 
higher color sensitiveness but poorer keeping quality, and 
is not a practical method for field operations. Greatly 
enhanced sensibility may be given by treatment with am- 
monia, but this again is a method for laboratory rather 
than field use. 

Resolving Power. — A question which arises in connection 
with all photography of detail is the size of the grain of the 
photographic emulsion. Dependent on the size of the grain 
is the resolving power, or ability to separate images of closely 
adjacent objects. This varies with the speed, fast plates 
being of coarser grain than slow ones; with the exposure; and 
with the method and time of development. In general, it 



236 AIRPLANE PHOTOGRAPHY 

may be said that the resolving power of the plate does not 
enter practically into aerial work, because the resolving 
power of all plates so far found usuable corresponds to a 
smaller distance than the size of a point image as limited by 
the performance of the camera lens and the speed of the 
plane. Remembering that xo nim. is a fair value for the 
size of a point image as rendered by the lens, the role of plate- 
resolving power is shown by consideration of the following 
table. Resolving powers are given in terms of lines to the 
millimeter just separable. 



Emulsion. 


Resolving Power. 


Seed Graflex 


25 


Eastman Aerial Film 


37 


Hammer Ortho 


44 


Cramer Isonon 


48 


Cramer Spectrum Process 


57 


Eastman Portrait Film 


61 



Tabulation of Requirements for Aerial Emulsions. — 
In terms of the sensitometric quantities just discussed the 
general requirements for aerial plates may be listed as follows : 

1. Speed. The speed usually connected with the contrast 
and density required for the exposure times available is 
about 150 H & D. Faster plates in general have too low 
contrast, but the highest speed that will give the necessary 
contrast is desired. 

2. Contrast. The contrast capable of development with- 
out fog should be from 1.5 to 2. This contrast should be 
produced by light of daylight quality, and, in orthochromatic 
and panchromatic plates, with the yellow or orange filters 
intended to be used with them. This contrast means a 
gamma infinity approaching 2.5. 

3. Speed of development. A gamma of nearly 2 should be 
developed in 2j/^ minutes at 20 degrees C. in the developers 
recommended below. 



PHOTOGRAPHIC EMULSIONS 237 

4. Fog. Not over .25 for this degree of development, and 
not over .40 for six minutes development. 

5. Color sensitiveness. This should in general be as high 
as possible. In terms of certain representative filters (de- 
scribed in a subsequent chapter) color sensitiveness should 
be such that with the white light speed above specified the 
relative exposures through the filters shall not be greater 
than as follows: 





No filter 


Aerol 


Aero 2 


#21 


#23a #25 


Panchromatic plate 


1 


3 


4.5 


7 


9 12 


Ortho plate 


1 


2.5 


3.5 


6 





Relative Behavior of Plates and Films. — The advantages 
of film from the standpoint of weight and bulk have been 
discussed in connection with aerial cameras. Were there no 
other considerations film would unquestionably be the most 
appropriate medium for aerial photography. There is, how- 
ever, the question of ease of handling, to be treated in a 
subsequent chapter, and the question whether the purely 
photographic characteristics of film are satisfactory. Can 
the same speed, contrast, and color sensitiveness be obtained 
on film as on glass .^^ Is the picture so obtained as permanent 
or reliable as the plate image .^ 

It must be confessed that up to the present emulsions 
on film have not proved the equal of those on glass. It has 
been found by emulsion manufacturers that the same emul- 
sion flowed on film and on glass gives better quality on the 
glass. Emulsions specially prepared for film fall somewhat 
short of the best plate emulsions. It has also been found 
harder to color-sensitize film, and to insure good keeping 
quality in the color sensitized product. 

In addition to the question of photographic quality there 
arises the matter of shrinkage and distortion. These are 
negligible with plates, but are a more or less unknown quan- 
tity in film. Irregular shrinkages of as much as two per cent. 



238 AIRPLANE PHOTOGRAPHY 

are found on experiment. This defect, of course, would be an 
obstacle only in exact mapping work. 

Positype Paper. — The need sometimes arises in military 
operations to secure prints ready for examination within a 
few minutes after the receipt of the negatives. Even the 
15 or 20 minutes within which a negative can be developed 
and a wet print taken may be considered too long. While 
such occasions are probably more apt to occur in popular 
magazine stories than in actual warfare, it is important to 
have available methods of producing prints with an absolute 
minimum of delay. This need is met to some degree by a 
direct print process, commercially exploited under the name 
of ''Positype." 

In this process the exposure is made directly on a sensi- 
tized paper or card, which is developed, the image dissolved 
out, the residue exposed j and again developed; thus furnish- 
ing a positive picture (reversed right and left). The time 
necessary to develop a print ready for examination need 
not be more than three minutes. Only a single print is 
available, but this is all that would be called for under the 
extreme conditions suggested. If later, copies are desired 
they may be made by the same process. 

Plates and Films Found Satisfactory for Aerial Work. — 
The following plates and films have been found particularly 
good for aerial photography. The list is not intended to be 
complete. Furthermore, it may be expected to be soon super- 
seded, as the efforts of various manufacturers are directed 
toward developing special aerial photographic plates. 

Among orthochromatic plates : The Cramer Commercial 
Isonon, the Jougla Ortho. 

Among panchromatic plates: The Ilford Special Pan- 
chromatic, the Cramer Spectrum Process. 

Film: Ansco Speedex, Eastman Aero. 



CHAPTER XIX 
FILTERS 

The Function of Filters in Aerial Photography. — The use 
of color screens or filters has been very common in ordinary 
landscape photography, for the purpose of securing approx- 
imately correct renderings of the brightnesses of colored 
objects. Plates of the non-color-sensitive type have their 
maximum of sensitiveness in the blue of the spectrum (Fig. 
105) and in consequence blue skies photograph as white, 
while other colors are likewise reproduced on a totally wrong 
scale. Filters for correct brightness rendering are calculated 
for a given color sensitive plate so that the resultant reaction 
to the light of the spectrum copies the sensitiveness of the 
eye, which is greatest in the yellow-green. Such filters for 
use with the common orthochromatic plates are of a general 
yellow color. 

Filters for aerial work are meant to serve quite a different 
purpose. Correct tone or color rendering is of quite second- 
ary importance to another use of filters, namely, to cut or 
pierce aerial haze. It is quite a matter of accident that the 
same general color of filter is called for both to give correct 
color rendering and to pierce aerial haze, namely, yellow. Yet 
on closer analysis it is found that quite different types of 
yellow filter are demanded, spectroscopically considered. 

Figure 106 (Ki and K2) shows the spectral transmission 
curves of the Wratten Ki and K2 filters, intended for correct 
color rendering with orthochromatic plates. The absorption 
increases gradually toward the blue. In the same figure is 
shown on an arbitrary scale the spectroscopic character of 
typical haze illumination, increasing in brightness inversely 

239 



240 



AIRPLANE PHOTOGRAPHY 



as the fourth power of the wave-length, that is, with great 
rapidity in the blue and violet. It is evident from this that 
a much more abrupt absorption than that of the Ki or K2 
filter is desirable, because in the green of the spectrum the 
haze light is comparatively weak, and more will be lost by 
any absorption in this region through decreasing useful 



/60 




Violet: 



6lue 



'S'O/UL 

Green 



Yellow OrarjQe 



''f f,,^-^T 



Fig. 106. — Characteristics of various filters. 



photographic action than will be gained by cutting out the 
haze. This latter consideration is important. The use of 
any filter means an increase of exposure; the use of yellow 
filters multiphes it several times. Careful experiment has 
shown that no filter of depth less than K li, to use the Wratten 
filters as a basis for discussion, are of real value in haze 
piercing. The filter ratio, or ratio of exposures with and 
without filter, is 4.7 for the K 1^ with the Cramer Isonon 



FILTERS ^41 

plate — a figure which shows the importance of securing the 
necessary haze-piercing character with the minimum absorp- 
tion of useful photographic Hght. 

Practical Filters. — Since the character of the absorption 
of the "K" filters is not all that could be desired, new filters, 
both of dyed gelatin and of glass, have been produced. The 
glass, a Corning product having a very sharp-cut absorption, 
has not yet been produced on a commercial scale with the 
high transparency in green, yellow and red that selected 
samples have shown. The United States Air Service has 
adopted filters of a new dye, called the EK, from the name 
of the company in whose laboratory it was produced. These 
filters are standardized in two depths of staining, called the 
"Aero No. 1" and "Aero No. 2." Their spectral transmis- 
sion curves appear in Fig. 106, along with those of certain 
darker filters useful only with panchromatic plates for 
exceptionally heavy haze. The characteristic of these Aero 
filters is their great transparency through all the spectrum 
except the blue, whereby the greatest haze-cutting action is 
attained together with a low filter factor. The filter factors 
of the Aero No. 1 and No. 2 with Cramer Isonon plates are 
3 and 5, respectively. 

Effects Secured by the Use of Filters. — The efficiency of 
yellow filters for haze-cutting is best shown by photographs 
taken at high altitudes with filters and without. Such illustra- 
tions are given in Figs. 107 and 108, where the first photograph 
is one taken at 10,000 feet without a filter, the second taken 
at the same altitude under the same conditions, but with 
an orange filter. Both are on panchromatic plates, and it 
will be seen that even with these plates the filter makes all 
the difference between a useless and a useful picture. But 
it must be clearly understood that the difference here lies 
between a plate sensitive chiefly in the blue and violet, and 
16 



U^ AIRPLANE PHOTOGRAPHY 

a plate affected only by the yellow, orange and red. The 
difference is not between what the eye sees and what a plate 
with a filter sees, as is sometimes supposed. As shown in 
Fig. 108, a filter enables the plate to photograph through the 
haze between clouds, but not through the clouds themselves. 
In general, no filter and plate combination which is feasible 




Fig. 107. — ^A photograph taken at 10,000 feet, without a filter. 

for aerial exposures is capable of showing more than the eye 
can see if yellow or orange goggles are worn. To do this it 
would be necessary for the photographic action to take place 
by deep red or infra-red light, which would demand expos- 
ures now out of the question. 

Filters are almost always necessary in photographing 
from high altitudes or in making distant obliques. At times, 
particularly after a heavy rain, the air is clear enough so 



FILTERS 243 

that filters may be dispensed with. Clearing weather was 
therefore chosen whenever possible for making obliques of 
the battle front. 

Filters for the Photographic Detection of Camouflage. — 
In the photographic as in the visual detection of camouflage, 
the problem is to differentiate colors which ordinarily look 




Fig. 108. — Photograph taken at same time and over same neighborhood as Fig. 107, but with an 

orange filter. ^ 

alike, but which are actually of different color composition. 
Particularly important are the differences between natural 
foliage greens and the paints used to simulate them. If these 
differ in their reflection spectra, a proper choice of filter will 
show up the two greens as markedly different. Two kinds 
of difference may be produced ; either the two colors may be 
changed in relative brightness, or they may be altered in 



244 AIRPLANE PHOTOGRAPHY 

hue. Thus foliage green, due to its possessing a reflection 
band in the red of the spectrum, which is absent in most 
pigments, may be made to appear red while the camouflage 
remains green or turns black. Filters which cause changes 
of color are of course of no use for photographic detection 
of camouflage, since the photographic image is colorless. 
Brightness differences are alone available. 

Those same filters which have been worked out primarily 
for producing brightness differences in visual detection of 
camouflage could be used photographically, provided the 
plates employed were color sensitive, and were as well 
screened to imitate the sensibility of the eye. But the most 
useful visual filters are those causing color differences to 
appear; more than this, the visual camouflage detection filters 
as a class have low light transmissions, so that their useful- 
ness in photography is doubtful. Little work has actually 
been done with camouflage detection filters for photography. 
Yet in spite of this photography has been of real service in 
this form of detective work. Its utility for the purpose comes 
from the fact that the natural sensitiveness of th e plat e to blue, 
violet and invisible ultra-violet acts to extend the range of 
the spectrum in which differences between identical and 
merely visually matched colors may be picked up. Conse- 
quently the plain unscreened plate has proved a very effi- 
cient camouflage detector — so efficient in fact that all camou- 
flage materials have had to be subjected to a photographic 
test before acceptance. Fig. 171 shows how an ordinary 
photograph reveals the unnatural character of the camouflage 
over a battery. 

Methodsof Mounting and Using Filters. — Themost prim- 
itive way of mounting a gelatin filter is to cut a disc from a 
sheet of dyed gelatin and insert it between the components of 
the lens. For this purpose the gelatin must be perfectly flat, 



FILTERS ^45 

which is insured by its method of preparation and test. One 
disadvantage of this method is that the filter can be inserted 
and removed only upon the ground. It is less satisfactory 
the larger the diameter of the lens, and the wastage of filters 
due to insertion and removal is apt to be high. The camera 
should be refocussed after filters of this kind are inserted. 

Glass filters, ground optically true, or gelatin filters, 
mounted between optically flat glass plates, are the most 
convenient and satisfactory. They may be mounted in 
circular cells to screw or attach by bayonet catches to the 
front of the lens. Or they may be mounted in rectangular 
frames to slide into transverse grooves in the camera body. 
Fig. 44 shows the mount of this latter form adopted in the 
larger United States Air Service cameras. This is partic- 
ularly convenient if it is desired to insert or change the 
filter while in the air — a practice not generally considered 
feasible in war work with the photographically inexperienced 
observer, but likely to be common with the employment of 
skilled photographers for peace-time aerial photography. 

German cameras are reported in which the glass filter is 
carried behind the lens, on a lever which also carries a clear 
glass plate of the same thickness, to be thrown in when no 
filter is needed, thus maintaining the focus. The perform- 
ance of the lens will be impaired by this scheme, unless it is 
specially calculated to offset the effect of the glass introduced 
in the path of the rays behind the lens — optically true glass 
has no effect on definition if placed in front of the lens. Glass 
filters may also be placed in close contact with the plate 
or film, in which case they must be much larger, but do not 
need to be of as good optical quality. 

Self»screening Plates. — Mention must be made of a 
quite different mode of realizing the filter idea, a method 
available where the sensitive plate is always to be used with 



Ue AIRPLANE PHOTOGRAPHY 

a filter. This is to incorporate a yellow dye in the gelatin 
of the plate itself. The dye must be one which has no direct 
chemical effect on the plate, but which acts simply as a 
coloring agent for the gelatin. " Self -screening " plates, as 
they are called, have been produced by the use of the dye 
called "filter yellow" and have found some use in ortho- 
chromatic photography. They effect a useful saving of light 
through the elimination of the reflection losses at the sur- 
faces of glass and gelatin filters. The filtering action of the 
dye in the plate is somewhat different from its ordinary 
one, since the deeper portions of the sensitive film are sub- 
ject to greater action than the surface, and this tends to 
diminish contrast. 



CHAPTER XX 
EXPOSURE OF AERIAL NEGATIVES 

The principal factors governing the length of exposure in 
the airplane camera have already been discussed under 
various headings. These are briefly, the nature of the aerial 
landscape, the practically attainable lens apertures, the 
form of the camera support, the speed of the plane, and the 
characteristics of plates, films and filters. It is convenient 
however, to re-assemble this information in one place, in 
such form as to apply to the practical problem of determin- 
ing the exposure to be given in any specific case. 

Limitations to Exposure. — In the ordinary photography 
of stationary objects, exposure is a variable entirely at the 
operator's command. Plates of any speed may be selected, 
so that attention may be focussed on latitude, color sensi- 
tiveness, and other tone rendering characteristics. The 
exposure may be made of a length sufficient to insure all the 
useful photographic action lying in the "correct exposure" 
portion of the sensitometric curve. The exposure ratio of 
any filter it is desired to use is a matter of indifference — ^its 
effect on color rendering need alone be considered. 

Airplane photography is sharply distinguished from 
ground "still" photography by its severe limitations as to 
the amount of ^the exposure. The actual duration is defi- 
nitely restricted by the high speed of the plane. In peace 
work this can be offset in part by using slower planes or by 
flying against the wind. The practical limitation to xJij- 
second, set by war-time requirements as to definition of fine 
detail, may be increased to ^ of a second, or even more, 
where mapping of grosser features is the object. A common, 

247 



248 AIRPLANE PHOTOGRAPHY 

but entirely avoidable limitation, is that due to vibration 
of the camera. By proper mounting this may be entirely 
overcome, leaving the ground speed of the plane the only 
source of exposure-limiting movement. The amount of light 
reaching the plate constitutes a primary factor in exposure, 
and this is a matter of lens aperture. Generally, lens aperture 
is smaller the larger the plate required to be covered, and the 
greater the focal length. Because of their larger aperture, 
the short-focus lenses which will be favored for peace-time 
large-area mapping will permit more and longer working 
days than have been the rule in long-focus war photography. 
The necessary use of j&lters, particularly at the high altitudes 
which would be chosen in mapping, in order to economize 
in the number of flights needed to cover a given area, intro- 
duces an inevitable decrease in the amount of light available 
at the plate, as compared with siu-face photography under 
the same illuminations. 

Broadly speaking, it may be said that all the demands 
made in reference to aerial photographic exposure work are to 
decrease the amount of light reaching the plate. Any surplus 
offered, as by the midsummer noon-day sun, must be immedi- 
ately snapped up, either by decreasing the exposure to get 
greater sharpness, or by introducing filters to get greater 
photographic contrast. The absolute exposure of the plate 
tends to be kept at the irreducible minimum. As already 
stated, it lies, with present photographic materials, on the 
"toe" of the "H & D" curve, just reaching up into the 
straight line portion. 

Estimation of Exposure. — ^According to the foregoing 
argument the problem of estimating an aerial exposure 
resolves itself largely into one of deciding how short this 
may be made. Or, if the light is strong, whether it is suffi- 
cient so that a filter may be introduced without demanding 



\ 



EXPOSURE OF NEGATIVES 249 

more than the xoo^ second or thereabouts which is dictated 
by the motion of the plane. 

Deciding upon exposures in the field has been largely 
a matter of experience and judgment. A majority of the 
cameras in use during the war were not furnished with shut- 
ters calibrated in definite speeds. Consequently, the sergeant 
upon whom the decision usually devolved became a store- 
house of knowledge as to the slit widths and tensions appro- 
priate to each individual camera. This knowledge had to 
be acquired from the results of actual photographic recon- 
naissances, or from special test fiights, both of them wasteful 
methods. But the chief objection to this state of affairs 
lies in the fact that the knowledge thus acquired is of no use 
to anyone else, nor is it applicable to other types of camera. 

The first essential to placing exposure estimation upon a 
sound basis is therefore an accurate knowledge of shutter 
performances. Either the shutter speeds should be placed 
upon the camera by the manufacturer and periodically 
checked, or a regular practice should be followed of calibrat- 
ing shutters, either at a base laboratory or even in the field. 

Assuming that the speeds of all shutters are accurately 
known, the process of estimating the requisite exposure 
becomes less a matter of mere guesswork and more nearly 
a matter of precision. For this purpose data on the variation 
of light intensity during the day and during the year (Figs. 
101 and 102) should be taken as a guide. These data refer 
of course to visual and not to photographic light, but since 
it is always necessary to use color filters, which make the 
active light of approximately visual quality, this is no valid 
objection. The effects of clouds and mist must of course be 
learned largely by experience, but with the above daylight 
data at hand, anyone in possession of definite information 
on the correct exposure with a given plate for a known day 



250 



AIRPLANE PHOTOGRAPHY 



and hour need not go far wrong in estimating exposures at 
any other time in definite fractions of a second. 

Exposure data charts. Fig. 109 shows a chart, pre- 






OaOBER 



SEPTBMBBi 



AU6(/Sr 



JULY 



Ji/A/£ 



MAY 



APfiiL 



MARCH 



FEBRUARf 



dAA/UAfiY 



DECEMBER K\\ 




W 18** 19 



Fig. 109. — Chart showing aerial exposures for all times of the day and year. Data on basis of 
F/5.6 lens, Jougla orthochromatic plate, and clear sunlight, no filter. Exposures to be doubled and 
tripled for overcast and cloudy weather. 

pared in the French service, indicating aerial exposures for 
all hours of the day throughout the year. These are for clear 
sunlight, for a lens of aperture F/5.6 and for "ortho" plates 



EXPOSURE OF NEGATIVES 251 

without a filter. They are based on what is probably an 
over-estimate of the actual speeds given by the French 
shutters. For "light" clouds the exposures are to be 
doubled, for "heavy" clouds quadrupled, and for forests 
and dark ground "lengthened." Charts of this form should 
be extremely useful, but they were actually not of great 
service because of the prevalent lack of knowledge of true 
shutter speeds. 

Exposure meters. Aerial photography offers an excellent 
opportunity for the use of exposure meters, particularly 
those of the type in which a sensitive surface is exposed to 
the light for a measured time sufficient to darken a pre- 
determined amount. The sensitive paper of the meter may 
either be exposed from the ground to the direct light of sun 
and sky, or from the plane to the light reflected from the 
ground. The first method will give figures subject to some 
correction for the character of the ground to be photo- 
graphed — whether fields, forests, or snow. The second 
method is to be preferred where the shutter speed can be 
adjusted in the air, according to the indications of the 
meter, or where the filter can be selected and put in place 
during flight. Trials with a commercial Wynne exposure 
meter, used in the latter manner, give as a working figure an 
exposure of .001 second for each 4 J^ seconds taken to darken 
the sensitometer strip to match the darker comparison 
patch. This relation applies to a lens of aperture F/4.5, on 
Cramer Commercial Isonon plates without filter. 



CHAPTER XXI 
PRINTING MEDIA 

Skilled photographers can examine a negative and can 
interpret its renderings with practically as much satisfaction 
as they get from a print, whereby a considerable amount of 
time can be saved in an emergency. The original glass 
negative should always be used when accurate measurements 
are to be made. These and a few other cases constitute the 
only use of a negative apart from its normal one, namely, 
for producing positive prints, usually in large numbers. The 
commonest form of print is on paper, although the most 
satisfactory print from the photographic standpoint is the 
transparency on glass or celluloid film. 

Transparencies. — Transparencies are made by the regu- 
lar photographic processes of exposure and development, on 
glass plates or films placed in contact with the negative, or 
in the appropriate position in an enlarging camera. The 
sensitometry and the terms used to describe the qualities 
of plate or film for this purpose are those already given in 
connection with the general discussion of plates and films. 
But the kind of emulsion to be selected is quite different 
from the aerial negative emulsion. There is here no practical 
limitation to the speed, contrast or latitude. Consequently, 
we can choose a positive emulsion on which the exposure 
through the aerial negative falls entirely on the straight line 
portion of the characteristic curve, thus reproducing all of 
its tones, and the contrast of the negative may be increased 
to any desired extent. The possibilities of positive emulsion 
are indeed rather greater than the usual aerial negative can 
utilize. A range of clearly graduated opacities of two or 

252 



PRINTING MEDIA 253 

three hundred to one is possible, so that not only can detail 
be well rendered in the high-lights, but also equally well in 
dark shadows where, indeed, an increase of illumination is 
necessary for it to be made easy to examine. This range is 
to be contrasted with the l-to-7 range in the aerial landscape, 
which may be doubled by a contrasty plate. In resolving 
power, the positive emulsion, which is slow, exceeds the 
negative emulsion. It easily bears examination through a 
magnifying glass, thus making any enlargement unnecessary 
in the printing process. 

Glass transparencies are of course impractical for general 
distribution, on account of their fragility. Heavy film trans- 
parencies are not open to this objection, and, especially in 
the form of stereos, constitute the most beautiful form of 
aerial photographic print. 

Paper Prints. — Prints on paper suffer by comparison 
with transparencies, in the range of tones which they exhibit. 
This lies between the white of the paper, which never has 
more than 80 per cent, reflecting power, and its darkest 
black, which differs with the kind of paper. In dull or mat 
papers the blacks will reflect as much as 5 per cent. ; in glossy 
papers, ordinarfly used for aerial negatives, the reflection 
from the black may be as low as one per cent., but in order 
to get the benefit of this the paper must be so held as not to 
reflect any bright object to the eyes. This deficiency in the 
range of paper gradations is not so serious with aerial nega- 
tives as with ordinary properly exposed negatives because 
of the small range of brightness in the aerial view. 

The sensitometry of papers is similartothatof plates, with 
the difference that reflecting powers take the place of trans- 
parency. As in the case of transparency emulsions there is 
in papers no dominating requirement for extreme speed, to 
which other characteristics must be subordinated. Yet 




^54 



AIRPLANE PHOTOGRAPHY 



speed is of sujQScent importance in handling large quantities 
of prints so that serial negative printing for military purposes 
has been done almost entirely on the rapid enlarging papers, 
rather than on the true contact printing papers, which 
are slower. 

The two principal types of rapid enlarging papers, the 




KUdto 

5*p«>ft t to S 


66« 


Expevart ta 


o.a.a. 


Tie* S«». 


Omma 


30 ••c. 
«fi • 

a • 


1.07 
1. 17 

I.2S 




L... 



FiQ. 110. — Characteristic curves of bromide paper. 



bromide and the "gas light," exhibit certain characteristic 
differences which are important to bear in mind in seeking 
to obtain any particular quality of print. Bromide papers, 
of which "Nikko" is a good example, show sensitometric 
curves rather like those of plates. That is, they increase in 
contrast with continued development. At the same time, 
as is shown in Fig. 110, they increase somewhat in speed 



PRINTING MEDIA 



^55 



with development; that is, under exposure can be compen- 
sated for to a small degree by protracted development. 
These characteristics of bromide paper can be utilized to 
secure prints of a quality quite different from that of the 
negative. Thus, if the negative has a long range of tones, a 



rnlarjl- 5 

Tl«a Her. 

30 aeo. 

45 " 
I «iB. 




3 (aosay oo 


:.-7 




" " ■ -^ 


68" 








cm.*. 700 








GemoA 








O.SO 
1. 02 
T.06 
I. SO 




// / y 


y/ I.» j 

/ 1 



Fia. 111. — Characteristic curves of gas-light paper. 

flat print can be secured by full exposure and short develop- 
ment. If, as is apt to be the case with aerial negatives, a 
print of greater contrast than the negative is desired, a short 
exposure with long development is called for. 

The sensitometric curves of a typical gas light paper 
"Contrast Enlarging Cyco," are shown in Fig. 111. Here 
the contrast is a fixed characteristic of the paper, and the 



^56 AIRPLANE PHOTOGRAPHY 

only effect of changing development is on the speed; that is, 
exposure and development are, within limits, interchangeable. 
Choosing a printing paper is a matter of deciding on the 
contrast required for the class of negative, and selecting a 
paper which will give this contrast with a good range of 
tones from a clear white to a deep black. The ideal paper 
would be one which was all straight line in the H & D plot. 
In such a paper there would occur no loss of contrast in the 
lighter tones when the high-lights were rendered by the clear 
white of the paper. Too great contrast with a short straight 
line portion, results in loss of detail at the ends of the scale. 
A negative possessing a very great range of tones cannot be 
correctly represented on one paper print — two printings 
are required, one for high-lights and one for shadows, but 
this difficulty is rarely to be faced in aerial views. The 
greatest demand for aerial printing papers has been for those 
of considerable contrast, because of the flat character of 
the negatives. 




CHAPTER XXII 
PHOTOGRAPHIC CHEMICALS 

General Considerations. — Developing, fixing and other 
chemicals for aerial work differ in no essential respect from 
those used in ordinary photography. Full discussions of 
these are to be found in numerous texts and articles. The 
aerial photographic problem is to select those most suited 
for the under-exposed flat negatives characteristic of photo- 
graphs from the air. At the same time selection from among 
the chemicals of appropriate quality must be governed by 
considerations of the conditions surrounding work in aerial 
photographic laboratories. These laboratories, especially 
in war-time, are apt to be most primitive in their facilities. 

Characteristics of Developers for Plates and Films. — 
From the standpoint of practicability, aerial negative devel- 
opers should have good keeping power, be slow to exhaust, 
and work well over a considerable range of temperatures. 
From the standpoint of the photographic quality desired in 
the negative, the developer should bring up the maximum 
amount of under-exposed detail. This means that it should 
impart the highest possible speed to the plate, with good 
contrast, and low fog or general reduction of unexposed 
silver bromide. 

There are many characteristics to study in a developer: 
its effect on inertia or speed, gamma infinity, fog, time of 
appearance, "Watkins factor," speed of development, tem- 
perature coefficient, dilution coefficient, keeping power, 
exhaustion, length of rinsing, stain, color coefficient and 
resolving power. These are defined and described as follows : 

Effect on inertia. The meaning of inertia has already 
17 257 



258 AIRPLANE PHOTOGRAPHY 

been given under the discussion of plate speed. While this 
is a constant, independent of time of development, for any 
one developer, it is altered appreciably bychange of the latter. 

Time-gamma relation. Contrast, symbolized by 7, has 
likewise been discussed under plate sensitometry. Viewed 
from the standpoint of the developer, the point of interest 
is the rate at which 7 varies with development, and the 
maximum contrast which can be reached or 7 infinity. 
Speed of development is commonly defined by the velocity 
constant, symbolized by /c, which is arrived at mathemati- 
cally from a consideration of the time of development to 
produce two different contrast values. High 7 infinity is 
desired for aerial negatives, and for rapid work k must 
also be high. 

Fog. The opacity due to chemical fog is to be kept at a 
minimum in aerial negatives, as it is chiefly prejudicial to 
under exposures. 

Time of appearance and Watlcins factor. The time of 
appearance, is measured in seconds. The Watkins factor is 
a practical measure of the speed of development, and is deter- 
mined by the ratio of the time of development required for 
a definite contrast, to the time of appearance. It is useful 
also as a guide to development time. 

Temperature coefficient. This is the factor by which the 
time of development at normal temperature (20 Cent.) must 
be increased or decreased in order to obtain the same quality 
negative, for a change of seven degrees either side of normal. 

Temperature limits are the temperatures between which 
development can be carried out with any degree of control 
or without serious damage to the negative. These factors 
are of great importance where climatic or seasonal changes 
have to be endured. 

Dilution coefficient. This is the factor by which the 



PHOTOGRAPHIC CHEMICALS 259 

development time is increased in order to maintain a given 
quality negative in different dilutions of the developer. It 
is useful in tank development. 

Keeping power. The keeping power of a developer, 
mixed ready for use, is determined by its ability to resist 
aerial oxidation. A developer of poor keeping power, which 
must be made up immediately before use, causes delay and 
waste of time whenever emergency work has to be done, 
whereas a developer of good keeping power may be left in 
its tank ready for instant use. 

Exhaustion of a developer is the rate at which it becomes 
useless for developing, due both to aerial oxidation and to the 
using up of its reducing power by the work done in develop- 
ing plates. It is conveniently measured by the area of plate 
surface developable before the solution must be renewed. 

Length of rinsing. The time required for rinsing between 
development and fixing bath plays a not unimportant part 
in total development time. Dichroic fog is caused with 
some developers if, due to insufficient rinsing, any of the 
caustic alkali is carried over to the fixing bath. Stains 
develop also if the fixing bath is old, or if light falls on the 
unfixed plate while any developer remains in the film. 

Color coefficient. The function of the sulphite, which 
forms a constituent of all developing solutions, is two-fold. 
It acts partly as a preservative, and partly to prevent the 
occurrence of a yellow color in the deposit. The yellow color, 
if present, increases the photographic contrast. This phe- 
nomenon has been purposely utilized, particularly in the 
British service, to give "stain" to negatives which otherwise 
would show insufficient printing density. The color index 
or coefficient of a negative (with a given printing medium) 
is the ratio of photographic to visual density. If we take a 
pyro developer containing five parts of pyro per thousand 



260 AIRPLANE PHOTOGRAPHY 

and ten parts of sodium carbonate, and then vary the amount 
of sulphite from none to fifty parts per thousand, the color 
index varies as follows: 

Sulphite 
Parts per Thousand Color Index 

50 , 1.16 

25 1.24 

15 1.30 

10 1.45 

5 1.80 

2.75 

The color index is somewhat different with various kinds 
of printing media. 

This staining effect is a variable one, depending upon 
length of development, dilution of the developer, length of 
rinsing, temperature, the fixing bath used (plain hypo being 
necessary for a maximum effect), the length of washing after 
fixation and the properties of the water used. Standardiza- 
tion of these conditions in the field is difficult; hence any 
developer which will give the same effective contrast without 
resorting to stain is to be preferred. 

Resolving "power. Some developing processes and condi- 
tions will introduce bad grain into the negative. Hence the 
resolving power which a developer brings up must be investi- 
gated among its other characteristics. 

Practical Developers for Aerial Negatives. — In the English 
service a pyro metol developer was generally used, producing 
stained negatives. The French, American and Italian prac- 
tice was to use metol-hydrochinon, vnthout staining. A 
special chlor-hydrochinon developer, worked out by the 
Eastman Research Laboratory for the United States Air 
Service, has probably the greatest merit of any yet tried. 
A comparison, given below, between it and a pyro metol 
formula used on a representative plate, illustrates the use 
of the various bases of study given above. 



PHOTOGRAPHIC CHEMICALS 261 

Solution A Ptro FoBMUIiA Solution B 

Pyro, 3.75 grams Sodium carbonate, 53 g 

Potassium metabisulphite, 3.75 g 
Metol, 3.05 g 
Potassium bromide, 1.5 g 
Water, 500 c.c. Water, 500 c.e. 

Use 1 part of A to 1 of B 
Chlorhydrochinon Formula 

Solution A Solution B 

Chlorhydrochinon, 25 g Sodium carbonate, 30 g 

Metol, 6 g Sodium hydrate, 10 g 

Sodium bisulphite, 2.5 g Potassium bromide, 3 g 
Sodium sulphite, 25 g 

Water to 670 c.c. Water to 330 c.c. 

Use 2 parts of A to 1 of B 





Pyro Chlorhydrochinon 


H & D speed 


150 180 


Gamma infinity 


1.45 2.12 


Fog (at maximum gamma) 


.32 .60 


Time of appearance 


5 seconds 5 seconds 


Watkins factor 


25 10 


Velocity factor "/f " 


.320 .400 


Temperature coefficient 


1.40 2.0 


Temperature limits 


4° to 32° C 4° to 32** C 


Keeping power 


45 minutes 8 days 


Exhaustion (100 c.c.) 


30 sq. in. 300 sq. inches 


Dilution coefficient 


2 2 


Color coefficient 


1.50 1.00 


Resolving power 


47 53 



Owing to the difficulty of securing pure chlorhydrochinon 
a metol hydrochinon of very similar properties has been 
worked out. Its composition is 



Metol 


16 grams 


Hydrochinon 


16 " 


Sodium sulphite 


60 " 


Sodium hydroxide 


10 " 


Potassium bromide 


10 " 


Water to 


1 litre 



To keep the ingredients in solution in cold weather, 50 
c.c. of alcohol should be included in every litre of solution. 
All things considered this is probably the most practical and 
satisfactory developer for aerial negatives. 



^62 AIRPLANE PHOTOGRAPHY 

Developers for Papers. — The following formula has been 
found very satisfactory for papers : 



Metol 


.9 gram 


Hydrochinon 


3.6 " 


Sodium carbonate 


20.0 " 


Sodium sulphite 


14.0 " 


Potassium bromide 


.5 to 1.0 " 


Water to 


1 litre 



Fixing Baths. — ^For plates the following fixing and hard- 
ening bath is recommended : 



Sodium thiosulphate (hypo) 


350 grams 


Potassium chrome alum 


6 " 


Sodium bisulphite 


10 " 


Water to 


1000 CO. 



During hot weather, the above quantities of chrome 
alum and bisulphite are doubled. 
For papers the following: 

Hypo, 35 per cent. 100 volumes 
Acid hardener 5 " 

The acid hardener is constituted as follows : 

Alum 50 grams 

Acid acetic 28° 400 c.c. 

Sodium sulphite 100 grams 
Water to 1 litre 

Intensification and Reduction. — These processes have 
been little employed in air work. Reduction is rarely neces- 
sary, for obvious reasons. Intensification would often be of 
value, but the common practice, which saves some time, is 
to use printing paper of strong contrast for those negatives 
which are deficient in density and contrast. When intensi- 
fication is desirable or permissible, either the ordinary mer- 
cury or uranium intensifier may be used. 

Water. — In the field it is found necessary in many cases 
to purify the water that is to be used in mixing up chemicals. 



PHOTOGRAPHIC CHEMICALS 263 



Water may contain suspended matter or dirt, dissolved 
salts, and slime. It is important to remove the suspended 
matter, as it may cause spots on the plates and papers, 
while any slime would coagulate, forming a sludge in the 
developer which would also tend to settle on the plates and 
cause marks during development. The dissolved salts may 
or may not cause trouble. Two methods of purification 
are possible: 

(a) Filter the water through a cloth into a barrel, add 
about one gram of alum for every four litres of water, and 
allow to settle over night. Draw off the clear liquid from a 
plug in the side as required. 

(b) Boil the water and allow it to cool over night. If the 
water contains dissolved lime, boiling will often cause this 
to come out of solution. 



V 

METHODS OF HANDLING PLATES, FILMS 

AND PAPERS 



CHAPTER XXIII 

THE DEVELOPING AND DRYING OF PLATES 

AND FILMS 

Field Requirements. — ^Developing, fixing, drying and 
printing in the field demand simple and convenient apparatus 
that may be carried about and installed with the least 
amount of labor. On top of these requirements military 
needs impose others that are more difficult. Speed is, on 
occasion, imperative. A print may be required within a 
few minutes after landing, and many thousands within a 
few hours. Quantity production must be achieved under 
the most primitive conditions. Nothing, in fact, shows the 
calibre of the photographic officer better than his choice of 
workplaces as the army moves forward. Ingenuity and 
practical judgment are at a premium. Cellars, stables, dog 
kennels, or huts hastily built from packing cases, must be 
equipped and in working order over night. All the facilities 
offered by a great city are urgently needed — water, electric 
light, power for driving fans — but must be dispensed with 
if the photographic section is to be convenient to the air- 
drome, whose portable hangars are most apt to be pitched 
in the open country. Water must be carried, electricity 
generated, and to the photographic problem is added the 
military one of concealment and protection. Dugouts and 
bomb proofs must be built for supplies, and "funk holes" 
for the men. Entire imderground emergency extensions 
have sometimes been built in stations occupied for extended 
periods, for airdromes are a favorite bombing target. 

For getting the exposed plates to the photo section, 
messengers, on motorcycles if possible, are employed. In 

267 



268 



AIRPLANE PHOTOGRAPHY 



some cases, where hangars and photographic hut are forced 
to be widely separated, recourse has been had to parachutes 
(Fig. 112), a device also employed to distribute prints to 
infantry during an advance. 

For warfare of movement, especially in sparsely settled 




Fig. 112. — Receiving pictures from plane by parachute. 



or devastated country, where cellars are unavailable, the 
dark room must be taken along. Motor trucks and trailers 
(Figs. 113, 114, 115), the former for hauling supplies and 
electric light generating plant, the latter fitted as a complete 
developing and printing laboratory, form the headquarters 
of each photographic section in the field. Usually altogether 
too small for the amount of work required, they were ex- 



PLATES AND FILMS 



269 



tended by tents and lean-to's, or ingeniously used as a 
nucleus for the organization of the favored stable or cellar. 
Methods of Plate Development. — ^Where speed is not 
required the simplest and commonest mode of developing 
plates is in the tray, one plate at a time. Common practice 
is to examine the plate at intervals during development, 
and discontinue the operation on the basis of its appearance. 




Fig. 113. — Mobile photographic laboratory. 



This is only possible if the plates used are insensitive to some 
light by which the eye can see. Deep red light is suitable 
for ordinary and most orthochromatic plates. A faint blue- 
green may be used with some panchromatic plates. The 
best practice, however, is to develop by time in total darkness, 
whereby all chance of dark room fog is avoided. Develop- 
ment time for plates of the average exposure of the one to 
be developed is either known from previous experience, or is 
found by trial on the first one. Development by time 



270 



AIRPLANE PHOTOGRAPHY 



results in negatives of densities varying with the exposures, 
but, as was brought out in the discussion of sensitometry, 
this difference can be compensated for by the choice of the 
paper used for printing, and by its treatment. 




Fig. 114. — Interior of photographic trailer, developing room. 

Where larger quantities of plates are to be handled tank 
development is adopted. In ordinary tank development the 
plates are placed in grooved tanks, into which is poured first 
the developer, next the rinsing water, and then the hypo. 
It has been customary in tank development as practiced for 
peace-time work to use dilute developer, requiring from ten 



PLATES AND FILMS 



271 



to thirty minutes, but speed requirements in war-time aerial 
photography dictate the use of full-strength quick-acting 
developer. An improvement on the simple grooved tank is 
provided by metal cages or racks, each holding a dozen or 




Fig. 115. — Interior of photographic trailer. Enlarging camera and printer. 

more plates, which may be introduced or removed from the 
tank as a unit (Fig. 116). 

The core rack system combines certain of the features of 
both tray and tank development. Each plate is inserted in 
a separate metal frame with projecting lugs to rest on the 



£72 



AIRPLANE PHOTOGRAPHY 



top of the tank and so suspend the plate in the solution. 
The process of development is the same as in the tank system, 
but any individual plate may be examined and removed. 

Film Developing and Fixing. — The problem of quicky 
handling roll film of large size is one upon whose solution 
depends in large degree the feasibility of film cameras for 






Fig. 116. — Tank and rack for tank development. 

aerial work. It presents many difficulties: a long film is 
unwieldy, is inherently subject to curling, and takes up much 
space if it is handled entire. For small scale operations roll 
film can be cut into short strips and developed either by 
drawing through a tray or, if cost of developer is no object, 
in a deep tank. In order to make the cutting apart of expos- 
ures easy in the dark, film cameras should make some form 
of punch mark in the film between the exposed parts, or the 
space between exposures should be uniform, so that a print 



PLATES AND FILMS 273 

trimmer set to a definite mark may be used. Racks for hold- 
ing two or three feet of film, folded back on itself and clasped 
by spring clothes-pins, are fairly practical. One object of 
the use of film, however, is to greatly increase the number of 
possible exposures; and where hundreds instead of dozens 
of exposures are to be developed, this method takes up 
entirely too much time. 

Following the practice in moving picture development, 
filTYi developing machines of various designs have been devised. 
Among these may be described the G. E. M. machine; the 
Ansco machine; the Eastman apron machine; the Brock 
frame and tank apparatus; the Eastman reel machine; and 
a modification of the latter by the United States Air Service. 

The G. E. M. film developing apparatus, similar in idea 
to the Eastman "apron " method of film developing, as exem- 
plified in the familiar amateur film developing machines, has 
the film wound in a spiral on a long linked metal frame or 
chain. After being wound it is placed in a tub of developer, 
from that to a tub of water, thence to a tub of hypo, and 
finally to a tub of water, where it is washed in several changes. 
The objections to the method are that it takes up much 
floor space for the various tubs, and that it requires such 
large quantities of solution. To develop a thirty -five foot 
length of 18 X 24 centimeter exposures requires approximately 
28 gallons of developer; for the rinsing, 28 gallons of water, 
and the same for hypo, and at least three times that for 
washing. In all 168 gallons of water must be brought to the 
developing hut or lorry. 

The Ansco machine makes use of an idea frequently 
applied in the moving picture industry. The film is carried 
spirally, upon two cross-arms which bisect each other at 
right angles, and which contain vertical pins around which 
the film is looped, beginning at the center and working out. 

18 



274 AIRPLANE PHOTOGRAPHY 

After it is wound it is placed in a tub of developer, as in the 
G. E. M. machine. It has an advantage over this apparatus 
in that the shape of the tubs or tanks is square instead of 
round. But it is equally extravagant of space and water. 

This same criticism may be made of the Eastman apron 
apparatus for film developing. This is similar to the G. E. M. 
machine, but differs from it in using a perforated celluloid 
apron to support the film during the various operations, 
instead of a metal chain. 

The Brock developing outfit consists of a rectangular 
wooden frame and a three-compartment tank. The frame, 
which is approximately 3 by 4 feet in size, is used as a support 
for the 4 inch wide film, which is wound spirally around it, 
between guiding pins. A special support is provided, on 
which the frame may be rotated as the film is fed off the 
camera spool. The frame, with the film on it, is lowered 
successively into the three narrow but deep compartments 
of the developing tank. The first compartment holds 
developer, the next water, the next hypo. The amount of 
developing solution required is rather large (96 gallons of 
water in all for a strip of 100 4X5 inch exposures), but 
because of the small surface exposed to the air, it keeps for 
a considerable period. The chief demand for floor space with 
this apparatus is for feeding the film on to the frame. 

In the Eastman twin reel machine the film is wound on a 
wooden drum or reel of large diameter, to form a helix. The 
drum is suspended so that the bottom edge touches the devel- 
oping solution, and, upon revolving the drum, every portion 
of the helix of film is brought into contact with the developer. 
By shaping the developing tank so that it closely conforms 
to the shape of the reels, a high economy in quantity of 
developing agent can be achieved. When developing action 
is finished, the developer is emptied out, rinse water put in; 



PLATES AND FILMS 275 

hypo follows, and then comes the final washing with water. 
With this apparatus the whole cycle is completed, for the 35 
feet length of film above considered, with sevengallonsof water. 

The Air Service apparatus differs from the above only 
in the drying method, which will be described below. 

Heavy cut film, such as is marketed under the name of 
Portrait Film, has not thus far been used in aerial work, 
except for printing transparencies. It is conceivable, however, 
that film in the cut form may be used in some future design 
of camera. This may be developed expeditiously in a tray, 
six or eight films being handled at a time, in a pile, pulling 
out the lower one frequently and placing it on top. The 
core rack system is also available for film in this form, special 
racks with clips to hold the film being necessary. 

Plate Drying. — The drying of negatives on glass is a 
comparatively simple matter, owing to the rigid nature of 
the emulsion support. A large number of plates may be 
placed in a compact mass in the ordinary plate racks of 
commerce with the wet sides accessible to a draft of air. 
Two dozen plates separated from each other by a quarter 
of an inch and left to dry spontaneously in a room of ordi- 
nary humidity and living temperature will dry in two hours 
and a half. If the surface be wiped with soft cheese-cloth 
or chamois, so as to absorb all the surface moisture before 
the plates are placed on the rack, this time may be appreci- 
ably reduced. By placing the plates in a forced draft of air, 
from an electric fan, this time may be reduced to an hour. 

Extra rapid drying of plates may be accomplished by 
placing them in a bath of alcohol before putting them in the 
racks. The alcohol displaces all the water in the film, and 
is itself very quickly dissipated into the atmosphere when the 
plate is taken from the tray. The plate must be left in the 
alcohol tray long enough for the substitution of the alcohol 



276 AIRPLANE PHOTOGRAPHY 

for the water in the film to take place. Five minutes is long 
enough. The alcohol before use must be as nearly free from 
water as possible. The best way to make sure of this is to 
place in the bottle of alcohol some lumps of calcium oxide, 
which will take up the water and form calcium hydroxide, 
which settles at the bottom of the bottle. 

Another method of quick plate drying takes advantage 
of the extraordinary greediness of potassium carbonate for 
water. The wet plates are placed in a saturated solution of 
potassium carbonate and left for a minute. If a plate be now 
taken from the solution and its surface wiped with a soft 
cloth, it will be found that the film has a greasy, slippery 
feeling, but that it contains no water and can be printed from 
at once. Plates so treated should be washed, however, at 
some time in the succeeding four months, or the traces of 
potassium carbonate left in the film cause deterioration. 

Film Drying. — Unlike the drying of plates, drying of film 
negatives is a very puzzling problem, and may be considered 
as the crux of the successful use of film in aerial cameras. 

Apron and similar machines have very poor drying effi- 
ciency if the film is left in place, for not only the film but the 
apron or chain must be freed of water. This may be has- 
tened, as in the G. E. M. machine, by blowing air through 
with fans, but even with their help drying a 35 foot film is a 
matter of two hours or more. Passing the film through 
wringers or a squeegee to remove excess water is a consider- 
able aid; the film may either be re-wound on a dry reel, to be 
put in a forced draft of air, or may be hung up in short lengths 
or festooned, either method taking up a great deal of space. 
.The use of alcohol is not advisable as it may abstract cam- 
phor from the celluloid and cause the film to become distorted. 

The Eastman twin reel machine had an upper reel joined 
to the lower or developing reel, with a chain and sprockets, 



PLATES AND FILMS 



m 



so that the upper reel revolved at the same time and rate of 
revolution as the lower, when the lower was being revolved 
at the gentle speed appropriate to the developing process. 
Fans blew a draft of air over the upper reel. This method 
necessitated over an hour for drying. 




Fig. 117, — U. S. Air Service film developing machine for film 24 centimeters wide. 

The Air Service model of film developing and drying 
machine (Fig. 117) introduces an essential modification in 
the drying scheme of the Eastman apparatus. The upper 
reel is quite independent of the lower reel and is revolved at 
a high rate of speed, so that a whirling action is introduced 
into the drying. Large rotating fans at the same time drive 



^IfS AIRPLANE PHOTOGRAPHY 

a considerable volume of air across the film surface, and the 
combination of the two agencies makes it possible to dry 
35 feet of 18X24 centimeter film in 20 to 30 minutes. This 
for large numbers of pictures makes the use of film even 
quicker than that of plates. The only practical drawback 
to the apparatus is its bulk, which calls for a separate room 
or trailer. This, however, seems to be inevitable in the use 
of large roll film. 

Cut film can be dried with speed only if placed in a draft 
of warm air. Drying boxes, with a chute or chimney and 
with fans to drive the air through from an alcohol stove, 
will dry several dozen films in an hour. The films must not 
be closer together than about one inch, which makes the 
drying boxes rather cumbersome. 

Marking Negatives. — ^After development and drying, 
and before filing or printing, each plate should be marked 
with data for purposes of future identification. This is most 
easily done with pen and ink on the film side (in reversed 
lettering) either along an edge in the unexposed portion 
covered by the sheath or in a corner, so as to lose as little 
of the photograph as possible. Just what data shall be in- 
scribed is dictated by the purpose for which the negative was 
made. The date, altitude, time of day, true north (from 
known permanent features or from shadow direction and 
time of day), number of the camera used, the focal length of 
the lens. Other records, such as the plane and squadron 
numbers, or even the pilot's and observer's initials, may be 
called for (Fig. 75). For mapping work the scale of each 
of a set of negatives, once found, may be marked, either in 
figures or by means of a line of length corresponding to a 
fixed distance on the ground. Rectifying data can similarly 
be inscribed, so that the negative can be printed in the en- 
larging and rectifying camera with the minimum of delay. 



CHAPTER XXIV 
PRINTING AND ENLARGING 

Contact Printing. — Single prints are made most simply 
in a printing frame held at a short distance from a light 
source. When any quantity must be made, as in turning 
out prints at high speed for distribution to an army before 
an attack, printing machines are employed. These consist 
essentially of a light box, a printing frame of plate glass, 
and a pressure pad. In the commercial models, such as the 
Crown and the Ansco, which are equipped with electric light, 
merely bringing the pressure pad down and clamping it 
automatically turns on the light, while release of pressure 
terminates the exposure. 

The question of regulating the distribution of light is of 
considerable importance with negatives taken by focal-plane 
shutters of non-uniform rate of travel. In the Mclntire 
printer (Fig. 119), the separate electric bulbs are on long 
necks in ball and socket joints, so that they can be brought 
individually closer to the printing surface or farther away 
from it, thus permitting a wide range of "dodging." This 
printer also has an automatic time control for the light, a 
valuable device where many prints from the same negative 
are desired. 

These machines are well suited for printing aerial nega- 
tives, either plate or cut film, if used where a source of elec- 
tric current is available. The chief defect, which may be 
caused by faulty construction, is imperfect contact between 
paper and negative, a cause of serious unsharpness on prints 
destined for minute study in interpretation. 

The printing of aerial negatives may be done either on 

279 



280 



AIRPLANE PHOTOGRAPHY 




Fig. H8. — Printing machine. 



roll or cut paper, and if films are used, a further alternative 
is offered of handling it either in the roll or in cut form. 
Where many prints are to be made from one negative roll 



P-RINTING AND ENLARGING 281 

paper has some advantages, particularly if a developing and 
drying machine is available. But for moderate numbers the 
advantage is small, since cut prints can be developed quite 




Fig, 119. — Interior of Mclntire printer, showing lamps adjustable in position for "dodging." 

conveniently in goodly numbers in the ordinary trays. But 
the advantages of keeping film in the roll form are very 
great, both in respect to storage and in respect to handling 
during printing, as the rollers provide the necessary tension 
and prevent the film "getting away." 



282 



AIRPLANE PHOTOGRAPHY 



For the American Air Service, cut paper has been used 
exclusively. For film printing, the Ansco machine has been 




Fig. 120. — Film printing machine. 



equipped with roll pivots to take film 24 centimeters wide 
which may be advanced in either direction by turning large 
milled heads (Fig. 120). If we put rollers on the two remain- 



PRINTING AND ENLARGING 283 



ing sides of the box to handle paper we transform the printer 
into the same form as a French machine, in which paper and 
film are moved at right angles to each other. A disadvantage 
of this modification, however, is the difiiculty of examining 
the negative to be printed. 

Stereo Printing. — To make separate prints from the two 
elements of a stereoscopic pair and mount them side by side 
after proper orientation is too slow a process if quantities 
of prints are needed. One method of multiple production 
is to make a master stereogram, and then produce photo- 
graphic copies of it, but there is inevitable loss of quality 
in this copying process. An intermediate method is to print 
from both negatives on the same sheet of paper. In order 
to do this the negatives must be placed in rather large frames, 
with mats properly located to guide the placing of the paper. 
The Richard double printing frame is a practical device 
which simplifies the necessary manipulations. It consists 
essentially of a platform pierced with three illuminated 
openings. The two negatives are compared, superposed, 
and orientated over the central opening and then shifted 
laterally, one to each of the two side openings, which serve 
both as printing frames and masks. The printing back 
slides on a rod, permitting the paper to be lifted up and 
moved between exposures. Once the negatives are properly 
placed, stereo prints can be turned out quickly and easily. 

Enlarging. — In the French service contact printing was 
the rule during the war. The English practice, on the other 
hand, was to take small negatives — 4X5 inches, with 8 to 
12 inch lenses — and enlarge them, usually to 0'}/^XS}/2 inches. 
For this purpose a regular part of the English photo section 
equipment was the enlarging camera (Fig. 115). This may 
be briefly described as a short focus camera in which the 
subject to be photographed is a negative, illuminated by 



284 AIRPLANE PHOTOGRAPHY 

transmitted light, whose image is thrown by the camera 
lens on the paper or other sensitive surface. By making 
the distance between negative and lens less than that 
between lens and paper, the resulting print is an enlarge- 
ment, and vice versa. The scale of enlargement or of reduc- 
tion is varied over limits set only by the length of the camera 
and the amount of light available. 

The lens employed must of course possess sufficiently 
high quality to preserve all the sharpness of the negative, 
and focussing must be done with accuracy. Next to the lens 
the most important element is the light source. This may 
be of the point form, such as a concentrated filament electric 
lamp, an oxy-acetylene lime light, or an acetylene flame. 
The latter was extensively used in the English service, while 
acetylene generators for emergency purposes formed part of 
each American photo truck equipment. With point light 
sources we must use condensers to focus the light into the 
projecting lens. Much less efficient, but the only recourse 
where large condensers are not available, is a diffusing glass 
behind the negative, illuminated either by a bank of electric 
lamps with mirrors or by a U tube mercury vapor lamp, 
where proper current can be got. 

The device for holding the printing paper must permit 
quick changing, but insure good contact. We may use 
either a spring plate to hold the paper against plate glass 
from behind, or else a weight acting on a lever arm of 
sufficient length. 

The need for some automatic means of focussing an 
enlarging camera has been very generally felt. An illustra- 
tion of such an enlarging camera is that put out by Williams, 
Brown & Earle, of Philadelphia, known as the "Semper- 
focal" (Fig. 121). In this camera the movements of the lens, 
paper easel and negative are so inter-related and actuated 



PRINTING AND ENLARGING 285 

with respect to each other that the correct focus of the 
instrument is maintained for any degree of enlargement 
or reduction. This feature is a great help in making up 
mosaic maps, where prints of continuously varying scale 
ordinarily occasion serious delay for individual focussing. 

Determining the correct enlargement for each negative 
of a mosaic is perhaps the most important problem in the 
use of the enlarging camera for aerial work. The correct 




Fig. 121. — "Semperfocal" enlarging camera, with mechanism for holding image in focus at 

any enlargement. 



setting of the camera may be found by either of two methods : 
the negative may be previously scaled and marked with a 
line on its edge, which must be projected to a definite size; 
or the true location of several points in the picture as 
obtained from an accurate map may be marked on the 
enlarging camera easel according to the desired scale, and 
the negative image projected to coincide with these. In 
either case, if an exact scale is desired, allowance must be 
made for paper shrinkage, a matter which must be deter- 
mined by previous experiment. 



286 AIRPLANE PHOTOGRAPHY 

Rectifying. — Negatives taken when the plane is not 
flying level will be distorted (Figs. 134 and 135). Contact 
prints from these will not fit into a mosaic, and no mere 
enlargement or reduction will make them available. It is 
necessary with these negatives to resort to a rectifying 
camera. This is an enlarging camera built so that the nega- 
tive and print easel may be inclined about vertical and 
horizontal axes, thereby purposely introducing a distortion 
sufficient to offset the distortion of the negative. Thus, if 
the bottom of the printing surface is moved away from the 
lens, that part of the picture will be enlarged; if moved 
toward the lens, reduced. 

For small rectifications the common practice is to tilt the 
printing surface alone, a method that is practical as long as 
this tilting does not affect the focus so much as to require 
prohibitive stopping down of the lens. For great distortions, 
such as that inherent in the principle of the Bagley camera, 
it is necessary to tilt both negative and print in order to 
preserve an approximate focus, a given portion of the nega- 
tive moving toward the lens as the corresponding portion 
of the print is moved away. Both schemes for rectification 
are shown diagrammatically in Fig. 122. 

Developing and Drying Prints. — The developing of prints 
follows closely that of cut or roll film, and so need not be 
treated separately. 

The drying of emulsions on paper is more easily accom- 
plished than the drying of emulsions on glass, for two 
reasons: the emulsions on paper are much more thinly 
coated, and there is diffusion of moisture into the atmosphere 
from front and back of the printing medium. In the field 
a common method has been to soak the prints in water-free 
alcohol and then burn off the alcohol, thus securing a dry 
print within two or three minutes after the conclusion of 



PRINTING AND ENLARGING 287 

washing. A later method very generally employed is to 
cover wooden frames three or four feet above the ground 
with chicken wire or muslin, and on these lay the prints after 
soaking them in alcohol. Below the frames currents of warm 



B --- 




Fig. 122. — Diagram showing enlarging with and without distortion: A, enlarging without dis- 
. tortion; B, distortion for rectification of print, by inclining printing surface; C, distortion, for recti- 
fication of print, by inclining both negative and printing surface. 



air rise from pans of burning alcohol, previously used to 
soak the prints and now useless as alcohol because of their 
high water content. 

Before putting them in alcohol it is advisable to squeegee 
all the surface water from the prints. This may be expedi- 



S88 AIRPLANE PHOTOGRAPHY 

tiously done by removing them in mass from the final wash 
water upon a large ferrotype plate, and either running the 
plate and prints together through a wash wringer with light 
pressure, or covering the whole with a sheet of blotting paper 
and pressing out the water underneath by means of a rubber 
squeegee vigorously applied. 

For base work one of the modern automatic print-drying 
machines used in commercial photography would be desir- 
able. Glossy surfaces are given prints by the usual ferrotype 
plate method. But this is too time-consuming for war 
practice, and besides has but doubtful advantage where 
papers of the glossy type are chosen. 



VI 

PRACTICAL PROBLEMS AND DATA 



CHAPTER XXV 
SPOTTING 

" Spotting, ' ' as distinct from mapping or from the photog- 
raphy of continuous strips, is the photography of a definite 
individual objective. In mihtary work spotting or "pin 
pointing" includes the photography of particular trenches 
or pivotal points in a trench system before an attack (Fig. 
123), of roads or bridges along which an advance must pass 
(Fig. 124), of batteries or big guns which are the subject of 
artillery fire (Fig. 125), both before and after their bombard- 
ment (Fig. 126), of gun puffs or exploding bombs (Fig. 131). 

The technique of spotting consists largely in getting prop- 
erly over the target and then securing the exposure at just 
the right moment. This is chiefly a question of proper 
piloting; but the aid which can be offered to the pilot by 
camera auxiliaries designed particuarly for spotting needs 
is very large. 

Discussion of the task of the pilot who must steer a 
photographic plane accurately over a previously selected 
point of interest cannot be undertaken without raising the 
question of who should take the picture, pilot or observer .f^ 
In the English service the most general practice was for the 
pilot to be charged with the responsibility both of covering 
the objective and of exposing. If a propeller drive was used 
on the camera, this left to the observer only the task of chang- 
ing magazines. If the camera was hand operated the plates 
were changed either by the observer, or else, as was fre- 
quently the case, distance operating devices were attached, 
so that the pilot even then did everything except change the 
magazines, and the observer was kept free to watch the sky 

291 



292 AIRPLANE PHOTOGRAPHY 




"S 



b. 



SPOTTING 



29S 




294 



AIRPLANE PHOTOGRAPHY 




a 



to 
to 
IS 

to 

a 

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o 

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SPOTTING 



295 



for enemy aircraft. A very desirable adjunct to the camera 
when plates are shifted automatically or by the observer is 
a distance indicator, to show the pilot when the shutter is 
set. Electrical indicators for this purpose have been devised. 
If the camera is completely hand operated, as were most 
of those in the French and German services, there is little 




FiQ. 126. — Example of spotting. Battery before and after bombardment. 

choice but for the observer to perform the entire operation. 
The exposing operation could have been delegated to the 
pilot, but such was not the custom with the French or with 
the American squadrons using French apparatus. In this 
method of operation the observer depends on the pilot to 
get the plane over the target, while the pilot depends on the 
observer to get the picture when the target is covered. 
Ample opportunity is thus offered for misunderstanding 



296 



AIR PLAN EPHOTOGRAPHY 



and disagreement. This can be avoided only by excel- 
lent sights properly aligned, for both pilot and observer, 
and by some means of communication between the two 
men concerned. 

The simplest means of communication is of course direct 
conversation. But this is only possible in those planes, such 




Fig. 127. — Photograph, made with long focus lens to determine the results of aerial bombing. The 
"Tirpitz" battery of long range naval guns directed on Dunkirk. 

as the DH 9, in which pilot's and observer's cockpits are 
immediately together, so that, by shouting, any desired 
information can be conveyed with fair ease. When the dis- 
tance is increased to four or five feet, as in the DH 4, the 
loudest shouts are totally lost in the roar of the engine and 
the blast of the wind. Speaking tubes and telephones are 
now fairly good, but are none too comfortable or convenient 



SPOTTING 



297 



to have strapped on one's head and face. A primitive 
device used to some extent in the war was merely a pair of 
reins attached to the pilot's arms, by which he could be 












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Pig. 128. — Diagram showing relationship between focal length and area covered by plate. 

directed which way to steer. There is much to be said for a 
simple semaphore system, where an indicator in the observer's 
cockpit actuates a similar dial in front of the pilot, indicating 
"right" or "left," "picture obtamed," "try again," etc. If 
the observer has a sight by which he can see far enough 



298 



AIRPLANE PHOTOGRAPHY 



ahead to correct the pilot's error of pointing, the need for an 
accurate sight for the pilot is diminished. 

In considering the question of sights, attention may 
again be called to the poor "visibility" from the pilot's seat 
in the present prevailing type of two-seater tractor plane. 



OVERLAP PHOTOGRAPHS 

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Fig. 129. — Diagram giving data on area covered at various altitudes by representative lens- 
Blind directly in front, beneath, and to either side (Figs. 7, 
8 and 9), it is no unusual thing for a pilot to entirely miss an 
objective, such as a railway line, which he can only estimate 
to be beneath him by judging its distance from those objects 
to either side which he can actually see. The English prac- 
tice of leaving a clear space of six inches to a foot between the 
fuselage and the beginning of the wing fabric, allows the 
pilot to look down over the side, a decided advantage. But 



SPOTTING 



299 



for photographic purposes nothing can compare with a good 
negative lens carrying fore and aft lines or wires, so that the 
pilot can see his object ve in ample time to head directly for 
it. The lens should either be large enough so that its rear 



\,PloceI\L ogainstfU^htand 
read the Area couered against 
Arrow A, 



ZJ%ceAnvaA oqainstrfieArea 

found from mof) ploth'ng and read 
rhe fiejght against f^cal Length. 




Zy.FlacePL^, 
against Heig fit and note 



4^. Place Ft. 

ggoinst Height and note 



^gure ggoins f Arrow CFhce halt figure againstArrow C.Tlace half the 



thegrounds/ieed ggoinst disfigure 
and the fime interval for overlaps 
is seen in outerdiat oft/tosite ArrowB 



ground ^eedogoinstthis figure and die 

time interi/af for i^ertico/ stereos is 

seen in outer diat opposite Arrvu/JD 



Pig. ISO. — Burchall Slide Hule, for calculating intervals between exposures, and for other aerial 

photographic data. 

edge gives the view directly downward, or supplemented by 
an additional lens pointing directly down, so that the cover- 
ing of the target is assured. To locate such a lens in the front 
cockpit, free of all controls, is a very hard task; even so its 
view is likely to be badly interrupted by the landing gear. 
Nevertheless, so important is it, both in photography and in 



300 



AIRPLANE PHOTOGRAPHY 




Fig. 181. — ^Aerial bombardment of Trieste. Note falling bombs in center of picture; and exploding 

anti-aircraft shells over the water. 

Italian official photograph. 



SPOTTING 



301 



bombing, to have a sight by which the plane can be accu- 
rately directed that designers of planes should recognize this 
need and make every effort to provide a suitable location. 

Sights for the observer have been discussed already. Here 
again the negative lens is to be preferred, but while the pilot's 
lens needs only directing lines in the axis of the plane (unless 




Fig. 132. — Example of spotting requiring exposure at exact instant. Explosion following burst of 

bomb in ammunition dump. 

British official photograph. 

he takes the picture), the observer's lens needs both an accu- 
rate center mark and an additional upper or lower sighting 
point. Accurate alignment of these marks with the camera 
axis must be arranged for in precise spotting. 

Accurate spotting work requiring the delineation of fine 
detail calls for cameras of considerable focal length. The 
camera of longest focal length used in the war was the French 
120 centimeter (Fig. 41). This was employed with great 
success in such work as regulating the fire of heavy railway 



302 



AIRPLANE PHOTOGRAPHY 



guns brought into range only at night, to fire a few shots at 
chosen angles. Photographs taken the next day would then 
show the exact spot where each shell fell, and the damage it 
did, to serve as a guide for the next night's operations (Fig. 
127). The field of these cameras is quite small — 8 to 12 
degrees — and so not only must sighting be exact but the 




Fig. 133.— The same subject a few minutes later. Height of smoke shown by shadow. 

British official photograph. 

area covered on the ground must be accurately known. This 
is to be calculated from the altitude, focal length, and plate 
size, by the relation — 

distance on ground _ altitude 
plate length focal length 

Data derived from such calculations may be incorporated 

in tables, or graphically in diagrams such as Figs. 128 and 129. 

These calculations and others required in mapping and 

stereo-work are simply and quickly made by slide-rule 



SPOTTING 303 

devices. One of these, the Burchell Photographic Slide Rule, 
developed in the English service, is shown in Fig. 130. This 
consists of two dials, the center one of which is mounted — 
usually by a pin pushed into a cork behind — so as to turn 
freely, to permit its being set for altitude, focal length, ground 
speed, plate size, etc., whereupon the area covered, or the 
appropriate interval between exposures may be read off. 

Cameras for spotting work should be capable of exposure 
at the exact moment desired. For if the camera is ever to 
catch the gun as it discharges, the bomb as it falls (Fig. 131), 
or the shell as it explodes (Fig. 132), the photograph must be 
taken within the instant. Automatic cameras, exposing at 
regular intervals, while adequate for mapping, are not fitted 
for many kinds of spotting. 



CHAPTER XXVI 
MAP MAKING 

Technique of Negative Making. — Stated in its simplest 
terms, the whole problem of making a photographic map 
from the air consists in taking a large number of slightly 
overlapping negatives, all from the same altitude, with the 
plane flying uniformly level. When trimmed and mounted 
in juxtaposition, or pasted together so as to overlap in their 
common portions, the prints from these negatives constitute 
a complete pictorial map. There is thus furnished by a few 
hours' labor topographic information which would be the 
work of months to obtain by other means. 

The making of map photographs involves all the special 
technique of spotting, with much in addition. The pilot's 
task is not merely to go over one object; he must navigate a 
narrow path, at a constant altitude, on an even keel. If he 
is to make not merely a ribbon, but a map of considerable 
width, he must take successive trips parallel to the first, 
each displaced just far enough from the previous course to 
insure that no portion is missed — a difficult task indeed. 

It is the observer's duty to so time the intervals between 
exposures that they overlap enough, but not so much as to 
be wasteful of plates or film. He must also change maga- 
zines or films so quickly as to miss no territory, or if some be 
missed, his is the task of directing the pilot back to the point 
of the last exposure, where they begin a new series. 

Level flying is entirely a pilot's problem. Its importance 

will be realized when we consider the accompanying diagrams 

(Figs. 134 and 135), where the effect on the resultant picture 

is shown of climbing, gliding, or banking to either side 

304 



MAP MAKING 



305 



Prints from negatives distorted in this way neither will be 
true representations of the territory photographed, nor will 
they match when juxtaposed. In fact, they can be utilized 
only if special rectifying apparatus is available for printing. 



/^ffCH/rfE ^Ly/ryc^ lei/el 



MfJCH/riE B/7/y/c//vd 




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Fig. 134. — Diagram showing effect of banking on aerial photograph. 

Flying at a constant altitude is similarly necessary if the 
prints are to be utilized without enlargement or reduction 
in order to make them fit. 

Assuming a skilled pilot who will do his part, the next 
step is to calculate the exposure intervals in order to insure 
an adequate overlap. If a negative lens is installed which 

20 



806 



AIRPLANE PHOTOGRAPHY 



has been marked with a rectangle the size of the camera 
field, the simplest method is to estimate the proper instant 
for exposure by watching the progress of objects across the 
lens face. This of course requires constant attention, and 



CLfM3frr<i 



D///rf<^ 




^//r-/'a//rr /^/s^ez> 



^//r-^o/^r Af/a^s^o 



Fig. 135. — Diagram showing effect of climbing and diving on aerial photograph. 



it is easier to do this only occasionally, in order to determine 
the ground speed in terms of camera fields traversed per 
minute. Thereafter exposures are to be made by time, as 
determined by a watch or clock. Any desired degree of 
overlap can be chosen, and either estimated, or more or less 



MAP MAKING 307 

accurately fixed by lines marked on the negative lens at a 
shorter distance apart than the edges of the field. The 
most usual overlap is 20 per cent., except for stereos, which 
call for 50 to 75 per cent. 

In the absence of a negative lens or some other sight to 
show the whole camera field, it is necessary to resort to cal- 
culation from the speed and altitude of the plane, the focus 
of the lens and the dimensions of the plate. If A is the alti- 
tude, a the focal length of the lens, cZ the diameter of the 
plate in the direction of travel (usually the short length is 
chosen for economy of flights to cover a given width),/ the 
fractional part by which one negative is desired to overlap 
the next, and V the ground speed of the plane, then we have, 
by simple proportion, that the interval between exposures, 

t, must be — 

.AdO-f) 
^ aV 

If A = 2000 meters, d = lS centimeters, /= 3^, a = 50 centi- 
meters, and V = WO kilometers per hour, this relation gives — 

^ 2000 X .18 X .8 X 3600 ,^ ^ . 

^ = Xx-mooo = ^^-^ ^"^^^^^ 

The principle of overlapping map exposures is shown in the 
accompanying diagram (Fig. 129), together with data cal- 
culated as above for a 4X5 inch plate. 

It is particularly to be noted that it is the ground speed 
of the plane that is used. This may be calculated by know- 
ing the air speed and the wind velocity and direction. Fig. 
136 shows the method of doing this graphically. First an 
arrow is drawn representing the direction it is desired to fly. 
Next a second arrow is drawn of length to represent the wind 
velocity. This must be inclined toward the first arrow in 
the direction of the wind, and its head is to touch the head 
of the first arrow. Then with the farther end of this second 



308 AIRPLANE PHOTOGRAPHY 

arrow as a center, describe a circle of such a length as to 
represent the air speed of the plane, in the same units as the 
wind velocity. Connect the point where this circle cuts the 
arrow of flight direction to the center of the circle by a 
straight line. This line constitutes the air speed arrow, 
giving the direction it is necessary to fly, at the given air 
speed, to make the course desired. The length of the flight 
direction arrow between its head and its point of intersection 
with the air speed arrow gives the ground speed. 

When the wind is ahead or astern this calculation reduces 
to the simple subtraction or addition of the wind velocity 




Vire-cTion vf/ fliqht- , 

I^X- Ground speed >| 



Fig. 136. — Diagram showing method of calculating ground speed from air speed and wind velocity. 

to the air speed of the plane. Whenever possible, mapping 
should be done up and down the wind (Fig. 137). If the 
plane is "crabbing," the above calculations for overlap are 
only valid if the camera can be turned normal to the direc- 
tion of travel over the ground. If the camera cannot be so 
turned the corners of the successive pictures overlap instead 
of their sides, with quite unsatisfactory results (Fig. 138). 

Calculation of the distance apart of the parallel flights 
necessary to make a map of any width is done by the use of 
a formula similar to the longitudinal overlap formula above, 
distance figuring instead of time. Using the same symbols, 
and denoting the distance by Z), we have — 

D = MHO 



MAP MAKING 



309 




Fig. 137. — Overlaps made when flying with or against the wind. 




Fig. 138. — Unsatisfactory overlaps made when plane is "crabbing." 



310 AIRPLANE PHOTOGRAPHY 

With the same figures as before, but substituting 24 centi- 
meters for the plate dimension, this relation gives — 

^ 2000 X .24 X .8 ^^„ ^ 
D = — • — = 768 meters 

It is of course largely a pilot's problem to steer the plane 
over parallel courses at a given distance apart, although the 
observer, noting conspicuous objects through a properly 
marked negative lens, may direct the pilot by any of the 
means of communication already mentioned. 

An alternative method of securing parallel strips, which 
is to be highly recommended where enough photographically 
equipped airplanes are available, is for several planes to fly 
side by side, maintaining their proper separation (Fig. 139). 

Cameras and Auxiliaries for Map Making. — Mapping can 
be done quite satisfactorily by hand operated or semi- 
automatic cameras, provided the observer has not too many 
other duties. On the other hand, the operation of exposing 
at more or less definite intervals of time, irrespective of the 
object immediately presented to the camera, is a largely 
mechanical one. It naturally suggests the employment of an 
automatic mechanism, whose speed of operation only is it 
necessary to watch. 

If a non-automatic camera is used the timing of exposures 
may be done by watching a negative lens, as described above, 
or by reference to a clock, assuming that the ground speed is 
known through calculation. A very practical advance over 
the ordinary use of a clock is to attach a stop-watch to the 
shutter release, so that it is turned back to zero and re-started 
at each exposure (Fig. 70). In passing, it may be noted that 
if the stop-watch hand makes an electric contact which throws 
the shutter release, then the device constitutes an attachment 
for turning any semi-automatic camera into an automatic. 



MAP MAKING 



311 



The most suitable cameras for mapping are unquestion- 
ably those of the entirely automatic type. The use of such 
cameras always demands a knowledge of the ground speed. 




Fig. 139. — Planes starting out to make a map by flying in parallel. 

This demand has led to many suggestions for ground speed 
indicators. The common idea of these is to provide a moving 
part on the plane — either a disc of large diameter, or a chain, 
or a revolving screw — whose speed may be varied until any 
point upon it appears to keep in coincidence with a point on 



312 AIRPLANE PHOTOGRAPHY 

the moving landscape below. The ground speed is then to 
be read off a properly calibrated dial. Or, as a further step, 
the frequency of the exposures may be directly controlled 
by the ground speed indicator mechanism. The entire 
control of the camera would then consist merely in occasional 
adjustment of the ground speed indicator. 

While entirely possible in theory, these devices are not 
easy to work with in practice, because the plane is always 
subject to some pitching and rolling, which make it difficult 
to hold any object constantly on the moving point. This is 
especially true at high altitudes, where the apparent motion 
of the earth is quite slow compared to the swervings of the 
plane. This objection is in part removed if the ground speed 
indicator is carried by a gyro stabilizer. 

Ordinary mapping does not demand such exquisite 
rendering of detail as does trench mapping. Nor is it neces- 
sary to fly in peace-time at such high altitudes as in war. 
In consequence, mapping cameras are preferably of the short 
focus, wide angle type, say, 25 centimeter focus for an 18 X24 
centimeter plate. Film is to be preferred over plates because 
of the greater number of exposures it is possible to make on a 
flight. The shutter of the mapping camera must be ex- 
tremely uniform in its rate of travel so that the elements of 
the map may match in tone (Fig. 140). A mount which 
permits the camera to be turned normal to the direction of 
flight, such as the British turret mount (Fig. 87), is particu- 
larly desirable if flying across the wind is necessary, as will 
often be the case in mapping strips between towns or between 
flying fields. Devices to indicate compass direction and 
altitude are called for in new and poorly mapped territory, 
and may be expected to receive intensive study in the future. 
The question of their utility is, however, bound up with the 
whole question of the sphere of aerial photographic mapping. 



MAP MAKING 



313 




314 AIRPLANE PHOTOGRAPHY 

Up to the present this has been almost entirely a matter of 
filling in details on maps obtained by the regular surveying 
methods, or of making pictorial maps for aviators. To what 
extent primary mapping can be done by the airplane is yet 
to be determined. , 

At this point mention must be made of special cameras 
for securing extremely wide angle views, thereby minimizing 
the number of flights. The Bagley camera, devised by Major 
Bagley of the U. S. Engineers, is an example. It has three 
lenses, a middle one pointing directly downward, and one to 
either side at an angle of 35 degrees. The pictures obtained 
with the side cameras are of course greatly distorted, and 
must be rectified in a special rectifying camera. The result- 
ant definition is not good, but as the maps are made on a 
much smaller scale than the original pictures, this is not a 
serious objection. It is a matter for the future to decide 
whether the additional labor on the ground necessary for 
the rectifying process is to be more expensive than the extra 
flights which must be made with the ordinary types of cam- 
eras covering a smaller angle. 

Printing and Mounting Mosaics. — ^With an ordinary set 
of overlapping negatives the first step toward producing a 
map is to scale the negatives. For this purpose one should 
be selected which by comparison with a map shows no dis- 
tortion, and which is on the desired scale, or is known to 
have been made at the average altitude of flight. A sketch 
map of the territory should then be drawn, on this scale, 
based on available maps. This sketch is preferably made 
on a large ground glass illuminated from behind (Fig. 141). 
On this all the negatives should be laid, and their proper 
relative positions sought. When this is done it is evident at 
once whether all the territory has been covered, and whether 
there are any superfluous negatives. Each negative should 



MAP MAKING 



S15 



then be examined as to its scale and distortion. If it can be 
made to fit the scale by simple enlargement or reduction, a 
line can be drawn on one edge of a length indicating its 
scale. This line will later be used as a guide in the enlarging 
camera. If the picture is badly distorted it must either be 
replaced by another negative, or if rectifying apparatus 
is available, it must be set aside for the making of a 
rectified print. 



1 


mamammmmmmm^ 


Q 


1 




3 




■J 



Fig. 141. — Scaling negatives for mosaic map-making. 

The next step is to make prints from the negatives, which 
may be done either by contact, or, necessarily if differences 
of scale must be compensated, in the enlarging camera. If 
prints to an exact scale are required the shrinkage of the 
paper must be determined and allowed for. The prints must 
all show the same tone, and must be uniform from edge to 
edge. If the focal-plane shutter is not uniform in its travel, 
as is frequently the case, this means that the print must be 
"dodged," or exposed more at one edge than the other, by 
locally shielding the plate and paper during exposure. A 



S16 



AIRPLANE PHOTOGRAPHY 



case of the step-like effect caused by uneven shutter action 
is shown in Fig. 140. The effect due to uneven shutter action 
is of course absent with a between-the-lens shutter, which 
constitutes a strong argument in favor of that type for use 
in mapping cameras. 

When the prints are made they must be mounted together 
on a large card or cloth background. For a very small 




Fig. 142. — Arranging prints for a mosaic map. 

mosaic they may be juxtaposed by simple examination, 
matching corresponding details in successive prints. For a 
mosaic of any size an accurate outline map must be drawn 
on the surface to which the prints are to be attached. The 
prints are then laid out on this outline, moved to their 
correct positions, and held down by pins (Fig. 142). When 
they are all arranged the final mounting may be begun. 
The excess paper, beyond what is necessary for safe overlaps, 



MAP MAKING 



317 



may be trimmed off, exercising judgment as to which print 
of each adjacent pair is of the better quality, and utilizing 
it for the top one at the overlapping junction. If one print 
shows serious distortion it may be placed under its fellows 
on all four edges, thus minimizing its weight. The edges 
are best made irregular by tearing. Straight edges are apt 
to force themselves on one's attention in the final mosaic 
and give an erroneous impression of the existence of straight 
roads or other features. Both forms of edging are shown in 
Figs. 124 and 143. 

An alternative method of securing the final print mosaic, 
where film negatives are used, is to trim successive film 
negatives so that the trimmed sections will exactly juxtapose, 
instead of overlap. The sections are then mounted, by 
stickers at their edges, on a large sheet of glass, and printed 
together. Captured German prints show that this was the 
method commonly used with the German film camera (Fig. 62) . 

It will be noted that the procedure which has been 
described and illustrated by Figs. 142 and 143 assumes the 
previous existence of a map accurately placing at least the 
chief features of the country covered. This draws attention 
at once to the limitations and true sphere of aerial photo- 
graphic mapping at the present time. With the cameras 
thus far it is not possible, nor is it attempted, to do primary 
mapping of unknown regions. Distortions due to lens, 
shutter, film warping and paper shrinkage considerably 
exceed the figures permitted in precision mapping. From 
the standpoint of geodetic accuracy the cumulative errors 
of deviations in direction, altitude and level, peculiar to 
flying, would soon become prohibitive. 

The great field for aerial photographic mapping in the 
near future lies in filling in detail on maps heretofore com- 
pleted as to general outlines, or, as in the war, on maps far 



818 



AIRPLANE PHOTOGRAPHY 




MAP MAKING 



319 



out of date. The war-time procedure in country largely 
unknown, such as Mesopotamia, was probably closely that 
which will be necessary in peace. Conspicuous points in the 
landscape were first triangulated from friendly territory, and 
from these the outline map was drawn, whose details were 
to be supplied by aerial photographs. Much of the "map- 
ping " of cross country aerial routes so far done is frankly of 
a pictorial nature, showing conspicuous landmarks and good 
landing fields — extremely valuable and useful, but not to be 
confused with precision mapping. In assembling mosaics of 
this kind the elaborate procedure described above is not 
followed. The process is the simple one of juxtaposing 
adjacent prints as accurately as possible by visual examina- 
tion. Errors are of course cumulative, but as long as exact 
distances are not in question this is no matter. 



CHAPTER XXVII 
OBLIQUE AERIAL PHOTOGRAPHY 

Oblique views from the airplane are of very great value. 
While vertical views are more searching in many respects, 
they do nevertheless present an aspect of the earth with 
which ordinary human experience is unfamiliar. Conse- 
quently they are difficult to interpret without special train- 
ing. They suffer, too, from the military standpoint, from the 
limitation that it is with vertical extension just as much as 
with horizontal that an army has to contend in its progress. 
Elevations and depressions of land show on an oblique view 
where they would be entirely missed in a vertical one. Eor 
illustration, study the picture of part of the outskirts of 
Arras (Fig. 144), presenting moat, walls and embankments, 
all of which would be serious obstacles, but would hardly be 
noticed on a vertical view. Pictures taken from directly 
overhead are eminently suited to artillery use, but oblique 
views of the territory to be attacked, taken from low alti- 
tudes, formed an essential part of the equipment of the 
infantry in the later stages of the war. 

Pictorially, oblique views are undoubtedly the most 
satisfactory. The most revealing aspect of any object is 
not one side or face alone, but the view taken at an angle, 
showing portions of two or three sides. Best of all is that 
taken to show portions of front, side and top — the well- 
known but heretofore fictitious "bird's-eye view" (Eig. 145). 
This possibility is ordinarily denied the surface-of-the-earth 
photographer, but the proper vantage point is attained in 
the airplane. 

Aerial obliques may be taken at any angle, although a 

320 



OBLIQUE PHOTOGRAPHY 



321 



distinction is sometimes made between obliques of high 
angle and panoramic or low angle views (Fig. 146). In 
addition to ordinary obliques, a very beautiful development 
is the stereo oblique. Both kinds of oblique photography 
call for special instrumental equipment and technique. 




Fig. 144. — The outskirts of Arras. Low oblique showing contours. 

Methods and Apparatus for Oblique Photography. — The 
simplest method of taking oblique pictures from a plane is 
to use a hand camera pointed at the desired angle. Its limi- 
tations are in the size and scale of the picture obtainable, 
and in the inherent limitations to the method of camera 
support. A step in advance of this is to mount the camera 
above the fuselage, on the machine gun ring or turret, in 
place of the gun. Considerably greater rigidity is thus 

21 



3^£ 



AIRPLANE PHOTOGRAPHY 




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a 



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OBLIQUE PHOTOGRAPHY 323 

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324 AIRPLANE PHOTOGRAPHY 

obtained, and heavier cameras can be utilized, although the 
wind resistance is a serious factor. Excellent obliques have 
been made in this way, even with 50-centimeter cameras, 
but the scheme is impractical in military planes, because of 
the removal of machine gun protection. 

If the camera is fixed in the fuselage in its normal vertical 
position, obliques may be and have been taken by the simple 
expedient of banking the plane steeply. This is not to be 
recommended as a standard procedure, especially for taking 
a consecutive series of exposures. 

The most satisfactory arrangements for taking obliques 
are two; first, to mount the camera obliquely in the plane, and 
second, to use a mirror or jprism, in front or behind the lens 
of the vertically mounted camera. The first method 
has been employed chiefly by the Erench, the latter 
by the English, whose gravity fed cameras could not be 
mounted obliquely 

Taking up first the oblique mounting of cameras, we 
find two ways of doing this: longitudinal mounting and 
lateral mounting. In longitudinal mounting the camera 
projects forward and downward, usually from the nose of a 
pusher or bi-motored plane. With this form of mounting 
(Fig. 147) it is necessary of course to fly directly toward the 
objective. If this is a portion of enemy trench, which must 
be photographed from a height of 400 or 500 meters, the 
plane will be directly on top of its objective a few seconds 
after the exposure is made, and be a conspicuous target, in 
imminent danger of destruction. Moreover, only a single 
short section of the trench would be obtained for each cross- 
ing of the line. The one case where resort to this method is 
practically forced is with the 120-centimeter cameras which 
simply cannot be slung athwart the plane. There is a slight 
advantage in this method of carrying in that the motion of 



OBLIQUE PHOTOGRAPHY 



325 



the image is less if the objective is approached, instead of 
being passed at the side, and so longer exposures can be made. 
The longitudinal mounting has, however, been very generally 
superseded by the lateral. 

Methods for mounting cameras obliquely for taking pic- 
tures through the side of the plane have been discussed in 
detail in connection with camera mountings and installations 
(Fig. 93). The chief difficulties are want of space, obstacles 




Fig. 147. — 120-centimeter camera mounted obliquely in the fore-and-aft position. 

at the side such as control wires and longerons, and failure 
of the camera to function properly at an angle. Even in the 
broad circular sectioned fuselage of the Salmson plane, 
quarters are so cramped that the French 50-centimeter 
camera when obliquely mounted cannot be used with the 
12-plate magazine, and recourse is made to thin flat double 
plate-holders. Holes in the side of the fuselage should clear 
all wires and should command a view unobstructed by the 
wings — which often means that the camera must be carried 
behind the observer's cockpit, irrespective of the suitability 



S26 



AIRPLANE PHOTOGRAPHY 



of that space from other standpoints. Cameras dependent 
for their action on gravity, such as the deRam and EngHsh 
L type, are unsuited for oblique suspension. 

For cameras which, because of their method of operation 

or shape cannot be slung ob- 
liquely, the only way to take 
obliques is to employ mirrors 
(Fig. 148) or prisms. These 
must be of the same optical 
quality as the photographic 
lens. They are both necessarily 
of considerable weight because 
they must be of large area of 
face to fill the entire aperture 
of an aerial lens. Mirrors are 
lighter than prisms, but must 
be quite thick to prevent dis- 
tortion of the surface due to any 
possible strains to their mount. 
Right angle glass prisms have 
been used by the English with 
the 8 and 10 inch L cameras. 
The prisms were uniformly 
tilted to an angle of 123^^ de- 
grees from the horizontal. 

Glass mirrors can be silvered 
either on the rear or front sur- 
face. If on the rear, both 
surfaces must be accurately 
parallel, which means much greater labor and expense than 
if the front surface can be utilized. The difficulty with front 
surface mirrors is that the metallic coating is easily tarnished 
or scratched, especially if silver is used, which is almost 



L 


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1 

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X 


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1 




r 

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Fig. 148.- 



-Mirror on camera cone for taking 
oblique views. 



OBLIQUE PHOTOGRAPHY 327 

imperative, since all the other metals have considerably 
lower reflecting powers. (Gold might serve both as mirror 
and color filter, because of its yellow color.) Placing the 
mirror inside the camera body in part obviates this trouble, 
but means the use of a special elbow lens cone. In any case 
the mirror or prism occasions at least a 10 per cent, loss of 
light. Pictures taken by reflectors of any kind are reversed, 
and must either be printed in a camera, or on transparent 
film which may be viewed from the back. 

The most usual condition for making obliques is to fly 
very low (300 to 600 meters), with the line of sight of the 
camera from 12 to 45 degrees from the horizontal. This 
low altitude necessitates very short exposures, to avoid move- 
ment of the image. The picture may be taken either the long 
or the short way of the plate, depending on the character of 
the object and the information desired. It is to be noted 
that successive oblique pictures cannot be mounted to form 
a continuous panorama — this being possible with obliques 
only if they are taken from one point, as from a captive 
balloon. If successive views are made on a straight flight 
at intervals so as to exactly juxtapose in the foreground, 
they overlap by a large margin the middle, and a point on 
the horizon, if that shows, will be in the same position in 
every picture. Mosaics of obliques could be made only by 
some system of conical mounting. 

Sights for Oblique Photography. — ^Any of the sights 
previously discussed for vertical work, such as the tube 
sights, are applicable to obliques. They must, however, be 
suited for mounting at an angle, in a position convenient for 
the observer. In addition, provision must be made for 
adjusting the angle so that the lines of sight of camera and 
finder are parallel. Mounting outside the fuselage is prac- 
tically the only feasible way, and is less objectionable with 



328 AIRPLANE PHOTOGRAPHY 

oblique than with vertical sights, as oblique sighting does 
not require the observer to stand up and lean over the edge 
of the cockpit. Windows in the side of the fuselage, either 
of celluloid or non-breakable glass, are a great aid to oblique 
observation. Marks upon the transparent surface can be 
utilized for the rear points of a sight of which the front point 
is a single fixed bead or rectangle. 



CHAPTER XXVIII 
STEREOSCOPIC AERIAL PHOTOGRAPHY 

One of the most striking and valuable developments in 
aerial photography has been the use of stereoscopic views. 
Pairs of pictures, taken with a considerable separation in 
their points of view and studied later by the aid of the stereo- 
scope, show an elevation and a solidity which are entirely 
wanting in the ordinary flat aerial vista. Often, indeed, these 
attributes are essential for detecting and recognizing' the 
nature of objects seen from above. Stereoscopic aerial 
photography has been justly termed "the worst foe of 
camouflage." 

Principles of Stereoscopic Vision. — ^The ability to see 
objects in relief is confined solely to man and to a few of the 
higher animals in whom the eyes are placed side by side. 
When the eyes are so placed they both see, to a large extent, 
the same objects in their fields of view. Owing to the sepa- 
ration of the eyes the actual appearance of all objects not too 
far away is different, and it is by the interpretation of these 
differences that the brain gets the sensation of relief. Thus 
in Fig. 149 the two eyes are shown diagrammatically as 
looking at a cube. The right eye sees around on the right- 
hand face of the cube, the left eye on the left-hand face of 
the cube. The two aspects which are fused and interpreted 
by the brain are shown in the lower diagram. 

Stereoscopic views or stereograms, made either by pho- 
tography or, in the early days, by careful drawing, consist of 
pairs of pictures made of the same object from two different 
points. For ordinary stereoscopic work these points are sepa- 
rated by the distance between the eyes, approximately 65 mil- 

329 



330 



AIRPLANE PHOTOGRAPHY 



limeters or 2^ inches. These two pictures are then so viewed 

that each eye receives its appropriate image from the proper 

direction, whereupon the object dehneated stands out in rehef. 

Fusion of the two elements of the stereoscopic picture 

can take place without the assist- 
ance of any instrument, if the 
eyes are properly directed and 
focussed, but this comes only with 
practice. Holding the stereogram 
well away from the face the eyes 
are directed to a distant object 
above and beyond, in order to 
diverge the axes. Then without 
converging, the eyes are dropped 
to the picture, which should spring 
into relief. It is necessary in 
moving the eyes from the distant 
object to the near stereogram to 
alter their focus somewhat, de- 
pending on how near the stereo- 
gram is held; and the success of 
the attempt to fuse the images 
depends on the observer's ability 
to maintain the eyes diverged for 
a distant object while focussing for 
a near one. Near-sighted people 
(on taking off their glasses) fuse the 
Fig. 149— The principle of stereoscopic stcrcoscopic iuiagcs quitc casily, 

vision. , ^ . ^ - 

smce their eyes do not locus on 
distant objects even when diverged for them. Transparen- 
cies are easier to fuse than paper prints, but in any case where 
a stereoscope is not used the separation of image centers 
should not be more than that of the eyes. 







STEREOSCOPIC PHOTOGRAPHY 331 

Stereoscopes. — The easier and more usual method of 
fusing the stereoscopic images is by a stereoscope. The 
simplest form consists merely of two convex lenses, one for 
each eye, their centers separated by a distance somewhat 
greater than that between the eyes. Their function is to 
bring the stereogram to focus, and, by the prismatic action 
of the edges of the lenses, to converge the lines of sight 




Fig. 150. — Common form of prism stereoscope. 



I 



which pass through the centers of the two pictures to a point 
in space in front of the observer. The two lenses should be 
mounted so as to provide for the adjustment of their sepa- 
ration to fit different eyes and print spacings. The most com- 
mon form of stereoscope is that designed by Holmes, for 
viewing paper print stereograms (Fig. 150). It has pris- 
matic lenses of an appropriate angle to converge pictures 
whose centers are three inches apart, instead of the lesser 
distance appropriate to stereograms intended for fusing 



332 



AIRPLANE PHOTOGRAPHY 



without an instrument. No adjustment is provided for 
varying the lens separation, but the print can be moved to 
and fro for focussing. 

Another form of stereoscope, one of the first produced, is 
the mirror stereoscope (Fig. 152), now used extensively for 



f-' 




Fig . 151 . — Box stereoscope. 



viewing stereo X-ray pictures. It consists of two vertical 
mirrors at right angles to each other, with their edge of 
contact between the eyes. The two prints to be studied are 
placed to right and left, an arrangement that permits the 
use of prints of any size. The convergence point is controlled 
by the angle between the mirrors. The Pellin stereoscope 



STEREOSCOPIC PHOTOGRAPHY 333 



(Fig. 153) utilizes two pairs of mirrors in a way to permit the 
use of large prints. The prints are, however, placed side by 
side on a horizontal viewing table, which avoids certain 
difficulties of illumination met with in the simpler mirror 
form. The box form of stereoscope (Fig. 151) using either 
prisms or simple convex lenses, is particularly adapted for 
viewing transparencies, although the insertion of a door at 
the top provides illumination for paper prints. The Schweiss- 




I ) ( \ 

Fig. 152. — Diagram of mirror stereoscope. 

guth design (Fig. 154) is intended primarily as an aid to 
selecting the portions of the prints to be cut out for mounting. 
The platform on which the pictures rest is composed of two 
long rectangular blocks, on which are plates of glass raised 
sufficiently to permit the prints to be slid underneath. The 
space between the blocks allows the unused portion of the 
photograph to be turned down out of the way. Prints of 
any size can thus be moved about until the proper portions 
for stereo mounting are found. Either block can be moved 



334 



AIRPLANE PHOTOGRAPHY 



in its own plane and also to and from the eye, whereby two 
prints of somewhat different scales can be fused. 

The Taking of Aerial Stereograms. — The normal separa- 
tion of the eyes is altogether too small to give an appearance 
of relief to objects as far away as is the ground from a plane 
at ordinary flying heights. In order to secure stereoscopic 
pairs it is therefore necessary to resort to a method originally 





Fig. 153. — Pellin double mirror stereoscope. 

employed for photographing distant mountains and clouds. 
This is to take the two pictures from points separated by 
distances much greater than the inter-ocular separation — ^by 
meters instead of millimeters — corresponding to the positions 
of the eyes on a veritable giant. In the airplane this is accom- 
plished by making successive exposures as the plane flies over 
the objective, at intervals to be determined by the speed, the 
altitude and the amount of relief desired (Fig. 155). 

An all important question which arises immediately is: 
What separation of points of view shall we select .^^ If the 



STEREOSCOPIC PHO TO GRAPH Y335 

exposures are too close together there will be little relief; 
if too distant the relief will be so great as to be unnatural, 
even offensive. Obviously we cannot here establish a cri- 
terion of natural appearance, since the natural appearance 




Fig. 154. — Schweissguth stereoscope, used for selecting portions of prints to be mounted. 

to ordinary human eyes is devoid of relief. We may, how- 
ever, define correct relief as that obtained when the apparent 
height of elevated objects is right as compared with their extension 
or plan. 

In order to secure this condition it is necessary, first, that 
each element of the stereoscopic pair be correct in its per- 



336 



AIRPLANE PHOTOGRAPHY 



spective. This is fortunately an old photographic problem, 
already well understood. Its solution is to view the photo- 
graph from a distance exactly equal to the focal length of the 
camera lens. Since the normal viewing distance is not less 
than 25 centimeters, lenses of this focal length at least are 
requisite for correct perspective. Secondly, it is necessary 
for correct relief that the two views be taken with a separa- 
tion equal, on the plane of the plate, to the separation of the 

STEREOSCOPIC PHOTOGRAPHS 

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FiG. 155. — Method of taking stereoscopic pictures. 



eyes, or 65 millimeters. If d is the interocular distance, a 
the viewing distance, identical with the focal length of the 
lens used, and A the altitude, then Z), the distance between 
exposures, is given by the relation — 

d ^D 

a A 

For a = 9^5 centimeters, 7=^5 approximately Vi, or the 



interval between exposures must be a quarter the altitude. 
With a 50 centimeter lens this becomes 3^, and so on. These 
figures show the fallacy of the suggestion sometimes made 



STEREOSCOPIC PHOTOGRAPHY 337 

that we take stereoscopic pictures by two cameras placed one 
at the extremity of each wing. 

When lenses of more than 25 centimeters focal length are 
employed, the stereoscope should be one capable of throwing 
the convergence point farther away than the customary 
25 centimeters. In the simple lens type of instrument we 
can do this by bringing the centers of the lenses closer 
together, and by making the focus agree with the convergence 
point by adjustment of the distance between lenses and 
stereogram. If enlargements are used they should be treated 
in all respects as originals made by lenses of the greater foci 
corresponding to the scale of the enlargement. 

When all the conditions are covered, the appearance pre- 
sented in the stereoscope is that of a model of the original object 

at a distance a, and ^ times natural size. If pictures are 
made at exposure intervals less than those indicated for 
correct relief, they show insufficient relief. This does 
not, however, give an unnatural effect, because anything 
between no relief and "correct" relief appears natural with 
large objects which are not ordinarily seen in relief by eyes 
not Brobdignagian. Conversely, stereograms made with too 
large exposure intervals show exaggerated relief. Yet this is 
often no objection. It is indeed rather an advantage if we 
wish to bring objects of interest to notice. Consequently, 
so long as the exaggeration of relief is not offensive, the 
permissible limits of exposure interval are pretty large. 
Actually, the eye tolerates such great deviations from strictly 
normal conditions that satisfactory stereoscopic effects are 
obtained for pictures viewed at very different distances from 
the focal length of the taking lens, and with the axes of the 
eyes parallel or even diverging, although there is some strain 
whenever focus and convergence points differ. On the whole, 
therefore, it may be said that the conditions above laid down 



338 AIRPLANE PHOTOGRAPHY 

for correct relief are only a normal, to be approximated as 
nearly as is practicable. 

Having established the correct relation of taking points 
for stereos the next problem is how to determine these when 
in the plane. The simplest way is by means of a stereoscopic 
sight This consists essentially of two lines of sight (fixed by 
beads, crosses, or other objects), inclined toward each other 
at the angle determined by the ratio of the ocular separation 
to the focal length of the lens. If the back sight is made a 
single bead or cross, the rest of the stereo sight will consist 
of two beads or crosses, separated from each other by the 
ocular distance of 65 millimeters, and distant from the back 
sight by the focal length of the lens (Fig. 157). The first 
picture is taken when the object is in line with the forward 
pointing line of sight, the second when it lies along the back- 
ward pointing one. Like other sights, the stereoscopic sight 
may be attached either to the camera, or if this is fixed in 
position, to any convenient part of the plane. A very simple 
sight for vertical stereoscopic photography consists of 
an inverted V painted on the side of the fuselage, so 
that the eye can be placed at the vertex and sighted along 
either leg. 

The common method of determining the space between 
exposures is by the time interval. If V is the speed of the 
plane, and t the desired time interval, we have, from the 
last equation — 

._ D _dA 

^~ V ~ aV 

If ^ = 2000 meters, d = 65 millimeters, and a = ^5 centimeters, 
and if the plane is traveling 200 kilometers per hour, the time 
interval must be — 

.065 X 2000 X 3600 ^ , , 

= 9.4 seconds 



.25 X 200,000 



STEREOSCOPIC PHO TO GRAPH Y339 



At 1000 meters altitude the interval will be half this, and so 
on in proportion. If the pictures are taken with a 50 centi- 
meter focus camera, and are hence to be viewed at 50 centi- 
meters convergence distance instead of at 25, the time will 
again be halved. These relations are clearly shown in the 
diagram (Fig. 156). Here the ^left-hand portion shows how 



HE16HTIN METERS BASE 

O WOO 2000 3000 4000 5000 8000 \ 60 

I I |i I II III nil I I I III nil I M II I I il 



tNTERVAL IN S£CONM 



-1500- 




Fig. 156. — Chart for calculating intervals between exposures for stereoscopic pictures. 

to find the stereoscopic base line at each altitude for each 
focal length; while the right-hand portion shows how to 
translate this into time interval for any plane velocity. 
The Burchall slide rule (Fig: 130) shows another way to 
arrange these data in form for rapid calculation. 

Plates used for stereoscopic negatives should be at least 
twice as long as the ocular separation, if correct relief is 



340 AIRPLANE PHOTOGRAPHY 

desired, and the full size of the stereoscope field is to be 
utilized. This relation follows at once if we consider that 
we wish to cut from each negative a rectangle 65 millimeters 
wide, and that the image of the target has shifted 65 milli- 
meters between exposures. If the plate is larger than this 
there is opportunity to select the view, or to pick several. 
If the plate is smaller the elements of the stereogram must 
be narrow strips. This, however, holds only for contact prints. 

The ordinary English practice in making stereo negatives 
is to take successive pictures with an overlap of 60 to 75 per 
cent. This practice is probably dictated by the 4X5 inch 
plate, since 60 per cent, overlap on 4 inches means a separa- 
tion of jui^ over af? inch and a half instead of 2^, but it 
leaves 2}^ iiiches of picture common to the two negatives. 
With ^ overlap the common portion is 3 inches, which per- 
mits of cutting 2^ inch prints, and allows some latitude for 
irregular motion of the plane or for chance error in calcula- 
tion of intervals. Data on the basis of ^ overlaps for a 
4-inch plate* are shown in connection with Fig. 155 which 
shows in diagr^mm9.tic form the variation of exposure inter- 
val with hei^t, together with other points of interest. 

Elevation Possible to Detect in Stereoscopic Views.— «- 
Can the actual difference it^ elevation be discovered by the 
use of stereoscopic views .f^'* An approximate idea may be 
obtained from the following considerations: Suppose we 
have two small point-like objects, one above the other, such 
as a street lamp globe and the base of the lamp pillar. In a 
view taken from directly overhead these will be superposed, 
and so will not be capable of separation. But, as the point of 
view is shifted sideways, the two objects separate, until a point 
is reached where they can just be distinguished as double. 
When this condition holds for either picture of the stereo- 
scopic pair it will be possible to obtain stereoscopic relief. 



STEREOSCOPIC PHOTOGRAPHY 341 

Now the separation which can just be distinguished is 
commonly assumed to be one minute of arc. This angle 
corresponds to about ^x^ the distance from the eye to the 
object. If the object is assumed at a distance a from the 
face, and on a line with one of the eyes, which are 
separated by the distance d, then (all angles being small) 
the object must be of height -A- times the horizontal distance 

which corresponds to one minute. For 25 centimeters' view- 
ing-distance this quantity is about 4, so that the least per- 
ceptible elevation is 3/0Q or about 9-^. The stereogram 
having been made under conditions giving correct relief, 
this fraction is also the fraction of the altitude of the plane 
when the photograph was taken which may be detected. 
An object as high as a man (6 feet) should be visible as a 
projection in a stereoscopic view taken at 6X900 =5400 feet. 
This relation — q^-q — holds (irrespective of the focal length 
of the lens), as long as the conditions for correct relief 
are maintained. 

Stereoscopic Aerial Cameras. — Cameras for aerial stereo- 
scopic photography need in no way differ in construction 
from those made for mapping or spotting, provided only 
they permit exposures to be made at short enough intervals. 
The addition of special sights, as already discussed, consti- 
tutes the only real difference between single view and stereo- 
scopic aerial cameras. But even without such sights ordinary 
aerial cameras are applicable to stereo work by the usual 
procedure of determining the exposure spacing by time. 

One scheme employed for taking low stereos, where the 
interval is only two or three seconds, is to mount two cameras 
in the plane, exposing them one after the other at the correct 
interval. Another method which has been tried with success 
is the use of a double focal-plane shutter in a single lens 
camera (Fig. 157). The two shutters are side by side, with 



S42 



AIRPLANE PHOTOGRAPHY 



their slots parallel to the line of flight. To take a stereo 
negative we expose first the shutter nearer the tail of the 
plane, and then the other, after an interval which can be 
calculated from the speed and altitude, or, better, determined 
by a stereoscopic sight. The two views are thus obtained 
on a single plate. Prints from these negatives are transposed 




Fig. 157. — ^Aerial hand camera fitted with two complementary shutter slits and double sight, for 

stereoscopic photography. 

right and left, and, if the prints are viewed in an ordinary 
stereoscope, have to be cut apart and transposed for mount- 
ing, or else this may be done to the negatives. 

In this connection attention may be drawn to an alterna- 
tive method of viewing stereograms, which may be used on 
transposed prints — a method which needs no instrument, 
and so has sufficient advantage to even warrant mounting 



STEREOSCOPIC PHOTOGRAPH Y 343 

ordinary stereoscopic pairs in the transposed position for 
observation. This method consists in crossing the optic 
axes, in the fashion illustrated in Fig. 158. A finger is held 
in front of the face in such a position that the left stereogram 
element and the finger are seen in line by the right eye; the 
right element and the finger by the left eye. The proper 
position is found by alternately closing each eye, and advanc- 
ing or retracting the finger. Then both eyes are opened and 
converged on the finger tip, which is thereupon dropped, 
leaving the picture standing out in relief. An opportunity 
to try this method is afforded by Fig. 159. 





Fig. 158. — Method of fusing transposed stereoscopic images by crossing the optic axes. 



Stereo Obliques. — The theory of making oblique stereo 
pictures is identical with that of other stereos. The only 
problem peculiar to obhques is that of making the exposures 
at short enough intervals apart. This problem is due largely 
to the fact that oblique views are ordinarily taken from low 
altitudes, for the purpose of "spotting" particular objects, 
rather than for mapping the gross features of an extended 
area. The same problem of how to secure a short exposure 
interval is met with when we attempt to take vertical stereos 
from a low altitude, but as already discussed, it is much 
preferable from the pictorial standpoint that pictures of 
definite small objectives be made obliquely. 

Another reason for taking stereo obliques from points but 



S44 AIRPLANE PHOTOGRAPHY 

little separated is of some interest in connection with the 
discussion above given of "correct" and "natural" relief. 
When the relief is "correct" the object appears, as already 
stated, to be a small model in its true proportions, standing 
at the convergence distance. When the eyes are converged 
to a small object 25 to 50 centimeters away all objects 
beyond are hopelessly transposed and confused. This does 
not happen when we look at large distant objects, since their 
background is at a distance effectively but little beyond them. 
As a result, when a stereo oblique is made in "correct" relief 
of such an object as the Washington monument with build- 
ings beyond, the confusion of the background presents an 
appearance entirely contrary to our visual experience with 
objects as large as the neighboring buildings are known to 
be. This effect may be avoided by choosing a uniform back- 
ground such as grass, or by taking the pictures very much 
closer together, at the expense of "correct" but at a gain 
in "natural" relief. 

Stereo obliques can of course only be made with any 
facility by laterally pointing cameras. From the calculations 
already given it appears that a "correct" stereo oblique of 
an object 500 meters away will mean exposures only two or 
three seconds apart, too short an interval for any of the 
ordinary plate-changing and shutter-setting mechanisms; 
and the case is even worse should less relief be desired. One 
solution of this problem has been the use, already mentioned, 
of two cameras mounted together, either side by side or one 
over the other, with separate shutter releases. Both releases 
may be controlled by the observer, using a sight, or else 
pilot and observer may work in harmony as has been recom- 
mended in the English service, where the pilot releases one 
shutter and the observer counts time from the instant he 
sees the first shutter unwind and releases the second. 



STEREOSCOPIC PH O TO GRAPH Y345 

A very satisfactory apparatus for the taking of stereo 
obliques consists of a 10-inch focus hand-held camera (Fig. 
157), provided with a two-aperture focal-plane shutter. The 
right-hand half of one curtain aperture is blocked out, the 
left-hand half of the other. The first pressure on the exposing 
lever exposes one-half of the plate, the second the other. A 




Fig. 159. — Oblique stereogram made with stereoscopic aerial camera (Fig. 157). To be viewed by 

crossing the optic axes (Fig. 158). 

stereoi^copic sight of the type already described is placed on 
the bottom. To make an oblique stereo negative the camera 
is held rigidly by resting the elbows on the top of the fusel- 
age and the first exposure is made when the object comes in 
line with the rear sight and the leading front sight. The eye 
is then moved so as to look along the line of the rear sight 
and the following front sight, and when the object is again 
in alinement the second pressure is given the exposing lever. 



346 AIRPLANE PHOTOGRAPHY 

Fig. 159 shows a stereo oblique made by this camera. The 
elements are transposed right and left, and the stereogram 
mfiy be viewed by crossing the optic axes as shown in Fig. 
158, or the two pictures may be cut apart and remounted. 

The Mounting of Aerial Stereograms. — The first step in 
making the printed stereogram is to select two pictures taken 
on the same scale, but from slightly different positions. 
These may be two chosen from a collection made for other 
purposes, or else a pair taken at distances calculated to fit 
them for stereoscopic use. The next step is to mark the 
center of each picture, either with easily removed chalk or 
witn a pin point. They are then superposed, and afterward 
carefully moved apart by a motion parallel to the line joining 
their centers when superposed. The final step before mount- 
ing is to mark out and cut the two elements, their bases being 
parallel to the line of centers, their horizontal length the dis- 
tance between the optic axes of the stereoscope (or as near 
this as the size of the prints will permit). They are then 
mounted on a card, with their centers separated by approxi- 
mately Q5 millimeters. The right-hand view is the one show- 
ing more of the right-hand side of objects, and vice versa. 
This process of arranging, cutting, and mounting is shown 
clearly in Fig. 160. In this case the stereoscopic elements 
lie symmetrically about the line joining the centers of the 
original prints. This is not necessary, as they may be selected 
from above or below this line so long as their bases are par- 
allel to it. A simplification of this method consists in super- 
posing the two prints, laying over them a square of glass of 
the size to which they are to be cut, then turning it so that a 
side is parallel to the line of centers, and cutting around it 
through both prints with a sharp knife. The principle and 
results are of course the same with both methods. 

If large numbers of stereoscopic prints are required it is 



348 AIRPLANE PHOTOGRAPHY 

necessary, for economy of time, either to photograph a fin- 
ished stereogram and make prints from this copy negative, 
or to set up special printing machines. Under the general 
discussion of printing devices a stereoscopic printer is 
described (the Richard) in which the two negatives are 
placed so that stereo prints can be got by two successive 
printings on one sheet of paper. 

Uses of Stereoscopic Aerial Views. — ^Attention has 
already been called to the characteristic flatness of the aerial 
view. Neither the picture on the retina nor that on the 
photographic plate affords any adequate idea of hills and 
hollows. Unless shadows are well defined, small local ele- 
vations and depressions cannot be distinguished from mere 
difference in color or marking. Even in the presence of 
shadows it is often only by close study that differences of 
contour are noticeable. But with stereoscopic views these 
features stand out in a striking manner. Taking our illus- 
trations from military sources, we may note the use of stereo- 
scopic pictures to detect undulations of ground in front of 
trenches (Fig. 161). They reveal the hillocks, pits, small 
quarries, streams flowing behind high banks, and other 
features which make the attack hard or easy. Commanding 
positions are shown, the boundaries of areas exposed to 
machine-gun fire, and the defilades where the attackers may 
pause to reform. Concrete "pill boxes" are located in the 
midst of shell holes of the same size and outline, and can be 
differentiated from them. 

Railway or road embankments and cuts can be detected 
and studied to extraordinary advantage in stereoscopic pic- 
tures. Thus what appears to be a mine crater on a level 
road, easily driven around, may be a gap blown in an 
embankment, a serious obstacle indeed. Bridges, observa- 
tion towers and other elevated structures jump into view in 



STEREOSCOPIC PHOTO G RAPH Y349 




Fig. 161. — Typical stereogram of military detail. Fuse by looking at a distant object over the top 
of the page, and quickly dropping the eyes to the print. 

the stereoscope when often they have entirely eluded notice 
in the ordinary flat picture. Once presented in relief, camou- 
flaged buildings or gun emplacements, however carefully 
painted, are ridiculously easy to pick out. 

Practical peace-time applications of stereoscopic views 
can easily be foreseen following the lines of war experience. 
Such, for instance, would be the study of proposed railway or 
canal routes. A series of stereograms would obviate the 
necessity of contour surveys, at least until the exact route 
was picked and construction work ready to start. 

Apart from their utilitarian side, however, stereoscopic 
views have very great pictorial merit. Stereoscopic pictures 
of cathedrals, public and other large buildings, have often 
great beauty, and afford opportunities for the study of form 
given by no other kind of representation, short of expensive 
scale models. They may very well lead in the near future to a 
revival of the popularity of the stereoscope. 

Impression of Relief Produced by Motion. — ^An appear- 



350 AIRPLANE PHOTOGRAPHY 

ance of solidity can be obtained in moving pictures by the 
simple expedient of slowly moving the camera laterally as 
the pictures are taken. As an illustration, if the moving 
picture camera is carried on a boat while structures on the 
shore are photographed, when these are projected on the 
screen they appear in relief, due to the relative motion of 
foreground and background. As relief of this sort is not 
dependent on the use of the two eyes, it demands no special 
viewing apparatus. This idea has been utilized to a limited 
extent in ordinary mov^ing picture photography by intro- 
ducing a slow to-and-fro motion of the camera, but this can 
hardly be considered satisfactory, since this motion is so 
obviously unnatural. 

In moving pictures made from the aiplane the normal 
rapid motion of the point of view is ideal for the production 
of the impression of relief in the manner just described. For 
instance, in moving pictures of a city made from a low flying 
plane, the skyscrapers and spires as they sweep past stand 
forth from their more slowly moving background in bold and 
satisfying solidity. In fact, such pictures probably constitute 
the most satisfactory solution yet found of the vexing prob- 
lem of "stereoscopic" projection. No better medium can be 
imagined for the travel lecturer to introduce his audience to a 
foreign city than to throw upon his screen a film made in a 
plane approaching from afar and then circling the archi- 
tectural landmarks at low altitudes. 



CHAPTER XXIX 

THE INTERPRETATION OF AERIAL 
PHOTOGRAPHS 

Oblique aerial photographs if on a large enough scale are 
even easier to interpret than are ordinary photographs taken 
from the ground, since they practically preserve the usual 
view, and add to it the essentials of a plan. With verticals, 
however, this is far from the case. In them all natural objects 
present an appearance quite foreign to the ordinary mortal's 
previous experience of them. This may be easily demon- 
strated by taking any aerial view containing a fair amount 
of detail and trying systematically to identify each object. 
A necessary preliminary to doing this accurately is acquaint- 
ance with and study of the ground photographed, or of 
similar regions, and of objects of the same character as 
those likely to be included. 

The interpretation of military aerial photographs is of 
such importance, and has become such an art, that it is the 
function of special departments of the intelligence service. 
Extended courses in the subject are now given in military 
schools. This instruction must cover more than the inter- 
pretation of aerial photographs as such. General military 
knowledge is essential, so that not only may photographed 
objects be recognized, but the significance of their appear- 
ance be realized. Whether attack or retreat is indicated; 
whether a long range bombardment is in preparation, or a 
mere strengthening of local defences. 

The natural difficulties of interpreting aerial views are 
enormously increased by the unfamiliar nature and fre- 
quently changed character of the military structures, and 

I 351 



352 AIRPLANE PHOTOGRAPHY 

particularly by the attempts made to conceal these from 
aerial observation by selection of surroundings and by cam- 
ouflage. The small scale of the photographs, in which a 
machine gun shows as a mere pin point, adds to the uncer- 
tainty, with the net result of making interpretation a task 
of minute study and deduction worthy of a Sherlock Holmes. 

Little detailed information on interpretation can be 
profitably written in a general treatise, partly because the 
illustrations available are of a highly technical military char- 
acter, partly because original photographs instead of half- 
tone reproductions are practically imperative for purposes 
of study. Nevertheless some general instructions, applicable 
to any problem of interpretation, may be given, as well as a 
few illustrations, drawn from military sources, which will 
serve to show the detective skill necessary. 

First of all it is important that the print or transparency 
be held in the right position. The shadows must always fall 
toward the observer; otherwise, reliefs will appear as hollows 
and hollows will show as hills. The reason for this is that the 
body ordinarily acts as a shield, preventing the formation of 
shadows except by light falling toward the beholder. Thus 
in Fig. 162 the slag heap looks like a quarry when the 
shadows fall away from one. The necessity for proper direc- 
tion of shadows is, it may be noted, in conflict with the 
ordinary convention for the orientation of maps — at least in 
the northern hemisphere. A city map, made by sunlight 
falling from the south, presents its shadows as falling away 
from the observer, when it is mounted with its north point 
at the top, as is customary. As a consequence buildings in 
aerial photographic mosaics of cities occasionally look 
sunken instead of standing out. 

The relation between the shape of the shadow and the 
object casting it must be well learned. This is a part of the 




28 



354 

r 



AIRPLANE PHOTOGRAPHY 



/B C" 

A D £ 




/B 






c 



A 



D £ 



EMBANK/^£/Vr 




FiQ. 163. — Guide to interpretation of trench details. 

training of every architectural draftsman, but the appear- 
ance of shadows from above has not heretofore been a matter 
of importance. The difference between high and low 



INTERPRETATION 



355 




Tft£/^CH .MORrA/f £-MPl.^C£l 

Fig. 164. — Guide to interpretation of shell holes and other pits. 

trenches, between cuttings and embankments, between shell 
holes, occupied or unoccupied, and "pill boxes," must be 
detected largely from the character of the shadows. Which 



i 



356 AIRPLANE PHOTOGRAPHY 

elevations and depressions are of military and which of 
merely accessory nature, whether this black dot is a machine 
gun or a signaling device, whether that dark spot is an active 
gun port or an abandoned one — these are all matters of 
shadow and of light and shade study. Several illustrations 
of these points appear in Figs. 163, 164 and 165. 

Shadows may be used to get exact information as to 
directions and magnitudes. If we know the time of day at 




Haysrac/< AJ. 6ufi em/olacemenf 

vsfHh cofictete 
L cen/te /JiVors J 

FiQ. 165. — Illustrating the importance of distinguishing between objects of similar appearance but 

different military importance. 

which a picture is taken, the direction of the shadows will 
give the points of the compass. A chart for doing this is 
shown in Fig. 166. The length of a shadow is a measure of 
the height of the object casting it, and the exact relation 
between the two dimensions is determined by the day and 
hour. Fig. 167 embodies in chart form the values of this 
relationship for all times of the year and day, while Fig. 168 
shows the kind of picture in which shadow data could be 
utilized to great profit. 

Minute changes, both in light and shade and in position, 
must be watched for with great care. Naturally growing 



INTERPRETATION 



357 



foliage and the cut branches used for camouflage differ in 
color progressively with the drying up of the leaves. Hence 
a mere spot of lighter tone in a picture of a forest, especially 



III 



VI 




Fig. 166. — Location of true north from direction of shadows. Place the dial on the photo- 
graph, the hour line corresponding to the time it was taken being pointed in the direction of 
the shadows. North lies between the two arrows, the exact direction being obtained by joining the 
center of the dial to the point on the figure of eight corresponding to the date on which the picture was 
taken. (Numbers on figure of eight represent the 1st of the month). 

if the picture is taken through a deep filter, becomes instant 
object for suspicion. The complete study of any position 
calls for photographs of all kinds — ^verticals, obliques, and 



358 



AIRPLANE PHOTOGRAPHY 



stereos. Stereoscopic views are the worst foe to camouflage. 
A bridge painted to look like the river beneath is labor 
thrown away if the stereo shows it to be a good ten feet 
above the real river! 




10" ir~i?" 15 tr n" 16^ ^ir_ i8"p9".2o . . I 

Fig. 167. — Length of shadow of object one meter high, at diflferent times of the day and year, for 

latitude of Paris. 

A few illustrations of the more ordinary and obvious 
objects whose detection is the subject of aerial photography 
are shown in accompanying figures. Fig. 169 pictures a 
typical trench system, with barbed wire. The trenches show 
as narrow castellated lines, from which run the zigzag lines 
of communicating trenches, saps, and listening posts. The 



INTERPRETATION 



359 




360 



AIRPLANE PHOTOGRAPHY 



minute pockmarks behind the main trench lines are shell 
holes and machine gun pits. The barbed wire shows as 
double and triple gray bands, intricately criss-crossed at 




Fig. 169. — Typical trench photograph showing first and second lines, communicating trenches, 
listening posts, machine gun emplacements, and barbed wire. 



strategic points. Another form of defence, intended for the 
same purpose as the barbed wire of the western front, is that 
furnished by overthrown trees in forest regions. Fig. 170 
reveals a mountain fortress surrounded by a zone of felled 



INTERPRETATION 



361 



trees, and indicates in striking manner the value of the infor- 
mation a single aerial photograph may furnish to an attack- 
ing force. Fig. 123 shows on a comparatively large scale 
opposing trench systems in which a natural obstacle — a 
river — separates the adversaries. Nicks and dots indicate 
machine guns to the skilled eye, and several rectangular 




FiQ. 170, — A mountain fort surrounded by felled timber. 

structures are revealed as concrete buildings which have 
survived unscathed the shell fire which has obliterated, and 
caused to be rebuilt, nearly every other element of the 
trench system. 

Isolated battery em.placements (Fig. 171) must be care- 
fully studied to learn if they are in use. The chief indication 
is given by the paths the men make in going and coming; 
these show as fine light lines, obliterated by growing vege- 



362 AIRPLANE PHOTOGRAPHY 




Fig, 171. — Three stages in the life of a battery. 



INTERPRETATION 



363 



tation if long disused. Another indication is the blast marks 
in front of the gun muzzles; occasionally the sensitive plate 
will catch the actual puff of smoke as the gun is discharged. 

Railways of various gauges show as thin lines, crossed 
by ties, and exhibiting the characteristic curves and switches. 
They are particularly important to detect because they 
naturally lead to guns or supplies of importance. Abandoned 
railways from which the rails and ties have been removed 
leave their marks on the ground and must be carefully 
distinguished from lines in actual operation. 

Aviation fields are easily recognized by the hangars, 
often with "funk hole" trenches alongside for the men to 
take shelter in during air raids (Fig. 172). Other character- 
istic features are the "T" which shows the direction of the 
wind to the returning pilot, and of course the planes them- 
selves, standing on the ground. But the field may be inac- 
tive, and the planes merely canvas dummies, so that to 
pierce the disguise, all paths, ruts, and other indications of 
activity must be minutely studied. 

Overhead telegraph and telephone lines are revealed 
when new by a series of light points (Fig. 174), where the 
posts have been erected in the fresh turned earth. Later, 
when the fields through which they pass are cultivated, the 
post bases show as islands left unturned by the plow. In 
winter the wires reveal their position by black lines in the 
snow caused by drippings. Buried cables are indicated 
while building by their trenches, and for some time after- 
ward by the comparatively straight line of disturbed earth. 

Just as the detective of classic story makes full use of 
freshly fallen snow to identify the footprints of the criminal, 
so does the aerial photographer utilize a snowfall to pierce 
the enemy's attempts at deception. Tracks in the snow 
show which trenches or batteries are in actual use. Melting 



364 AIRPLANE PHOTOGRAPHY 




M^^ 



fi#^#*" 






Fig. 172. — Aviation field, showing hangars, planes, landing "T" and refuge trench. 

of the snow in certain places may mean fires in dugouts 
beneath. Black smudges in front of trench walls show where 
guns are active. Guns, wire and other objects, however 



INTERPRETATION 



365 



? 



i 



I 








1 VictrnTY^^^f Que/iTi/i zi-aiT'^o^c/i 



J'iG. 174. — A fully interpreted aerial photograph. 



INTERPRETATION 367 

carefully painted to match the gray-green earth, stand out 
in violent contrast to this new white background (Fig. 173). 
After the aerial photograph has been interpreted the 
results of the interpretation must be made available to the 
artilleryman or the attacking infantryman. This may be 
done by legends marked directly on the photograph. Another 
method is to mount over the photograph a thin tissue paper 
or oilskin leaf, with the interpretation marked on it. A yet 
more elegant method consists in outlining all the chief fea- 
tures of the photograph in ink, writing in the points of 
importance in interpretation, and then bleaching out the 
photograph with potassium permanganate solution. Photo- 
graphic copies of the resultant line drawing are then mounted 
side by side with the original photograph. Fig. 174, which 
shows a fully interpreted photograph, is an example of this 
kind of mounting. 



CHAPTER XXX 
NAVAL AERIAL PHOTOGRAPHY 

The problems of naval aerial photography are qiute 
different from those of military aerial work, and on the whole 
they are more simple. At the same time, photography has 
played a considerably less important part in naval aerial 
warfare than in land operations. Photography as a necessary 
preparation for attack has not figured in naval practice 
nearly so much as have the record and instruction aspects. 
To some extent this is due to the nature of the naval opera- 
tions in the Great War, to some extent to the limitations of 
ceiling and cruising radius of the naval aircraft. 

A photographic reconnaissance, preceding and following 
a bombardment of shore batteries; a photographic record of 
the ships at anchor, as at Santiago; a photograph of the forts 
defending a channel, as at Manila; photographs, quickly 
developed and printed, of an approaching fleet — all these 
are possibilities of great usefulness in naval warfare between 
contestants both of whom "come out" and carry the strug- 
gle to the enemy's gates. But in the recent war the use of the 
submarine, operations under cover of fog, the striving for 
"low visibility," and the considerable distances to be 
traversed to reach the enemy lairs, have conspired to limit 
the development of photography as a major aid to naval 
combat. Probably when the whole history of the conflict 
is told we shall learn that the Zeppelins which cruised over 
the North Sea, keeping the Allied fleet under observation, 
had a regular routine of photographic work. In the Italian 
zone, where much of the enemy territory and several impor- 
tant naval centers lay at only short distances over the Adri- 



NAVAL PHOTOGRAPHY 



369 



atic, the naval photographic service more nearly rivalled 
that of the army than in the English, French and American 
zone of activity in the Channel and North Sea. 

The majority of the photographs made in the British 
service were obliques, taken by short focus (6 to 10 inch) 




Fig. 175. — A lighthouse, as the naval flier sees it. 



hand-held cameras. This type was employed partly because 
of difficulties to be noted presently, in using other forms of 
cameras, but more especially because such pictures sufficed 
for the kind of information desired. A hand-held camera 
formed part of the outfit of each flying boat and diri- 
gible, but, unlike land reconnaissance, planes ascending 
primarily for picture taking were unknown in their naval 

service. Usually no photographic objective was predeter- 
24 



370 AIRPLANE PHOTOGRAPHY 

mined — ^photographs were made only if objects of interest 
were come upon. Mapping also formed no part of the sea- 
plane's work. Four plates would be carried, instead of as 
many dozens in the land machine, and often these would 
come back unexposed. There were of course some photo- 
graphic flights planned out beforehand, for the purpose of 
photographing lighthouses and other landmarks whose 






Fig. 176. — A threatened submarine attack. Throwing out a smoke screen to protect a convoy. 

British official photograph. 

appearance from the air should be known to the naval avia- 
tor (Fig. 175). Among the accidental and record types of 
photograph come convoys (Fig. 176), whose composition and 
arrangement were made a matter of record, particularly if 
any ship was out of its assigned position. Photographs of 
oil spots on the sea surface, or other results of bomb dropping, 
were necessary evidence to establish the sinking of a subma- 
rine (Fig. 179). Pictures of all types of ships friendly, 



NAVAL PHOTOGRAPHY 



371 



neutral, and where possible, enemy — were a much needed 
part of naval equipment, in particular pictures of friendly 
destroyers and submarines, which should not be bombed 
by mistake. For safe navigation it was essential to have 
photographs of uncharted wrecks (Fig. 181), of buoys out 
of place and of ships failing to return signals or otherwise 




Fig. 177. — Submarine coming to the surface. 
U. S. Naval Air Service photograph 

to comply with rules. The great majority of the pictures 
were taken from altitudes of not more than 300 meters. 

Hand-held cameras for naval work have practically the 
same design as those for land work. In view of the smaller 
number of pictures taken on naval trips, and the consequent 
absence of any need for great speed in changing plates, the 
ordinary two-plate dark slide has been found satisfactory in 



372 AIRPLANE PHOTOGRAPHY 




Fig. 170.— Dropping depth bombs. 




Fig. 179. — The submarine destroyed. Destroyer on tell-tale oil patch. 
British official photographs. 



NAVAL PHOTOGRAPHY 



373 



the English service. But these are much less convenient 
than the bag magazines used in the U. S. Naval hand camera 
(Fig. 31). The sights on the naval hand camera are prefer- 
ably of the rectangular, field indicating type, especially useful 




Fig. 180. — A convoy at anchor in port. 



in photographing extended objects such as convoys. As the 
flying boat travels , comparatively slow, it is easy for the 
observer to stand up to take pictures, and the sight is con- 
veniently placed on top. But if held out over the side for 
verticals the sight must be on the bottom. Rectangular 
sights in both positions are provided in the English camera 



374 AIRPLANE PHOTOGRAPHY 

(Fig. 186). Naval cameras should be immune from moisture, 
which means doing away with all wooden slides or grooves. 
A praiseworthy practice is to carry the camera in a water- 
proof bag. 

Cameras other than of the hand-held form have been 
little used in sea planes, owing to the difficulties of installa- 




FiG. 181. — Airplane photography as an aid to salvaging. Position of wrecked merchantman twelve 
fathoms down revealed by photograph from the air. 
Photograph by British Air Service. 

tion. The hydro-airplane, consisting of an ordinary airplane 
fuselage mounted on two pontoons (Fig. 182), can carry the 
same kind of photographic equipment as the land machine. 
But if it has a single central pontoon this is not feasible. 
The hydro-airplane is, however, largely superseded by the 
flying boat (Fig. 183), whose fuselage, of boat form, rests 
directly on the water. In this type of sea plane, views taken 



NAVAL PHOTOGRAPHY 



375 




Fig. 18'^.— a sea plane. 






i^*-' HSkmb0.\: 






Fig. 183.— a flying boat. 



vertically downward are not easy to make. In the larger 
flying boats the hull projects out horizontally a matter of 
several feet beyond the side of the cockpit. An ordinary 



376 AIRPLANE PHOTOGRAPHY 




J<'IG. 184. — ^A dirigible or "blimp" — possibly the photographic aircraft of the future. 




Fig. 185. — English"Type 18" band camera on bracket for exposing through side window of flying boat. 

British official photograph. 



NAVAL PHOTOGRAPHY 



377 



outboard mounting is therefore out of the question. The 
camera must either be held out at arm's length or else 
mounted on a long bracket (Fig. 186). The usual place for 
carrying the camera is in the front cockpit with its magnifi- 
cent all-round view. Obliques can, too, be taken in great 
comfort from the side windows behind the wings, as shown 




Fig. 186. — Camera mounted in bracket from forward cockpit of flying boat. 
British official photograph. 

in Fig. 185. The possibility of cutting a hole in the bottom 
of a flying boat to take care of a vertical camera is not 
entertained in British and American naval cricles. Never- 
theless it is the regular practice in the Italian service, with 
their small high ceilinged flying boats. In them a round hole 
is cut in the floor, stopped with a plug and rubber gasket. 
After the boat rises into the air the hole is opened, and the 



378 



AIRPLANE PHOTOGRAPHY 



regulation Italian camera is set securely in a frame on the 
floor over the hole (Fig. 187). Photographs are taken to 
the capacity of the camera, and if it is desired another camera 
is put in its place, till all its plates have been exposed, and 
then even a third. Before coming down the hole must 
of course be closed again. Sliding doors have been de- 
signed to close this aperture, but have not proved sufficiently 



Po^azipne deh macxhtna fofo^rdfca ^u§li idrovolentt 




'y^ZZ^ 



Fig. 187. — Italian flying boat with camera mounted on the floor. 

water-tight, although such a device could undoubtedly be 
worked out. 

With its space for ^ve or more passengers, and with its 
low speed, the modern flying boat affords an excellent craft 
for photographic work. There is ample room for any size 
of camera, and for any style of mounting, if we assume that 
there is no objection to an opening in the bottom. The low 
ceiling of these ships, however, prevents their use for certain 
forms of aerial photography which should be of the greatest 
importance. Operations against shore stations — harbors. 



NAVAL PHOTOGRAPHY 



379 



docks, shipyards, ships at anchor, and fortifications — cannot 
be undertaken for fear of anti-aircraft guns and hostile land 
planes. The solution of the problem of carrying and launch- 
ing fast high flying planes from ships will immediately extend 
the usefulness of aerial photography to coastal work. In 
the recent war, such of this as was done, along the Belgian 
coast — ^the shore batteries, and the results of naval operations 
at Zeebrugge and Ostend — was done by land planes from 
territory held by the Allies. The photographic equipment of 
sea planes of the type suggested will of course present special 
problems, but the apparatus used will be apt to approximate 
closely to that of the land planes. 



VII 

THE FUTURE OF AERIAL PHOTOGRAPHY 



CHAPTER XXXI 

FUTURE DEVELOPMENTS IN APPARATUS 
AND METHODS 

Prophecy is an undertaking that always involves risk. 
The prophet's guess of what the future will bring forth is 
based only on the tendencies of the past, the most urgent 
needs of the present, and the activity of his imagination. 
He may easily — and he usually does — entirely overlook 
certain possibilities which may arise apparently from no- 
where and which profoundly affect the whole trend of devel- 
opment. Conditions which dominate at the present time — 
such as military necessity — ^may happily drop into the back- 
ground and free the science from some of its severest restric- 
tions. With this caution, some future possibilities in appa- 
ratus and methods may be presented along the lines already 
used in discussing the present status of aerial photography. 

Lenses. — ^From the military standpoint the next steps 
in lens design would be toward telephoto lenses on the one 
hand, and on the other toward lenses of short focus and wide 
angle. The telephoto lenses used for spotting would be of 
long equivalent focus — a meter and more — ^but of handy 
size, that is, not more than 50 centimeters over all working 
distance. The wide angle short focus lenses would be 
designed for low flying reconnaissance or quick mapping 
work. They would also be demanded for peace-time map- 
ping projects, where the largest possible amount of territory 
should be covered in a single flight. Both types of lens 
should be pushed to the extreme in aperture, for short ex- 
posures and the maximum of working days will always 
be demanded. 

383 



384 AIRPLANE PHOTOGRAPHY 

Cameras. — ^Peace-times will give the necessary oppor-. 
tunity to develop self-contained and therefore simply 
installed cameras. They will at the same time be made 
very completely automatic but simple to operate in spite of 
their complexity. Such cameras have, during the war, been 
the ideal of all aerial photographers, but the time has 
been too short since the necessary conditions have been 
understood for that lengthy development work and those 
complete service tests which are so necessary to develop 
all automatic apparatus. Several designs which are now 
being perfected may be counted on to take us a long way 
toward this ideal. 

On the other hand, that military ideal which leaves the 
camera operator the greatest possible freedom for other 
activities, is apt to be entirely reversed in peace. The 
camera operator can now be required to be an expert, who 
will be free to change plates or filters and to estimate expos- 
ures, instead of giving his best efforts to the problem of 
defence. For him a simple and reliable hand-operated or 
semi-automatic camera is entirely satisfactory, and the great 
expense of complicated automatic apparatus has no longer 
its former justification. 

Camera Suspension. — ^Perhaps the most pleasing pros- 
pect before the aerial photographer as he turns from war to 
peace work is that of having planes built for and dedicated 
primarily to photography. Instead of his camera being 
relegated to an inaccessible position, picked after the plane 
design has been officially "locked;" instead of yielding first 
place to controls, machine gun and ammunition; instead of 
being jealously criticised for the space and weight it takes 
up, the camera can now claim space, weight, and location 
suitable for any likely aerial photographic need. High speed 
no longer will be vital, and slower planes, permitting longer 



APPARATUS 385 

exposures in inverse ratio to their speed, will be chosen for 
photographic purposes. 

A development which is sure to intrigue many investiga- 
tors is the gyroscopically controlled camera. This has its 
chief raison d'Ure in precision mapping, whose possibilities 
from the air will undoubtedly be intensively studied at once. 
With the automatically leveled camera will come renewed 
attention to indicators of time, altitude, and direction, with 
the ultimate goal of producing aerial negatives that show 
upon their face the exact printing and arranging directions 
necessary to put together an accurate map. 

Sensitive Materials. — Manufacturers of plates and films 
will direct efforts toward producing emulsions cf good con- 
trast, high color sensitiveness and high effective speed, 
especially when used in conjunction with the filters neces- 
sary for haze penetration. Exposure data will be accumu- 
lated and exposure meters appropriate for aerial work will 
be developed. 

Color Photography. — Color photography from the air by 
any of the screen-plate or film-pack methods is probably 
out of the question because of the long exposures required. 
The screen-plates are unsuitable also because of the relatively 
large size of their grain compared to the detail of the aerial 
photograph. Ordinary three-color photography, using three 
separate negatives, is always subject on the earth's surface 
to the difficulty that the three negatives must be exposed 
from the same point of view, either in succession or by means 
of some optical arrangement which is costly from the stand- 
point of light. In photographing from the air this difficulty 
of securing a single point of view for the three photographs 
is absent. Three matched cameras, side by side in the fusel- 
age, have identical points of view as far as objects on the 
earth below are in question. Consequently, three-color 
£5 



386 AIRPLANE PHOTOGRAPHY 

negatives are entirely possible, and indeed will be simple 
to make as soon as plates of adequate color sensitiveness and 
speed are available. Probably the new Ilford panchromatic 
plate has the necessary qualities. 

Night Photography. — The searching eye of photography 
was so omnipresent in the later stages of the Great War 
that extensive troop movements and other preparations had 
to be carried out either in photographically impossible 
weather or else at night. The natural reply to the utilization 
of the cover of night is to "turn night into day" by proper 
artificial illumination. At first thought it might well appear 
that the task of- illuminating a whole landscape adequately 
for airplane photography is well-nigh hopeless by any artificial 
means. On one hand we have the very short exposures alone 
permissible; on the other the fact that the intensity of day- 
light illumination is overwhelmingly greater than those com- 
mon in the most extravagant forms of artificial illumination. 

Toward the close of the war, however, actual experiments 
made with instantaneous flashes of several million candle- 
power showed that if proper means were provided to insure 
the flash going off near the ground, and if its duration were 
made no longer than about -^ second, interpretable photo- 
graphs were obtainable on the fastest plates. It appears, 
therefore, merely a matter of manufacturers perfecting the 
technique of flash production, and of inventors providing 
the launching and igniting devices to push this kind of 
photography to the practical stage. The achievement of 
night photography cannot fail to have an enormous effect 
on future tactics. 

The technique of night photography may take either of 
two directions. On one hand we may develop flashes of the 
requisite intensity to give all their light in -^^0 second; on 
the other hand, it may prove more feasible to use flashes of 



APPARATUS 



387 



longer duration and to arrange for the camera shutter (of the 
between-the-lens type) to be exposed synchronously with 
the middle of the flash. One way, frequently suggested, to 
use these longer flashes would be to trail the charge on a long 
wire, through which the ignition is effected electrically. This 
is not likely to be satisfactory, however, for the resistance of a 
wire is so great that when the plane flies at any practical 
height, the trailed flash, if it reaches near the ground, will 
be forced to a very great distance behind. Probably the 
best solution will involve accurate synchronizing of the fuse 
in the freely dropped sack of flash powder with the exposing 
mechanism in the camera. 



CHAPTER XXXII 

PICTORIAL AND TECHNICAL USES 

Aside from their element of novelty, aerial photographs 
have undouted qualities of beauty and utiHty. The "bird's- 
eye view" has always been a favorite for revealing to the 
best advantage the entire form and location of buildings and 




Fig. 188.— Rheims Cathedral. 



of other large objects. Heretofore such views have usually 
had to be drawn by an imaginative artist. 

Aerial oblique views possess the virtues both of pictures 
and of plans. They are destined to be extensively used in 
the study of architecture (Eig. 188), Cathedrals, castles. 



USES 



389 



town halls, particularly those still in their cramped medieval 
surroundings where they can never be seen in their entirety 
from the ground, come forth in all their beautiful or quaint 
proportions from the airman's point of vantage. Stereoscopic 
aerial views are destined to occupy a valuable position also. 
Stereo prints of the famous buildings of Europe, taken from 




Fig. 189. — A portion of Vienna seen from the air, during a "propaganda raid." 
Italian official photograph. 



the air, will give to the prospective traveler or the arm-chair 
tourist a many fold more accurate idea of their construction 
than will any number of mere surface views. 

A vertical aerial photograph is most closely akin to a 
map, but has advantages over any ordinary surveyor's 
product. As a guide it is infinitely superior to the best 
draftsman's diagram, for it provides a wealth of detail 
whereby the traveller may definitely locate himself. At a 



390 



AIRPLANE PHOTOGRAPHY 




USES 



391 



<< 

& 




392 



AIRPLANE PHOTOGRAPHY 



single glance he notes the objects of interest within his radius 
of easy travel. The guide-book of the future will therefore 
be incomplete without numerous aerial views, both vertical 
and oblique. As an illustration of the peculiar merit of the 
view from the air, consider the photograph of Vienna made 
during d'Annunzio's "propaganda" bombardment. Or the 




Fig. 192. — A sea-side resort. 



picture of the Rialto bridge (Fig. 190). No ordinary photo- 
graph from land or water suggests the central roadway and 
no map shows the beauty of its elevation. Both are shown 
here, as well as an intimate view of the arched and pillared 
courtyard of the Fondaco de' Tedeschi to the right. 

Airplane photographs will undoubtedly be widely used 
in certain fields of advertising. Architects and real estate 



USES 



393 



agents may be expected to display their wares by the aid 
of aerial views. A well-planned country estate or goli course, 
or a suburban development (Fig. 191), can be shown with a 
completeness, both as to environment and stage of progress 
which no other form of representation can approach. A sea- 
side resort can now show the extent and grouping of its 




Fig. 193. — A bathing beach seen from the air. 



natural and artificial amusement features in a single picture 
(Fig. 192). Even the extent of its bathing beach under water 
is revealed to the aerial photographer (Fig. 193). Real 
estate agents can utilize aerial photographic maps of cities 
to great advantage. On these their properties can be pointed 
out, with the nature of their surroundings shown at a glance, 
together with their relation to transportation, schools. 



394 



AIRPLANE PHOTOGRAPHY 




Fig. 194. — Mt. Vernon from the air. 



churches, shopping districts, parks, or factories. The future 
purchaser of lots in a distant boom town will no longer be 



US.ES 



3^^ 



satisfied with a map outlining the streets with high-sounding 
names. He will demand an authentic aerial photograph, 
showing the actual number of houses under construction, the 
streets, gutters and sidewalks already laid, the size and 
planting of trees. 




Fig. 195. 



-A contrast in roofs. The Capitol retains its individuality, while the White House loses 
all character when seen from above. 



The study of landscape gardening is another field for 
which the aerial photograph is peculiarly fitted. A collection 
of oblique pictures of the chateaux and palaces of Europe 
showing their approaches and grounds, or of the historic es- 
tates of our own South, (Fig. 194), will be worth more to the 
prospective designer of a country estate than maps and 
ground pictures can ever be. Closely allied to landscape 



396 



AIRPLANE PHOTOGRAPHY 



gardening is city planning, for which the aerial map will be 
quite indispensable. The appearance of a city from the air 
may indeed become a matter of pride to its inhaj)itants, and 
not only the arrangement of streets and parks, but even the 
character of the roofs of the buildings, be the subject of 
study (Fig. 195). 




Fig. 196. — An aviation field under construction; early stage. 

Engineers and constructors will depend more and more 
on preliminary photographic surveys as a basis for locating 
their operations. At the later stages of their work they will 
use aerial photographs for recording progress. Periodic 
photographs of buildings in process of construction, such as 
are now made from the ground, are much more illustrative 
when made from the air. Only from above is it possible to 
obtain in a single picture the progress of the complete project, 



USES 



397 



such as the construction of an aviation field (Figs. 196 and 
197) or of a shipyard. The building of large structures — 
bridges, hotels, ships on the stocks — particularly demands 
aerial views if the foreground is not to eclipse the center of 
real interest. 




Fig. 197. — An aviation field under construction; later stage. 

News events will soon call for an aerial photographic 
service. Already we are seeing newspapers and magazines 
featuring aerial photographs of the entry into conquered 
cities and the parades of returning fleets. Accidents, fires, 
floods and wrecks, of either local or national interest, can 
best be represented by this newest form of photography. 

The photographing of wrecks, fires and floods suggests 
the importance of aerial views to insurance underwriters. 



398 



AIRPLANE PHOTOGRAPHY 




USES 



399 



wto require the most minute information on the character- 
istics of buildings in every neighborhood, and on the extent 
and nature of damage done. Marine insurance companies 
might with profit use the airplane camera to help estimate 




Fig. 199. — Waves set up by a ship — of interest to the naval architect. 

the chances of salvage of a stranded ship or a vessel 
foundered in shallow waters (Fig. 181). 

Numerous scientific uses for aerial views seem likely. 
Prominent among these is their use in geology, for the study 
of the various forms of earth sculpture. Pictures from the 
air of extinct volcanoes will give information as to their 



400 AIRPLANE PHOTOGRAPHY 

configuration that would otherwise require months of pains- 
taking survey to obtain. Aerial photographs of active vol- 
canoes (Fig. 198), showing the results of a succession of out- 
bursts — one obliterating the other — would prove of the great- 
est value, especially when studied in conjunction with other 
scientific data, the whole making a record unobtainable by 
any other means. 

In earthquake regions — notably Southern Italy and 
Japan — ^the changing coast lines, shallows and safe harbors, 
could be promptly ascertained after the subsidence of each 
fresh shock, with a consequent keeping open of trade routes 
and often the saving of life. River courses, glacier forma- 
tions, canons, and all the larger natural formations which 
man usually sees only in minute sections, and which he 
must build up in his mind's eye or by models, are today 
quickly and accurately recorded by the camera in the air. 
Such formations as coral reefs, whose configurations can 
now be accurately learned only by laborious surveys of a 
limited number, could be studied in quantity and with here- 
tofore unknown satisfaction as the result of a single expedi- 
tion with a ship-carrying seaplane and aerial camera. 

Another scientific field — probably one of many similar 
ones — lies in the study of the waves set up by ships (Fig. 199). 
These are of extreme importance in the realm of naval 
architecture, but before the day of the airplane could never 
be easily studied in full scale. 



CHAPTER XXXIII 
EXPLORATION AND MAPPING 

Aerial photographic mapping in war-time has been 
almost entirely confined to inserting new details in old 
maps. For such work some distortion or a lack of complete 
information on altitude and directions is not a serious mat- 
ter, because the known permanent outlines serve as a 
basis. Furthermore, in so far as outline maps are concerned, 
as distinguished from pictorial maps, these have been drawn 
on the ordinary scales, and with the ordinary conventions of 
engineering map practice. 

Aerial photography may be used in the future in prac- 
tically the same way, as an aid to the quick recording of 
those minute details which would ordinarily consume an 
enormous amount of labor to survey directly. The region 
shown in Fig. 200 affords a good illustration. A discouraging 
amount of time and effort would be required to map this 
section of Virginia by the usual methods, while the smallest 
curve of creek and shore is instantly and completely recorded 
on a single photographic plate. But there are other possibili- 
ties, diverging from this application both toward greater 
and lesser requirements for precision. 

Pictorial maps, in which the actual photographs figure, 
promise to be an essential part of the airman's equipment, 
whether he be pilot or passenger, mail carrier or sports- 
man. Without any pretention to detailed accuracy of 
location, these maps will show, in strip or mosaic form, the 
general appearance of the country to be traversed, with 
particular reference to good landing fields and other points 
of interest to the aviator. Vertical pictorial maps may be 

26 



402 AIRPLANE PHOTOGRAPHY 

supplemented by obliques giving the view ahead, whereby 
the pilot may direct his ship. Thus the Washington monu- 
ment as seen by the pilot from Baltimore is a truer guide 
than is the country beneath him. The crossing of mountain 
ranges is another case where the oblique picture will be more 
useful than the vertical (Fig. 201). 




Fig. 200. — ^An aerial photographic survey of ground difficult to cover by ordinary surveying methods. 

Contrasted with the merely pictorial maps will be pre- 
cision surveys. Whether it will prove practical to make 
these entirely from the air is still an open question. It is 
to be assumed that cameras can be constructed with lenses 
having negligible distortion of field, with between-the-lens 
shutters to obviate the distortions due to the focal-plane 
type, with auxiliary devices for indicating compass direction, 



EXPLORATION AND MAPPING 403 




404 AIRPLANE PHOTOGRAPHY 

altitude, and inclination, or with gyroscopic mounting so 
that an inclination indicator is unnecessary. The applica- 
tion of aerial photography to precision mapping will depend 
upon the perfection which such cameras attain, as estimated 
by the permissible errors in this form of mapping. Entire 
dependence on photography, as in uncharted regions, is 
likely to be worked up to slowly, beginning with a stage 
of rather complete triangulation of natural or artificial 
points — say three in each constituent picture — then through 
several stages each successively employing fewer and fewer 
well determined points. The photographic mapping of 
some of our Western States will be greatly facilitated 
by the 100-yard squares into which the land is divided 
and already marked in a manner which shows clearly in 
aerial photographs. 

A theoretical possibility is the plotting of contours from 
stereo-aerial pictures. Given two elements of a stereo- 
scopic pair, taken from points whose separation is known, 
the position of any point in space shown in the stereoscopic 
view can be determined by the use of the stereocomparator. 
This is an instrument already employed in mountain photo- 
surveying, which consists essentially of a compound stereo- 
scope in whose eye-pieces are two points movable at will 
so that the relief image formed by their fusion can be made 
to coincide with any chosen part of the landscape. 
The chief difficulty in the application of this idea to aerial 
work is to fix the base line. This problem may be met in 
some cases by using stereo obliques, and getting the base 
line by simultaneously made vertical photographs of well 
surveyed territory beneath. Possibly also methods can be 
developed by which photographs from two or more known 
altitudes may furnish the requisite data. 

City mapping is a field for which aerial photography is 



EXPLORATION AND MAPPING 405 



:3 

B 




4.06 



AIRPLANE PHOTOGRAPHY 




Fig. 203. — Mosaic map of the City of Washington. White rectangle shows portion included in 

next figure. 

peculiarly fitted (Fig. 202). A complete map of a large 
city is a labor of years. In fact, a modern city is always 



EXPLORATION AND MAPPING 407 




Fig. 204. — ^Portion of Washington mosaic, full size. 

dangerously near to growing faster than its maps. An 
aerial map, on the contrary, can be produced in a few hours. 
Paris was mapped with 800 plates in less than a day's actual 
flying. Washington was completely mapped in ^14. hours. 



408 AIRPLANE PHOTOGRAPHY 

with less than 200 exposures. The entire map is shown, on a 
greatly reduced scale, in Fig. 203, while Fig. 204 shows a 
small portion of it in full size, from which can be obtained an 
idea of the dimensions of the original. These maps, while 
not accurate enough for the recording of deeds and mort- 
gages, yet serve the majority of needs. There is indeed no 
reason why with long focus cameras, given several accurately 
marked points, the photographic map of a piece of real 
estate should not be made with all the accuracy needed, still 
leaving the whole process of partial surveying helped out by 
photography an enormously simpler one than the usual method . 
Rougher types of surveying, in open country, offer a 
most promising opportunity. Railway surveys, showing 
the character of the country: passes through mountain 
ranges : the available timber and other materials of construc- 
tion . Canal routes , with the available sources of water supply , 
and the best choice of course to avoid deep cuttings and 
aqueducts. Irrigation projects, with the natural lakes, 
river courses and valleys, which may be dammed to form 
storage basins. Coast, river and harbor surveys are possible 
by aerial means with a promptness and frequency which 
should entirely revolutionize the making of maps of water- 
ways. Shifts in channels and shallows, even of considerable 
depth, stand out prominently in the aerial photograph. 
The actual bottom, if not more than three or four meters 
down — as in a bathing beach — shows in the aerial photo- 
graph (Fig. 193), while the varying surface tints caused by 
light reflected from the bottom at far greater depths are 
readily differentiated by the camera from the air. An 
instantaneous photograph will thus perform the work now 
done by a week's soundings. Fig. 205, taken near Langley 
Field, shows how the aerial photograph may be used to chart 
natural channels, while Fig. 206 shows the dredged chan- 



EXPLORATION AND MAPPING 409 




Fig. 205. — Shallows and channels revealed by the aerial photograph. 



410 



AIRPLANE PHOTOGRAPHY 




EXPLORATION AND MAPPING 411 




412 AIRPLANE PHOTOGRAPHY 




Pi S 






I >! 

o' "^ 



EXPLORATION AND MAPPING 413 

r.els of the port of Venice. Navigation of such a river as the 
Mississippi with its shifting bars may come to be guided by 
monthly or even weekly aerial photo maps. 

Among other uses for aerial photography will be the loca- 
tion of timber. As one illustration, may be taken the dis- 
covery of mahogany trees. Their foliage at certain times of 
the year is of characteristic color. This may be recorded 
on color sensitive plates with a scientifically chosen filter, 
and the cutting expedition sent out with the photograph as 
a guide. In this as in other cases where rough or miexplored 
country is to be covered, it is a question whether the air- 
plane will after all be the most feasible craft, on account of 
its necessarily rapid rate of travel, and its need for known 
landing fields. The dirigible of large cruising radius, which 
can seek its landing field at leisure, is probably indicated for 
this kind of work. It may indeed, as already hinted, prove 
to be the chief photographic aircraft of the future. 

Archaeological surveys offer a fascinating opportunity 
for airplane or dirigible balloon photography to render 
scientific service. Buried in desert sands or overgrown with 
tropical vegetation the ancient cities of Asia Minor, of 
Burma, and of Yucatan evade discovery, and even when 
found remain unmapped for decades. Discovery and map- 
ing can now go hand-in-hand. The topography of barbaric 
or colonial towns and villages, whose importance could 
not warrant elaborate surveys, but which should neverthe- 
less be a matter of record, will be quickly and easily plotted 
by photography (Fig. 207). To this day who knows how 
the streets run in Timbuctu, and how, save from the air, can 
we ever map the teeming cities of China? He who would fol- 
low in the footsteps of Haroun-al-Raschid can even now ex 
plore the by-ways of Bagdad by the aid of the Royal Air 
Force photographic map! 



INDEX 



Aberrations, lens, 47, 54 
Aberration, spherical, 47, 54 

chromatic, 48, 54 
Acetylene light, 284 
Advertizing, use of aerial photography 

in, 392 
Aero 1 and Aero 2 filters, 241 
Airplane, as camera platform, 20 

types of, 24 
Air speed, 27, 308 

indicator, 33, 34 
Alcohol, use of in plate drying, 275 

use of in print drying, 286 
Altimeter, 33, 174. 

reading recorded on film, 135, 171 
Altitudes of flight for photography, 61, 

224 
Anastigmatic type of lens, 47 
Aperture, lens, 39, 44, 56, 57 
Archaeological surveying, 413 
Astigmatism, 49, 50, 54 
Auxiliaries, camera, 163 

installation of, 214 

Bagley camera, 286, 314 

Balance, of camera, 96 
of plane, 157, 208, 217 

Balloons, 15, 16, 18 

Banking, 27, 324 

Batteries, storage, 150, 156 

Bay, camera, 120, 210 

Bellows, camera, 65, 95 

Biplane, 24 

Blast marks in front of guns, 363 

Bleaching out print to leave interpreta- 
tion marks, 367 

Boat, flying, 25, 374 



Bowden wire, 103, 106, 108, 111, 114, 

116, 118, 126, 129, 136, 163 
Brightness, range of, 221, 225 
Burchell photographic slide rule, 303, 

339 

Camera, airplane, 39, 384 

automatic, 18, 43, 90, 116, 124, 125,311 

B. M., 120, 203 

Bagley, 286, 314 

Brock automatic plate, 126 

C type, 43, 87, 103, 109 

classification of, 43 

deMaria, 86, 89, 103 

deRam, 82, 93, 121, 129, 157, 205, 

214, 326 
E type, 43, 87, 103, 109 
elements of, 42 
film, see Film cameras 
Folmer automatic plate, 126 
hand held, 95, 321, 369 

English, 99 

German, 99 

U. S. Air Service, 100 
lea, 103 

Lamperti (Italian), 112, 211 
L. B., 120 

long focus, 103, 301, 324 
L type, 43, 82, 94, 102, 117, 162, 210 
M., 106, 203 
non-automatic, 43, 102 
Piserini and Mondini, 111 
semi-automatic, 43, 116, 149 
stereoscopic, 341 
Camouflage, filters for the detection of, 
225, 243 
stereoscopic views and, 329, 358 

415 



416 



INDEX 



Ceiling of plane, 27, 130 

Channels, detection of by photography, 

408 
Chemicals, photographic, 257 
Chlorhydrochinon developer, 261 
Chromatism, lateral, 49, 54 a 
City planning, use of aerial photography 

in, 396 
Clock-work for driving cameras, 149, 155 
Clouds, 224, 242 
Collimator, 66 

Color, coeflBcient or index of negative, 
259 

filters, see filters 

photography 385 

sensitive plates, 15, 174, 233, 237 

sensitiveness of film, 131 

sensitizing, methods of, 235 
Coma, 47, 48 

Communication, means of on plane, 296 
Compass, 33, 173 

reading recorded on film, 135, 171 
Cone, camera lens, 42, 114 

interchangeable, 108, 120 
Construction operations, aerial photo- 
graphic records of, 396 
Contact, imperfect in printing, 279, 284 

prints, 45, 279 
Contacts, electric, on plane, 163 
Contours by stereo aerial photography, 

404 
Control, distance, of camera, 110, 163 

speed, of camera, 136, 144, 157 
Controls, duplicate, 19, 25, 195, 209, 214 

of plane, 21, 26 
Convergence point, 337 
Cord for adjusting shutter aperture, 82 
Core rack development, 271 
Contrast, in brightness on earth's 
surface, 221 

in photographic emulsions, 15, 230, 
236, 258 



Counter, exposure, on magazine, 88 

on release, 164 
Covering power of lens, 44, 49, 50, 58 
Crabbing, 27, 308 
Cradles, camera, 195 
Cross wires, 23 

insertion of camera through, 42, 210 
Curtain, auxiliary shutter, 80, 84, 106 

speed of travel of shutter, 74 

uniformity of, 76, 82, 84, 86, 315 
Cylinders, relation between vibration 

and number of, 185 

Dark slides, double, 87, 99 
Daylight, intensity of, 222 
Definition, lens, 44 

Density, of air, effect of on propeller, 
154 
of photographic image, 228 
Developers, for plates and films, 257, 260 

for papers, 262 
Developing machines, film, 133, 273 
Ansco, 273 
Brock, 274 
Eastman, 274 
G.E.M., 273 
Development, core rack method of, 271 
factor, 228 
film, 272 

methods of, 267, 269 
of prints, 286 
speed of, 236, 267 
tank, 270 
time, 269 
DH4 plane, 210, 217, 296 

photographic, 213 
DH9 plane, 296 

Diaframs, to equalize illumination of 
plate, 78 
lens, 48, 58 
Dilution coefficient of developer, 258 
Dirigibles, 15, 16, 413 



INDEX 



417 



Distortion, absent with between-the-Iens 
shutter, 74 

barrel, 51 

due to camera tilting, 206, 286, 305, 
315 

in aerial maps, 317 

lens, 39, 44, 51, 54, 56, 62 

pin-cushion, 51 

produced by film shrinkage, 237 

produced by focal plane shutter, 74 

produced by glass plate in front of 
film, 131 

with wide angle lenses, 63 
"Dodging" in printmg, 279, 315 
Doors in plane for camera to work 

through, 214 
Drying of films, 267, 276 

of plates, 267, 275 

of prints, 286 

Earth, appearance of from plane, 30 

Eastman apron film developing machine, 
274 
twin reel film developing machine, 
274 

Efficiency, propeller, 155 
shutter, 70, 72, 76 

English aerial photographic practice, 
45, 46, 283, 291, 340 

EK filters, 241 

Electric, drive for cameras, 116, 119, 
123, 145, 149 
generator operated by motor, 146 
motor, characteristics of, 151, 156 
motor, service, 163 

Elevation possible to detect in stereo- 
scopic views, 340 

Emulsions, photographic, characteris- 
tics of, 227 

Enlarging, 45, 279, 283 
camera, 283 
27 



Enlarging versus cgntact printing, 59 
Exhaustion of developer, 259 
Exploration, use of aerial photography 

in, 401 
Exposure, data charts, 250 

distance between for mosaic maps, 
307 

distance between for stereos, 336 

estimation of, 248 

limitations to, 247 

meters, 251 

meter, Wynne, 251 

of aerial negatives, 247 

relation between motion of plane and, 
68, 185 

under, period, 230 

Field, angular, of lens, 5.7, 302 

flatness of lens, 56 
Field laboratory, mobile, 268 
Film cameras, 43, 130, 134 

Brock, 138 

Duchatellier, 131, 136 

F type, 134, 171 

G.E.M., 138 

German, 64, 76, 139, 317 . 

K type, 142, 203, 214 
Film, celluloid, 130 

backed, 133 

changing in the air, 137, 144 
color sensitiveness of, 237 
cut, 275, 278 
development of, 130, 272 
drying, 131, 276 
means for holding flat, 130, 131 
relative performance of compared to 

plates, 237 
satisfactory kinds for aerial work, 238 
shrinkage, 237 
Filters, 15, 58, 106, 174, 224, 233, 239, 

241, 243 
effects secured by use of, 241 



418 



INDEX 



Filters, gelatin, 244 
glass, 245 

holders for, 67, 106, 224 
ratio, 240 
Fixing bath, 262 

Flaps, auxiliary to shutter, 80, 84, 85, 
98, 99, 101, 119 
to protect lens, 214 
Flash lights, 18, 386 
Flying boat, 374 

Focal length, relationship of lens char- 
acteristics to, 55, 57 
requirements for, 39, 42, 58, 61, 103 
Focal plane, 50 
shutter, 70, 71 
distortion by, 74 
performance of, 86 
types and representative, 80 
Focus, depth of, 44 

effect of temperature on, 41, 65 
fixed, in aerial cameras, 40 
ground and air, 65 
Focussing, automatic, 284 

by parallax, 65, 66 
Fog, atmospheric, 224 

photographic, 237, 258 
French aerial photographic practice, 45, 

46, 283 
Friction disc speed control, 136, 159, 160 
Fuselage, 21, 87 ; 

shape and size depending on type of 
engine, 23 
Future of aerial photography, 383 

German aerial photographic practice, 

63, 103, 209 
Glass, optical, used in lenses, 44 
Gloves, handling apparatus through, 41, 

89 . . 

Governor for camera speed, 159 
Graphite, use of to prevent static dis- 
charge, 134 



Gravity, action of in aerial cameras, 41, 
112, 115, 119 
center of, should not change in 

cameras, 125, 207 
change of center of, in magazines, 92, 

112 
depended on in magazines, 88 
handles best at center of, 96 
pseudo, in moving vehicle, 188 
support at center of, 182, 203 
Ground speed, 27, 307 

indicator, 311 
Guide-books, illustration of, 15, 392 
Guides for inserting magazines, 41 
Gyroscope, 189 
Gyroscopically controlled instruments, 

29, 174, 192, 312 
Gyroscopic erector, 188 

mounting of camera, 187 
stabilizer. Gray, 190 

Hardener, acid, 262 

Hand operation of deRam camera, 121, 

125, 129 
Haze, 15, 30, 223, 233, 239 
Head resistance and weight, equivalent, 

156 
Heat, effect of on plate sensitiveness, 

175, 232 
Heater, electric, in camera, 142, 174, 232 
Horizon, photography of to indicate 
inclination, 174 

position of, 30 
Hurter and Driffield sensitometric 

curve, 227 

Illumination of field by lens, 50, 54, 56, 

57 
Image of point source, size of, 49, 54, 56 

size of in relation to focal length, 59 
Incidence, angle of assumed by plane at 

high altitudes, 206 



INDEX 



419 



Inclinometer, 33, 35, 171, 173 
Indicators, distance, 163, 295 
Inertia, 229, 257 
Installation, camera, 24, 208 
Instructions, operating, to be placed on 

apparatus, 42 
Instruments, airplane, 30 
Instrument board, 30 

photographing, 170 
Intensification of aerial negatives, 262 
Intensity of daylight, 222 
Interpretation of aerial photographs, 17, 

40, 351 
Interval between exposures, 40, 158, 
305, 307 

for stereoscopic pictures, 334, 338 

methods of regulating, 124 
Isochromatic plates, 233 
Italian photographic practice, 122, 377 

Jamming of cameras in operation, 119, 
120 

Ki and K2 filters, 239 

Keeping power of developer, 259 

Kites, 15, 16, 18 

Laboratory, mobile photographic, 269 
Landscape gardening, use of aerial pho- 
tography in, 395 
Latitude of plates, 229 
Lens, 42, 39, 44, 383 

aperture, 39 

characteristics, 46 

mounts, 65 

suitable for aerial photography, 62 

symmetrical, 52 

telephoto, 61 

testing and tolerances, 52 

unsymmetrical, 52 

wide angle, 62 
Levels, spirit, on camera, 95 



Light, distribution of in aerial view, 221 
trail method of testing camera mount- 
ings, 183 
Loop, centrifugal force in, 29 

Machine gun ring as camera mount, 321 
Magazines, 87 
bag, 88, 101 
Bellieni, 92 
Chassel, 92 
deMaria, 89, 98 
Ernemann, 89 
Folmer, 90 
Fournieux, 92 
Jacquelin, 92 
Piserini and Mondini, 90 
Ruttan, 92 
Magazine racks, 94 

installation of, 217 
Mapping, 64, 135, 185, 186, 304, 401 

precision, 317, 404 
Maps, mosaic, 17, 39, 64, 314 

sketch, 314 
Marking of negatives, 278 
Metol-hydrochinon developer, 261 
Mirrors for oblique photography, 324, 

326 
Monoplane, 24 
Motions of camera, 179 
Motive power for aerial cameras, 145 
Mounting, camera, 102, 193, 179, 183 
384 
bell-crank, 120, 198, 203 
Brock, 139, 207 
center of gravity, 205 
floor, 195 

G. E. M., 138, 207 
Italian, 207 
outboard, 194 
parallel motion, 198 
pendular, 185 
tennis ball, 196 



42a 



INDEX 



Mounting, camera, turret, 312 

of prints, 316 

of stereograms, 346 
Movement, of film during exposure, 75, 
142 

of image, permissible, 68 
Moving pictures from plane, 350 
Mud splashing on camera, 214 

Naval aerial photography, 368 
Negative lens sight, 168, 299, 305 
Night photography, 386 
Numbering devices in cameras, 169 

Oblique views, 39, 320 

angles at which taken, 40, 321 
exposures for, 69 
filters for, 242 
use of hand cameras for, 95 
Observer, function of in aerial pho- 
tography, 291, 295, 304 
Oil spray from motor, 214 
Opacity, 228 
Opening for camera, 211 
Opposite directions, shutter to move 

alternately in, 76, 139 
Orthochroraatic plates, 233 
Overlaps, for mapping, 307 
for stereoscopic views, 40 
on a turn, 64, 139 

Panchromatic emulsions, 233 

plates, 49, 238 
Panoramic views, 321 
Parallax method of focussing, 65, 66 
Parallel, flying in, 308 
Photographic planes, special, 213 
Pilot, function of, in aerial photography, 

291, 295 
Pin-points, 39, 291 
Pistol grip for hand cameras, 95, 96 
Plate holders, 87 



Plates, bathed, 235 

behavior of compared to film, 237 

color sensitive, 233 

iso and ortho-chromatic, 233 

panchromatic, 233 

satisfactory kinds for aerial work, 238 

seK screening, 245 

shape of, 63 

size of, 43, 62 
Plumb line, behavior in banking plane, 28 
Ply- wood veneer construction, 23, 211 
Positype paper, 238 
Potassium carbonate for drying plates, 

276 
Power required to drive cameras, 125 
Pressure, of shutter curtain, 131 

plate, for holding film flat, 131, 141 
Printing, 279 

contact, 279 

machines, 279 

media, 252 
Prints, paper, 253 

development of, 286 
Prisms for oblique photography, 324, 

326 
Propeller, characteristics, 152 

constant speed, 129, 159 

drive for cameras, 102, 116, 119, 135, 
136, 144, 157, 158 

position of, 120 

variable speed, 159 
Pump, for producing suction on film, 132 
Punch marks on film, 144, 272 
Pyro developer, 261 

Racks, negative, 271 

Real estate, aerial mapping of, 393 

Rectifying, 286, 305 

camera, 314 
Reduction of aerial negatives, 262 
Release, shutter, 96, 99 

duplicate, for pilot, 164 



INDEX 



421 



Release, shutter, time controlled, 124 
Relief, criterion for correct, 335, 344 
exaggerated, 337 

impression of, produced by motion, 
349 
Resolving power of plates, 59, 235, 260 
Richard stereo printing frame, 283, 348 
Rinsing of plates, 259 
Rubber, sponge, use of in camera mount- 
ings, 195, 203 

Safe lights, photographic, 269 

Safety catch on camera mounting, 203 

device on camera driving mechanism, 
162 
Salvaging of ships, aerial photography 

and, 399 
Seaplane, 25, 374 
Self screening plates, 245 
Semaphore signalling in plane, 297 
Semperfocal enlarging camera, 284 
Sensitized materials, requirements for, 

225 
Sensitometry, 227 

of papers, 253 
Shadows, compass directions from, 356 

proper direction for, in examining 
prints, 352 
Shaft, flexible, 119, 123, 142, 161 
Sheaths, plate, 87, 88, 93, 126, 170 
Shrinkage, film, 237, 317 

paper, 285, 315, 317 
Shutter, 42, 68 

between-the-lens, 58, 70, 112, 115, 
316, 387 

efficiency, 70, 72, 76 

focal plane, 70, 71, 73 

focal plane, double for stereo work, 
341, 345 

focal plane, moving alternately in 
opposite directions, 76 

focal plane, types of, 80 



Shutter, Folmer, 80 

lea, 81 

Klopcic, 78, 84, 98 

release, 96, 99, 124 

speed, 39, 40, 58, 70, 249 

testing, 76 

testing apparatus, 77 
Sights, 164, 166, 296, 301, 327 

adjustable for angle of incidence of 
plane, 169 

attached to plane, 167 

negative lens, 168, 299, 305 

rectangular, 98, 167, 373 

stereoscopic, 338 

to indicate size of field, 166, 373 

tube, 101, 166 
Single-seaters, carrying cameras in, 114, 

211 
Slide rule, photographic, 303, 339 
Solenoid, 151, 163 
Sound, not to be used for indication in 

plane, 164 
Spacing of pictures in film camera, 144 
Speaking tubes on plane, 296 
Speed, of development, 236, 267 

of plates, 228, 236 
criteria of, 230 
effect of temperature on, 175, 232 

variable, control of camera, 144, 151 
Spotting, 64, 125, 291 
Spring motors, 149, 155 
Springs, use of in aerial cameras, 41 

in magazines, 88 
Stabilized camera, 95, 187 
Static electric charges on film, 131, 133 
Stereocomparator, 404 
Stereo-oblique views, 115, 321, 343 
Stereoscopes, 331 
Stereo printing, 283 
Stereoscopic cameras, 341, 344 

effect; absence of at flying heights, 30, 
334 



422 



INDEX 



Stereoscopic photography, 329 
mounting, 346 

pictures, fusion of without instru- 
ments, 330, 343 
sights, 338 
views, 39, 40, 64 

uses for, 348 
vision, principles of, 329 
Stop-watch attached to shutter release, 

310 
Strap, on hand camera, to go around 
observer's neck, 99 
on plate magazine, 87 
to go over hand, on hand camera, 100 
Stream lines, 26 

lined hood, 214 
Suction for holding film flat, 131, 132, 
142 
advantages of continuous and inter- 
mittent, 132 
Surveying by aerial photography, 401 

Tank development, 270 

Tearing edges of prints, 317 

Telephones on planes, 296 

Telephoto lens, 61 

Temperature, coefficient of develop- 
ment, 258 
effect of on focus, 41 
effect of on mechanical functioning, 

41, 102, 125 
effect of on plate speed, 232 
limits of development, 258 

Test chart, lens, 52 

Threshold value, 230 

Thrust, propeller, 152 

Timber, location of by aerial photog- 
raphy, 413 

Tone rendering, correct, 226, 230, 239 

Touch, sense of, not dependable in 
plane, 41, 125, 164 



Trailer, photographic truck and, 268 
Transmission of power to camera, 161 
Transparencies, 252, 330 
Transparency, 227 
Trays, camera, 195 
Tri-color ratio, 235 
Triplane, 24 

Tuning fork, used in shutter tester, 79 
Turbine, wind, for driving camera, 116, 
127, 144, 147, 158 

Uniformity of curtain speed in focal 

plane shutter, 76, 82, 84, 86, 315 
Unit system of camera construction, 42 
Uprights, camera, in plane, 209 
Uses for aerial photography, 388 

Velocity constant, 258 

Veneer construction, 23, 209, 211 

Venturi tube, 132, 142, 144 

Vibration, 16, 18, 26, 40, 41, 58, 102, 179 

Volcanoes, photography of, 15, 399 

Water for mixing chemicals, 262 
Watkins factor, 258 

Weight and head resistance, equiva- 
lence, 156 

of film compared to plates, 101, 130 

of deRam camera, 123 

of hand cameras, 96 

of K type camera, 144 

of K film roll, 144 

of L type camera, 117 

of M type camera, 109 

of storage batteries, 157 
Wind, flying against, 68, 308 

motor, 146 
Windows in side of plane, 214, 328 
Wire, barbed, appearance of in aerial 

photograph, 360 






FACULTY OF FORESTRY 
WIIVERSITY OF TORONTO 



TR 

810 

183 



Ives, Herbert Eugene 
Airplane photography 



IVES, H.E. 



TR 

ma 



AUTHOR 



Airplane photography > 



183 



TITLE 



ri'S6168) 



DATE 



ISSUED TO 



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