The Photographic Negative Volume 3

125196

THE PHOTOGRAPHIC NEGATIVE

The

PHOTOGRAPHIC
NEGATIVE

HERBERT C. McKAY F. R. P. S.

IN FOUR VOLUMES
VOLUME 3

LITTLE TECHNICAL LIBRARY

COPYRIGHT, 1942 BY ZIFF-DAVIS PUBLISHING COMPANY

ALL RIGHTS RESERVED, NO PART OF THIS BOOK MAY
BE REPRODUCED WITHOUT PERMISSION* IN WRITING,

ZIFF-DAVIS PUBLISHING COMPANY CHICAGO NEW YORK

PRINTED IN THE U. S. A.

3.

Table of Contents

VII Chemical After-Treatment 40

Negative Characteristics; Methods of After-Treatment; In-
tensification; Reduction, Handwork; Use of Glass Plates
for Enlargements; Retouching with a Brush, Retouching
with a Pencil; Retouching to Get Cloud Effects.

VIII Characteristics of the Negative 46$

Development and Contrast; Defining the Terms: The Soft
Negative; Detail, Texture: Color; Resolving Power and
Grain; Negatives to Fit Your Needs; Thin vs. Heavy
Negatives; Experimenting with New Techniques; More
About the Emulsion; Color Sensitivity; Basic Nature of
the Spectrum; Types of Color-Blind Emulsions, Types of
Orthochromatic Emulsions; Types of Panchromatic Emul-
sions.

IX Sensitometry 53?

Some Definitions: The Densitometer; The Characeristic
Curve; Reading the Curve, Gamma; Gamma Infinity; Ex-
posure Range: H & D value; Some Further Considera-
tions of Sensitometry; The Comparison Scale; The Gray
Scale; The Photographic Image; Film-Speed Ratings; The
Eder-Hecht Sensitometer; Schemer Film-Speed Ratings;
Other Systems of Sensitivity Rating; Curves and Their
Interpretation; Wedge Spectrograms; The Filter Curve

405

VII.

CHEMICAL AFTER-TREATMENT

VII.

Chemical After-Treatment

A WIDE gap exists between the work of the aver-
age photographic amateur and that of the salon
exhibitor. The fact itself is obvious, but many ama-
teurs do not have the slightest idea as to how the gap
can be bridged. There is a large volume of amateur
work of highly creditable quality both as regards tech-
nique and esthetics, which, nevertheless, falls way short
of the salon standard. One of the most valuable train-
ing aids that any amateur can gain is that which comes
from examining a salon print and comparing it with a
contact print from the original negative.

A photographic negative has often been likened to
a stencil, and this is true insofar as documentary and
record photography is concerned. However, in the
process of picture making by photography the negative
is nothing more than the raw material from which the
final picture is made. It is not too farfetched to com-
pare the negative and the original subject. The original
is arranged and lighted by the photographer to the end

409

that he may obtain, in a mechanical reproduction, the
specific effect he desires. The negative, although it
permits no alteration of pose and lighting, is subject to
a wide variety of controls. This may and very often
does result in a positive which bears only a basic re-
semblance to a straight contact print.

The actual procedure of controlling the print pro-
duction may be divided into three parts. The first of
these is the removal of certain actual blemishes, or cor-
rection of other technical deficiencies of the negative.
The second is the alteration of the values of the nega-
tive, which includes decreasing or increasing the depth
of the tone, the emphasis of the lighting effect, the
removal or addition of complete images or portions
of such images, and a' similar definite alteration of the
main image itself. The third step is that in which
the effect of the projected image upon the sensitive
paper is controlled by various means during the actual
process of projection. The first two activities lie with-
in the province of our present discussion. The third,
being a subdivision of print making, does not fall within
the province of our subject, the photographic negative.

One of the characteristics noted in the completed
negative is its general density and contrast. It must
be borne in mind that we have several combinations of
four basic conditions. These are underexposure, over-
exposure, underdevelopment, and overdevelopment. A
negative may be underexposed and underdeveloped. It
may be underexposed and overdeveloped. It may be
overexposed and underdeveloped, or it may be overex-
posed and overdeveloped (see Fig, 71).

Negative Characteristics.

Underexposed-Underdeveloped. The image of this
negative is thin throughout. Most of the shadows are
non-existent, while the highlights are considerably

410

less than normal in density. If the conditions are ex-
treme, this negative must be abandoned and replaced
by another exposure. It would be desirable to build
up the image to a greater density, but this cannot be
done unless there is at least an infinitesimal deposit
upon which to build. When shadows are completely
transparent containing no deposit whatsoever it is
hopeless to try to remedy the condition.

Underexposed-Overdeveloped. This negative has
an image in which the highlights probably are opaque
and the shadows without detail. If the shadows are
completely clear there is no possible remedy, just as in
the preceding case. However, if the condition is one in
which there is^ detail in the shadow but it is unavailable
because no printing paper will accommodate the range
of the negative, it- would be desirable to build up the
shadow detail and cut down the highlights. This can
be done to a limited degree by a process of intensifica-
tion followed by selective reduction.

Overexposed-Underdeveloped. This negative often
appears to have very high quality. It is full of detail
both in the shadows and in the highlights which are
not blocked. The difficulty with this negative is that
it produces a gray or flat print that is, the shadows are
not dark enough and the highlights are not white
enough. This negative would be greatly improved in
an extreme case by removing the silver deposit to the
point where shadow detail was barely visible, and then
building up the contrast by disproportionately increas-
ing the highlight deposit. This is a very simple matter,
as it involves simple reduction followed by straight
intensification.

Overexposed-Overdeveloped. This negative is ex-
tremely dense, and in many cases the highlights
through a range of several tones exhaust the emulsion.
However, it is only the extreme case in which this is
true. Reduction of the image will usually result in a

411

!

z .

8, JT
3. -;

O

Is

1 e

e 3

1 1

-o

2

c

i

THE PHOTOGRAPHIC NEGATIVE-

reseparation of the highlights to a degree which will
make the highlight detail thinner. As there is an ab-
normal degree of contrast produced by the over-
exposure in this negative, it would be desirable to de-
crease the contrast as well as the density ; and this, too,
is a practical laboratory step.

These are the extreme cases of negative faults
resulting from incorrect exposure and development. It
may as well be stated frankly that the best remedy for
any of them is to make the negative again. As long as a
photograph can be duplicated without the expenditure
of too much time and effort, it is always very much
easier and gives a higher degree of satisfaction. It is,
as a matter of fact, easier to develop a negative than it
is to intensify or reduce it. In addition, the negative
which is correctly exposed and developed has a quality
which cannot be expected from one which was made in-
correctly in the first place, and upon which remedial
devices have been employed.

There are cases, however, in which the error is
slight, and corrective measures may be employed easily
and with complete satisfaction simply because the
amount of change from the original condition is com-
paratively slight. It is one of the remarkable character-
istics of the photographic process that a slight absolute
change in the negative will result in a marked ap-
parent change in the positive.

Correct Exposure-Underdevelopment. This nega-
tive is complete in all detail, but it is inclined to be
thin and lacks the necessary contrast to give a good
print. The image is characterized by satisfactory de-
tail in the shadow. A negative of this kind often can be
made to give just as good a print as a perfectly proc-
essed negative by the use of simple intensification.

Correct Exposure-Overdevelopment. This is a
very common fault among amateurs who do not con-
scientiously watch the temperature of their darkrooms.

416

-CHEMICAL AFTER- TREATMENT

The shadows are just 'about right, but the highlights
are so opaque that they cannot be printed. This is the
type of negative which produces the mediocre snap-
shots in which hands and faces, and to a greater degree
light-colored clothing, appear blocked up and chalky.
Many negatives can be rendered completely satisfac-
tory by the use of a reducing bath whose action is re-
stricted largely to the highlights and which has a mini-
mum effect upon the shadows.

Underexposure-Correct Development. This nega-
tive has extremely thin or even blank shadows, while
the highlights appear normal. The transition from
highlight to shadow is toore abrupt than it should be,
and half-tones in the print are considerably darker than
they should be. As we have stated before, shadows
which are non-existent on the original negative cannot
be brought into the picture by any method short of
actually sketching them in with brush or pencil. Of
all errors of exposure and development in the photo-
graphic process, we may regard underexposure as the
single one for which there is no satisfactory remedy.
The only thing to be done is to duplicate the negative
by making the exposure again and this is substitution
rather than remedy. If an underexposure has been cor-
rectly developed and will not give a satisfactory print
on any grade of paper available, it is ordinarily hope-
less. When the fault is slight, intensification may help,
but the result will not be first class.

Overexposure-Correct Development. This nega-
tive is heavy and requires a long printing time. But
unless the error has been extreme, a prolonged printing
exposure will usually give a fairly satisfactory positive.
Negatives of this kind are quite easily remedied by sub-
mitting them to the action of a reducer which attacks
the various deposits proportionately, so that while the
normal contrast is retained the over-all density is re-
duced considerably.

417

THE PHOTOGRAPHIC NEGATIVE

Methods of After-Treatment.

The intensification of a negative can be produced
by various chemical agencies in which the opacity of
the deposit is increased and the color of the deposit
is changed, or in which a supplementary deposit is
made upon the exisiting one.

The one big advantage of after-treatment lies in
the fact that by a careful selection of the method used
the degree of the result can be controlled and the con-
trast of the negative altered by using a solution which
acts more strongly upon the highlights or the shadows
respectively. Although it is ordinarily understood
that intensifiers are not subject to this control, it has
long been recognized that such a control is easy with
reducers. As a matter of fact, there is such a control
obtainable in intensification, so that a wide range of
different conditions may be compensated.

Also when we are concerned with the removal of
silver from the deposit to make it less opaque, we
have similar control.

There are reducers which act upon the shadows
more quickly than upon the highlights, and thus in-
crease the contrast of the negative while reducing its
density. Other reducers act uniformity throughout the
image, and do not alter the original specific contrast
of the negative. AL third type of reducer acts more
rapidly upon the highlights than upon the shadows,
and thus reduces the contrast as well as the density of
the negative as a whole.

It must be understood that any after-treatment en-
dangers the negative to a certain extent, and this is
particularly true in the case of reduction. To prevent
the losses often encountered, care should be taken to
use only pure chemicals of the best grade and to make
sure they are fresh. In some cases it is also advisable
to harden the negative before giving it any additional

418

CHEMICAL AFTER-TREATMENT

Negative fault and remedy chart used In laboratory shows effect
of various combinations of exposure and development. Upper
Jeft, print from negative being considered; upper right, photo of
9-step wedge; bottom, wedge showing theoretical result if print
were limited to 9 tones. This chart was made for a negative which
was given 1/10 normal exposure and twice normal development.

419

THE PHOTOGRAPHIC NEGATIVE-

chemical treatment. Both intensification and reduction
should be done when the negative is first removed from
the wash water. If the negative has been dried, it
should be soaked thoroughly before undergoing any
further chemical treatment.

Intensification and reduction were once almost rou-
tine steps in negative processing because there were
only one or two types of emulsion available and only
one printing medium. This made it necessary to bring
everything to a uniform quality. Today, with the wid!e
variety of emulsions and printing media, the need for
after-treatment is much less. VVhile every photo-
graphic technician should be familiar with the proc-
esses, they should be avoided except when absolutely
essential. This means, as stated before, that the best
way to intensify or reduce is to make a new exposure
and give it correct processing.

Intensification.

Before considering chemical intensification in de-
tail, we will examine superficially some of the methods
of intensification which are not chemical in the ordi-
nary sense of the word.

One of the simplest methods of mechanical intensi-
fication is to make a positive transparency from the
negative, using the thinnest obtainable film for the pur-
pose. From this film a second negative is made through
the back of the film so that it will register with the
original negative when they are placed face to face, This
second negative should be made on glass a lantern
slide plate can be used. The two images are carefully
registered, the original film is backed by a second sheet
of glass, and the whole bound together. This is a some-
what clumsy device, and when any degree of enlarge-
ment is to be used, accurate registration becomes al-
most impossible.

420

-CHEMICAL AFTER-TREATMENT

A similar method consists in making the best pos-
sible positive transparency from the negative. This
positive image is intensified or reduced as may be
necessary, and from it a duplicate negative is made.
This method is widely used when the original negative
is of such value that possible damage cannot be risked
by submitting it directly to chemical treatment.

Quite satisfactory jesults have been obtained by
making a series of duplications in this way, alternating
negative and positive and adding to the density and
contrast of each image through control of development
and exposure. This method really works and is prac-
tical. Another method, which is mentioned merely as a
curiosity because it is too involved for practical appli-
cation, consists of giving the negative a coat of water-
proof varnish. A coat of bichromated gelatin is ap-
plied and exposed through the negative, which is then
placed in warm water. The exposed gelatin remains
hard, while the unexposed areas swell. This relief
image is treated with a pigment such as lamp black,
which adheres only to the swollen gelatin. Variations
of this process include one which practically amounts
to making an oil print (negative) on the surface. All
these methods are simply variations of pictorial pig-
ment processes.

Another group of intensification methods includes
those in which intensification is obtained by changing
the color of the deposit. For example, a negative which
is toned by the ordinary sulfide sepia toning process
will become considerably intensified, although its
visual appearance would not suggest such a condition.
Dye toning can be used; in fact, any process which
alters the color of the deposit within the gelatin to one
that is less actinic than the original will prove effective.

Of these methods the repeated duplication and the
sulfide toning are the only ones actually worth a trial.
Serious intensification is based largely upon addition

421

THE PHOTOGRAPHIC NEGATIVE-

Superproportional type,
such as lead, which
increases contrast.

Contrast- re tain ing
type, such as chromium.

Flattening type, such
as mercuric iodide,
which has greatest
effect on shadows.

Hg. 72. Curves showing typical action of various intensifies
422

-CHEMICAL AFTER-TREATMENT

to or substitution of the original silver image. This
may be accomplished by first bleaching the silver, which
really means that the metallic silver is converted into
silver chloride or silver bromide. After bleaching, the
bromide or chloride is converted into some substance
which, by reason of its greater m^ss, altered color, or
both, has a greater density than the original.

Mercury Intensification. This process of intensi-
fication is based upon the use of mercury as the bleach-
ing agent. The method works very well, but when
miniature negatives are involved it should be avoided.
Most intensifiers which employ a mercury bleach pro-
duce a new image in which the grain is excessive.

The amount of intensification is not as great as
most people believe visual inspection is not a reliable
guide! While the usual mercury bleach followed by
ammonia blackening will, under ideal conditions, pro-
duce an intensification of from about 30 to 70 per
cent, depending upon the original density of the de-
posit, as ordinarily practiced it will give an over-all
increase of between 20 and 25 per cent. If the various
densities are measured individually, the lighter half-
tones will be found to have gained proportionally
more than either the extremely light areas (shadow
areas) or the very dense deposits (highlights). In
contrast to this, the mercuric iodide bleach followed
by sodium thioantimoniate will, under ideal conditions,
give upwards of 100 per cent increase, approaching or
even exceeding 1 50 per cent, with the greatest increase
in those areas somewhat darker than medium. There
is no particular point in discussing every variation
of the process of intensification, so we will consider
only those more commonly used.

The ordinary mercuric bleach contains approxi-
mately 130 grains of mercuric chloride and 25 minims
of either hydrochloric or nitric acid in 10 ounces of

423

THE PHOTOGRAPHIC NEGATIVE-

water. This is roughly equivalent to 30 grams of mer-
curic chloride and 5 cubic centimeters of acid in one
liter of water.

Mercuric chloride is the poisonous chemical ordi-
narily known as bichloride of mercury or corrosive
sublimate. It should be handled with extreme care
and the use of rubber gloves is advisable, particularly
if there are any scratches on the hands. The mercury
bleach is preferably used only in glass or hard rubber
trays. Mercury attacks ordinary metals so vigorously
that minute cracks in an enamel tray might cause dif-
ficulty. For the same reason articles of gold, silver,
aluminum, etc., must not be allowed to come in contact
with the solution or they will be blackened.

When the negative is immersed in the bleach it ap-
pears to become darker, and if examined by holding it
up to the light it will appear to have a bluish or even
purplish color. However, this stage soon passes, and
the image takes on a more or less white color. It is
during this period that the first degree of control may
be exercised. If the intensification is to affect the entire
image, the film is left in the bleach until the image ap-
pears completely white when examined through the
back of the film. In the case of a very thin negative
which is too contrasty (such as one which has been
underexposed and overdeveloped), the process of
bleaching is watched through the back of the film. The
shadows will be whitened completely while the high-
lights continue to have a considerable amount of black
deposit. If the bleaching is interrupted at this point
and the negative placed in the wash, the subsequent
blackening will have its full effect upon the shadows
while the highlights will be proportionately less in-
tensified.

After bleaching, the negative must be thoroughly
washed. The washing should continue for at least 15 to
20 minutes. This washing will do much to eliminate

424

-CHEMICAL AFTER-TREATMENT

later faults in the intensification. At this stage the
negative must once more be blackened that is, the
light-sensitive chloride or bromide must be changed
into a more stable and darker-colored material. The
bleaching and subsequent blackening must both be
carried out in a light which, although weakly actinic,
is subdued sufficiently so as not to produce the re-
versal effect known as "solarization." In short, this
submission to subdued light is in a sense an exposure,
and must, within wide limits, be controlled. An ex-
cessive exposure would mean the risk of reversal of
the image.

There are many methods used for blackening the
image, the choice of which will be determined by the
specific characteristics of each agent as described in the
following paragraphs.

Ammonia

Ammonia (strong) 2Vi drams

Water to make 10 ounces

This dilute solution of ammonia blackens the
image almost instantly. The ammonia dissolves a con-
siderable part of the chloride and a certain amount of
the mercury, and the residue is extremely opaque but
unfortunately it is not stable. For this reason ammonia
is suitable for use only with negatives which do not
have permanent value. Unlike many other forms of
photographic fading, this image, after fading, cannot
be restored by any known method. The solution must
not be used for more than one negative which, of
course, means one roll of film if the entire roll is to be
treated.

Ammonia produces a considerable intensification of
the heavier deposits, but has comparatively little action
upon shadow detail. Its greatest value then lies in the

425

THE PHOTOGRAPHIC NEGATIVE-

intensification of line-copy negatives which are to be
used for a specific limited time and then discarded. Be-
cause of the softening effect it is advisable to add about
2J per cent potassium alum to the mercury bleach
used. This means adding about J4 ounce to the 10
ounces of solution.

Sodium Sulfite

Sodium sulfite, desiccated V& ounce

Water to make 10 ounces

To the above solution sufficient sodium bisulfite
or acetic acid is added to make the solution turn blue
litmus paper pink. When sulfite is to be used for
blackening, it is advisable to add potassium bromide to
the bleach in an amount equal to that of the mercuric
chloride. That is, 130 grains of mercuric chloride are
dissolved in 5 ounces of water, an equal amount of
potassium bromide is dissolved in another 5 ounces,
and the two are mixed after the chemicals are thor-
oughly dissolved. This alteration of the bleach is ap-
plicable only to sulfite blackening. It is advisable be-
cause it not only increases the degree of intensification
but also helps to avoid streaks and other irregularities.

Redevelopment. The bleached image may be dark-
ened by redeveloping in any ordinary developer. Al-
though amidol does not give quite as much intensifica-
tion as some other developers, it is one of the best in
point of retention of the proportionate densities of the
original.

In all the foregoing methods of blackening it
is necessary to expose the bleached image to a certain
amount of white light because the blackening is a re-
duction in many ways analogous to development.
There is one process, however, which does not require
any exposure at all and which has two other very im-
portant characteristics. It is particularly adaptable

426

-CHEMICAL AFTER-TREATMENT

to photometric and other precision work. This process
is development in an iron developer and may be re-
peated a number of times with constantly increasing
intensification, provided the emulsion has been hard-
ened and is washed between the various steps. The
proportionate density values are maintained with such
accuracy that the intensified densities can be used as
bases for photometric measurements.

A two-solution developer is used, . Solution A is
neutral potassium oxalate, 2j4 ounces ; hot water, 20
ounces. Allow this solution to cool to room tempera-
ture and carefully decant the clear supernatant liquid
for use. Solution B is made up of ferrous sulfate, 2J4
ounces, citric acid, 10 grains ; water, 10 ounces. For use,
one part of solution B is poured slowly into three parts
of solution A with constant stirring (do not add A to
B). This produces the rerddish solution which is used
to redevelop the bleached image.

One of the most energetic intensifiers for use fol-
lowing a mercury bleach is sodium thioantimoniate ( or
Schlippe's salt), which is made up by dissolving 100
grains of the salt in 10 ounces of water.

We have discussed intensification by redevelop-
ment following the mercury bleach. There are several
other intensifiers, one of which makes use of mercury
in a single bath. This is a solution of mercuric iodide.
The solution can be made in two ways, the first of
which is ordinarily used when mercuric iodide as such
is not available.

Mercuric Iodide Intenslfler

Solution A*

Mercuric chloride 17% grains

Water to make 1 ounce

Solution B.

I'otBMium Iodide 44tfrains

Water to make 1<

427

THE PHOTOGRAPHIC NEGATIVE-

Make these solutions in any desired multiples of
one ounce. When the solutions are made, pour about
three quarters of Solution B into Solution A while stir-
ring vigorously. Pour the remaining fourth of Solution
B into A very slowly, using just enough to cause the
solution to clear. Discard any remainder of the iodide.

If it is preferred to use mercuric iodide the follow-
ing formula will be found satisfactory (Figs. 73, 74) :

Sodium sulfite, desiccated 1 ounce

Mercuric iodide 45 grains

Water to make 10 ounces

The sulfite is added to the solution because the
mercuric iodide, which is a very heavy red salt, is not
readily soluble in water but it does dissolve quite
easily in a solution of sulfite.

This intensifier is unusual in that slight traces of
hypo remaining in the negative will not have any effect
upon the intensified image. With most intensifiers a
bare trace of hypo will produce spots and streaks. The
negative, after washing or after soaking, is simply im-
mersed in the above intensifier and allowed to remain
until the desired degree of intensification has been
reached. The negative takes on a warm tone and then
turns to a distinct brown. When completely dry, the
negative may take on a brilliant yellow-orange color.
This is not objectionable and, in fact, adds to the print-
ing density of the negative, although it can easily be
prevented. Wash the negative for about 10 minutes in
running water after intensification. Then place it in
any ordinary developing solution for 10 to 12 minutes,
and wash again.

This intensifier is extremely easy to use. It is one
of the safest because it does not produce streaks and

430

-CHEMICAL AFTER-TREATMENT

spots. As its action is proportionately greater on the
shadow areas, it is valuable for improving thin, con-
trasty negatives which result from underexposure and
overdevelopment.

Other intensifies make use of chromium, copper,
lead, Uranium, and silver.

Chromium Intensifier

Solution A.

Potassium bichromate 1 ounce

Water to make 10 ounces

Solution B.

Hydrochloric acid G.P 1 ounce

Water to make 10 ounces

For the greatest intensification take 10 parts of
Solution A, 2 parts of Solution B, and 88 parts of
water. For medium intensification use 20 parts of
Solution A, 10 parts of Solution B, and 70 parts of
water. For only a slight intensification use 20 parts
of Solution A, 40 parts of Solution B, and 40 parts of
water. The intensification must be done under a sub-
dued light. This solution is a bleach in which the nega-
tive must remain until all traces of the original black
image have disappeared. After bleaching it is essential
that the negative be washed until all the yellowish
color has been removed from the emulsion. As the
bichromate color is quite obstinate, this washing can be
hastened by giving the negative a minute or so rinse
in a 5 per cent solution of ordinary sodium carbonate.

The image is redeveloped in any ordinary de-
veloper, although a metol type is one of the best. This
development should be continued for at least 15
minutes as the intensification increases after the initial
blackening, although there is no visual change in the
density. Intensification can be repeated, although very
little is gained after the second treatment. The intensi-

431

THE PHOTOGRAPHIC NEGATIVE-

fied image is quite permanent, and because of the warm
tone the solution may be used to intensify transparen-
cies, lantern slides, and paper prints. In fact the dilu-
tion suggested for slight intensification is often used
on prints simply to improve their color. This intensi-
fier is not quite proportional to the original as the
shadows tend to intensify proportionately more than
the highlights.

Copper Intensifier. This extremely active in-
tensifier is based upon a copper bleach followed by
blackening in silver nitrate The original method was
not entirely satisfactory with the modern type of emul-
sion, but an improved method was worked out in 1924
by Z. Zelger. The copper or bleaching bath which does
not bleach but turns the image a distinct yellow color
is in itself a two-part solution made by pouring Solu-
tion A into Solution B.

Copper Intensifier

Solution A.

Copper sulfate 11 grains

Glacial acetic add 1 dram

Water to make 5 ounces

Solution B.

Potassium iodide 11 grains

Ammonia (strong) 3 Vi drama

Water to make 2% ounces

When Solution A is poured into Solution B, a con-
siderable amount of heat is generated and the bath is
allowed to come to room temperature before using.
The cool solution will be a clear blue color and should
be tested for acidity with blue litmus paper. If the
color of the litmus paper is not changed to a delicate
pink, sufficient acetic acid must be added to accom-
plish this. Do not add so much acid that the litmus
will turn to a distinct acid red.

432

-CHEMICAL AFTER-TREATMENT

The negative is bleached until the image becomes
yellow throughout, which can best be ascertained by
examining it through the back. After the negative
is completely bleached it is immersed in the following
blackening solution :

Silver nitrate. , .... . . 10 grains

Sodium acetate 40 grains

Water to make 10 ounces

When the image has become completely black the
film is cleared for 2 minutes in a solution of 1 per cent
ammonia. After rinsing for a minute it is placed in the
following :

Sodium hydrosulfite ... J4 ounce

Sodium bisulfite 20 grains

Water to make. . . . ... .... 10 ounces

Instead of this final bath one can use an ordinary
amidol developer to which sufficient sodium carbonate
is added to cause a strip of red litmus paper to turn
blue. This intensifier increases the density of the nega-
tive about three times, and as a rule the process is
limited to the intensification of line originals as it has
a tendency to destroy delicacy of gradation.

Lead Intensifier. Lead is another intensifier which
is extremely active but which, like the silver-copper
method, is limited to line-copy or other negatives in
which gradation is of little value. The bleaching solu-
tion is -made up as follows :

Potassium ferrlcyanide 180 grains

Lead nitrate 260 grains

Glacial acetic acid 99 mtninrm

Water to make 10 ounces

433

THE PHOTOGRAPHIC NEGATIVE-

Bleach the negative in this solution and then place
the negative in three successive baths of 3 per cent
hydrochloric acid, allowing 5 minutes in each. The
negative must be handled carefully at this stage be-
cause the acid tends to make the gelatin extremely
delicate. Wash the negative thoroughly until all the
yellow color is removed, and darken by placing it in a
solution of YZ ounce sodium sulfide dissolved in 10
ounces of water. The two remaining intensifiers are of
greater general value than the two preceding ones.

Silver Intensifies Make up a solution of 1 part
formalin in 10 parts of water. Place the negative in this
bath for 5 minutes. Rinse for 2 minutes in running
water. Then place in the following bath :

Potassium bichromate .... . . 1 gram

Potassium bromide. ... ... .20 grains

Hydrochloric acid . . . 60 minims

Water to make. . . . .20 ounces

The time of immersion in this bath should be
exactly 1 minute at 68 F. If the time is prolonged
the image will start to bleach, and if this occurs the
original gradation will be lost. Rinse in running water
for two minutes. Then place in the intensifying solu-
tion. If this solution is made in the two stock parts
as given, and both preserved in dark amber bottles,
they will remain in good condition for many months.

Solution A.

Silver nitrate 400 grains

Water (distilled) to make 10 ounces

Solution B.

Ammonium thiocyanate 700 grains

Sodium thiosulf ate (hypo) 700 grains

Water to make . 10 ounces

434

-CHEMICAL AFTER-TREATMENT

Add one part of Solution A to one part of Solution
B while stirring the latter vigorously. When this mix-
ture is completed add one-quarter of a part of pyro solu-
tion and one-half part of 10 per cent Ammonia. The
pyro solution is a 10 per cent solution of ordinary pyro
with a sufficient amount of sodium sulfite added to
prevent its discoloration by contact with the air.

As in all direct silver work, the trays and other
utensils must be chemically clean. The negative is
placed in such a tray and the mixed silver solution
poured over it. Intensification will not start for about
1 to 2 minutes, and then proceeds gradually. The proc-
ess may be stopped at any desired point by placing
the negative in an acid fixing bath. When removed
from the silver solution, the negative will have a slight
brownish pyro stain. It should remain in the fixing
bath until this stain has been removed. Following this
the negative is washed thoroughly and at the conclu-
sion of the washing the surface is rubbed lightly with
a tuft of wet cotton to remove any deposit which has
been formed by ordinary settling. This process is the
most valuable method of intensification for miniature
negatives, as they will retain the grain pattern even
when the process is used with an ultra-finegrain nega-
tive. Silver intensification can be repeated to advan-
tage if desired.

Uranium Intensifier. It has often been stated that
the intensification of an image which was not re-
corded by the exposure is impossible. This of course is
quite true. However, by the use of uranium it is pos-
sible to build up to printing density an image which is
so weak that it is hardly visible. Therefore as.it tends
to increase contrast as well as build up an almost
invisible image, it is an ideal intensifier for a negative
which is so badly underexposed that it is quite un-
printable (see Figs. 75, 76). The intensifying solution
is made up as follows:

435

THE PHOTOGRAPHIC NEGATIVE-

Solution A.

Uranium nitrate .... 50 grains

Water to make . . ... 5 ounces

Solution B.

Potassium f erxicyanide ... 50 grains

Water to make ... 5 ounces

For use, mix 4 parts of Solution A, 4 parts of Solu-
tion B, and 1 part of glacial acetic acid. The negative
must be absolutely free from hypo, or stains will be
sure to resultr When intensification has proceeded far
enough, the negative is washed in soft or distilled
water. The image has been converted to a mixture of
silver ferrocyanide and uranyl ferrocyanide. As the
uranyl ferrocyanide is soluble in alkalis, hard water
cannot safely be used for washing.

If a yellow stain persists after about 15 minutes
cashing in running water, it may be removed by the
following clearing solution :

Potassium citrate. ... 20 grains

Sodium sulfate 100 grains

Water to make . . . . 8 ounces

If the intensification has been too great or for any
reason it is desired to return the negative to its original
condition, a wfeak solution (about 2 per cent) of am-
monia or sodium carbonate will remove the intensifica-
tion. Following this the negative should be placed for 2
minutes in a normal stop bath of 2J4 per cent acetic
acid and thoroughly washed. After washing, the in-
tensification may be repeated if desired.

Thus far we have considered most of the types of
intensification which are really of value (Fig. 72), and
we may summarize the results as follows :

For ordinary work a mercuric chloride bleach fol-

438

-CHEMICAL AFTER-TREATMENT

lowed by sodium sulfite will give a good degree of in-
tensification and a stable image.

For a high degree of intensification, including re-
peated intensification with an exact preservation of the
original density ratios, the mercury is used followed
by the iron developer. For most general purposes the
mercuric iodide intensifier is highly satisfactory be-
cause the results will not be spoiled if a trace of hypo is
present ; the amount of intensification is quite satisfac-
tory, and the increase in grain size is not nearly as great
as with the two-solution mercury intensifiers.

For finegrain intensification the silver method is
best, although it does require extreme cleanliness and
considerable care.

For extreme intensification of underexposed nega-
tives, uranium will probably be found the most satis-
factory.

For extreme intensification of line copy or any
original where the fidelity of gradation may be sacri-
ficed, the copper-silver or lead methods may be used.

So far as the effect upon contrast is concerned the
following facts should be remembered: Mercury fol-
lowed by ammonia intensifies the heavy densities with-
out affecting the shadow areas. In fact, the method
seems to have a tendency to reduce the shadows. Thus
this intensifier tends to increase contrast.

Mercury followed by sodium sulfite is fairly pro-
portional, although there is a tendency to increase con-
trast.

Mercury followed by iron developer is noted for
the fidelity with which it retains the original density
ratios.

Mercury followed by organic developers such as
amidol maintains a fairly satisfactory proportionality.
Mercuric iodide single-solution intensifier tends to re-

439

THE PHOTOGRAPHIC NEGATIVE-

duce contrast by acting more energetically 011 the
shadows than upon highlights. Chromium is another
intensifier which tends to work more energetically on
the shadow portions and thus reduce the contrast
which so often is excessive in weak negatives.

Copper-silver and lead intensifiers, as we have
seen, are not at all proportional and tend to exaggerate
the heavy densities at the cost of the thin areas, thus
making them most suitable for line copies.

Silver intensification preserves an almost exact
proportional ratio among densities. This is to be ex-
pected, since its action is very closely allied to that of
a physical developer, and physical development is
noted for its fidelity to the tonal values of the original.

It can be seen from the foregoing that the choice
of intensifiers is largely determined by the character
of the original negative both as regards its contrast and
its over-all density, as well as by the ultimate effect
which is desired. Broadly speaking, the intensifiers
using a mercury bleach are not wholly satisfactory.
Neither are they economical, because whatever the
blackening agent may be it must be discarded after hav-
ing been used on a single film. The needs of the aver-
age advanced amateur will be fully met by the use of
mercuric iodide, silver, uranium, and chromium.

It is suggested that the initial experiments with
intensifiers be conducted with ' discarded negatives.
Experienced amateurs proceed with intensification
without hesitation, but for some reason it seems that
the beginner almost invariably gets streaked and
spotted negatives. This is largely due to the presence
of hypo and to intensifying negatives which have been
dried and not sufficiently resoaked. Once you have be-
come accustomed to the process, however, you will not
hesitate to intensify any negative which may need such
treatment with the possible exception of extremely
valuable negatives which cannot be replaced.

440

-CHEMICAL AFTER-TREATMENT

Reduction.

Photographic reduction, not to be confused with
the process of chemical reduction which forms the
basis of development, is in a sense the opposite of in-

Rg. 77. Superficial reduction removes an equal amount of the
silver from all densities. It increases contrast by destroying
delicate tones without affecting greatly those of great density.

tensification. Its purpose is to reduce the density of the
image and, like intensification, this reduction may affect
the contrast in different ways. It may be stated at the
beginning that while intensification is a comparatively
normal process to be attempted without any hesita-
tion, the same thing cannot be said for reduction.
Farmer's reducer, which will be discussed later, is a
safe reducer; but other formulas which are used are
apt to result in a lost negative unless the chemicals are
positively known to be fresh and pure. Therefore the
amateur should be warned to resort to reduction only

441

THE PHOTOGRAPHIC NEGATIVE-

as a last measure only when nothing else will serve.
On the contrary, it is certainly advisable for the
amateur to try out the various types of reducer on
waste negatives so that he can become accustomed to
the process. Then when the necessity arises he will

Fig. 78. Proportional reduction removes the same percentage of
the silver from each of the different densities. The amount re-
moved in any one area is determined by the actual density itself.

have confidence in his ability to do the best that might
be expected under the circumstances.

Reducers are generally classified according to their
effect upon the resulting contrast of the negative.
These three classes are superficial, proportional, and
superproportional reducers (Figs. 77, 78, 79).

Superficial Reducers. This type reduces equally
all densities of the silver image as it penetrates. That
is, if the negative has twenty strata of silver particles
and the reducer is allowed to act for two units of time,
the densest highlights would still contain eighteen

442

-CHEMICAL AFTER-TREATMENT

strata of silver while the thin shadow areas, which con-
tain only one or two strata, would be entirely removed.
Thus the effect of this reducer is to increase the con-
trast of the negative by destroying completely the most
delicate tones while hardly making any perceptible

Fig. 79. Superproportional reduction removes a certain percent-
age of the deposit mass, the greater the original deposit the
greater is the reduction of density. Contrast is also reduced.

change in those highlight areas of great density.

The greatest value of this type of reducer is to
remove fog from the surface of negatives, particularly
those in which the image has sufficient body to per-
mit such removal. The superficial reducer is also^re-
ferred to in photographic literature as the subtractive,
cutting, subproportional, or surface reducer.

Proportional Reducers. The proportional reducer
takes away approximately the same percentage of
silver from each density. Thus it tends to retain a
normal degree of contrast. Although it is commonly

443

THE PHOTOGRAPHIC NESATIVE-

said that this reducer does not alter the original con-
trast of the negative in terms of relative values, it
does remove the printing difficulties encountered in
the extreme contrast of overdeveloped negatives.

Superproportional Reducers. This type of re-
ducer has a very slow solvent action upon the silver,
but as silver is removed it is formed into silver sulfate.
The presence of the silver sulfate accelerates the sol-
vent action and appears to do so in proportion to the
amount of sulfate present. Thus, the reducer will have
a greater proportional action upon the dense deposits
of the negative than upon the shadow areas and there-
fore it will produce a reduction of density and also a
reduction in contrast at the same time.

Farmer's reducer is one of the oldest reducers
known, having been originated by Howard Farmer in
1884. It is a superficial reducer, made up of equal

rrts of a 10 per cent solution of ordinary hypo and a
per cent solution of potassium ferricyanide. The
ferricyanide tends to discolor the emulsion, so just
enough sodium carbonate can be added to make the
solution distinctly alkaline. This addition not only
prevents discoloration, but extends the life of the solu-
tion and renders its action more regular.

The well-washed negative is placed in this solu-
tion, and the tray or tank agitated continuously
throughout the entire process. The film is removed at
intervals for examining by transmitted light, because
it is impossible to judge the progress of the operation
while the film reniains in the tray. When the film is
removed it must immediately be plunged into clear
water and thoroughly washed before examination ;
otherwise streaks will be certain to form. The nega-
tive must be removed from the reducing solution be-
fore it has reached the desired density. The reason
is that the reduction naturally continues for a short
period after the washing is started. The allowance

444

THE PHOTOGRAPHIC NEGATIVE-

which must be made for this continued action can only
be judged by experience (see Figs. 80, 81, 82).

Another superficial reducer is that of Belitzski.
It contains potassium ferric oxalate and oxalic acid,
and as it has very little if any advantage over Farmer's
we will not discuss it at length. The formula is as fol-
lows:

Belitzski's Reducer

Potassium ferric oxalate 150 grains

Sodium sulfite, desiccated 125 grains

Water to make 7 ounces

When the solids are completely dissolved, add 40
grains of oxalic acid. Shake the solution until it turns
to a distinct green. Allow solid particles to settle,
and pour off the clear supernatant liquid. Add 1%
ounces of hypo, and when this is dissolved the reducer
is ready for use.

A very good proportional reducer which is not
widely known is the ferric alum reducer. Ferric am-
monium sulfate is made into a 2 per cent solution in
rain water or distilled water. To this is added J4 per
cent pure sulfuric acid. After the negative has been
reduced, it is rinsed for 5 minutes in water containing
1/10 per cent sulfuric acid. Then it is washed thor-
oughly. Tap water may be used if it is definitely free
from any trace of chlorine or chlorides.

The most common proportional reducer is one
made by combining potassium permanganate and am-
monium persulfate. This solution is mixed im-
mediately before use, and should be prepared by dis-
solving each ingredient in about 5 ounces of water,
after which they are mixed and water added to bring
the total volume up to 20 ounces.

448

-CHEMICAL AFTER-TREATMENT

Potassium permanganate 1 grain

Sutf uric acid (155) l%fl. oz.

Ammonium persulfate 90 grains

Water to make 20 ounces

The negative is treated from 2 to 5 minutes, ac-
cording to the amount of reduction desired. After re-
duction, it is rinsed and placed for 5 minutes in a 10
per cent solution of sodium bisulfite. It is then washed
for about 20 minutes.

About the only satisfactory reducer of the super-
proportional type is ammonium persulfate, which is
obtained in the form of small colorless crystals. The
salt absorbs moisture from the atmosphere and be-
comes unstable. If the odor of ozone is noticed when
the bottle is opened, it is an indication that the salt
has started to decompose. The persulfate is highly
unstable in solution, and the reducing 1 bath should be
prepared immediately before use.

When using the reducer on a test negative pay
particular attention to the heaviest deposits. If there
is a slight white cloud formed, even one which is
barely distinguishable, it indicates the presence of
chlorine or chloride in the water. Such materials pres-
ent even in extremely minute quantity alter the degree
of proportionality in the reduction. When there is a
considerable amount of chlorine in the water a persul-
fate made with this water will not reduce.

The most common proportional reducer is one
is, the acidity of the bath in operation will affect the
velocity of reduction, so it is usual to make the bath
acid before using it. The solution is made as follows :

Ammonium persulfate 20 40 grains

Sulf uric acid (!#)) 1% drams

Water to make 2 ounces

449

THE PHOTOGRAPHIC NEGATIVE-

Reduction starts rather slowly and continues to
gain in speed. Therefore the negative must be watched
very carefully or the increasing velocity will carry the
reduction beyond the desired degree. Because of this
the negative must be removed before the desired de-
gree of reduction is obtained. The process is termi-
nated by immersing the negative for 2 minutes in a
10 per cent solution of sodium sulfite as a stop bath.
The negative is not rinsed between the reducing bath
and the stop bath. After removal from the sulfite bath
it is washed for 15 to 20 minutes.

So far we have been concerned primarily with an
alteration in the amount of silver in the emulsion.
There are times, however, when the negative has high-
lights of too great density, and shadows which are not
strong enough to permit any reduction. It is quite
possible to modify such a negative by treating it first
with an intensifier such as mercuric iodide and then
with a superproportional reducer, so that the end result
is a reduction of highlights with an intensification of
shadows. A similar result may be obtained mechan-
ically by making a very thin positive transparency and
binding this in register with the negative. The total
density is increased by this method, although contrast
is reduced. As has been stated before, this combina-
tion of negative and positive is difficult to handle in
the matter of registration, and is not at all suitable
for negatives which have to be enlarged to any consid-
erable extent.

Local After-Treatment.

There is one point where the old-time photogra-
pher was greatly superior to the modern one. This
was in his manipulation of after-treatment. A part of
his normal routine consisted of intensifying and reduc-
ing certain restricted areas of the negative. This is

450

-CHEMICAL AFTER- TREATMENT

not difficult; the only real trouble is that it requires
a tremendous amount of patience. Obviously when
both a reducer and an intensifier are to be used on a
single negative and to be used only in certain parts
of the negative the process is limited to single-solu-
tion baths. The combination most ordinarily used is
Farmer's reducer and mercuric iodide intensifier.

The negative to be worked upon should be at-
tached to a sheet of glass by means of rubber cement.
After the negative has been attached to the glass, a
band of this cement is run around the edge of the film
to form a seal. After attaching the negative to the
glass, it is placed in a tray of water at about 65 or 70
F. and allowed to remain for an hour or so. It is then
removed and the surface water wiped sufficiently with
a tuft of absorbent cotton. Free surface moisture is
then removed by blotting the negative with Textilex
paper.

The reducer or the intensifier is picked up on a
cotton swab wound on the end of a match or toothpick.
The swab should be thoroughly wet but not dripping.
The local area to be reduced is rubbed with this swab
until a shallow pool of the solution is formed on the
face of the negative. This is allowed to remain for
about ten seconds. It is then picked up with a cotton
swab which has been soaked and squeezed as dry as
possible, and the negative lightly rubbed with cotton
charged with fresh water. The negative is then blot-
ted and the procedure repeated. The operation can be
performed in much less time than is necessary to de-
scribe it, and the repetition is continued until the de-
sired density is obtained. If the solution is applied
and allowed to remain on the negative until the de-
sired effect is obtained, the negative will show a sharp
hard line around the edge of the area treated. How-
ever, by alternately treating and washing the area a
blended edge which is rarely noticeable is obtained.

451

THE PHOTOGRAPHIC NEGATIVE-

Local after-treatment, like all other forms of gen-
eral retouching, is not practical for small negatives.
When working with any negatives smaller than 5x7
inches and this includes practically all modern ama-
teur sizes it is advisable to make a transparency
upon glass 8 x 10 inches, or larger if a larger print is
desired. A certain amount of work may be done upon
this transparency, including the local intensification
and reduction as well as actual handwork. An en-
larged negative is made by printing this transparency
by contact upon a second transparency plate. This
negative is quite large enough for all ordinary after-
treatment.

It is common practice to build up extremely weak
shadows with a dye applied directly to the emulsion.
The amateur is not encouraged to attempt this for sev-
eral reasons. It is difficult to follow an outline with
sufficient accuracy or to blend the dye so that no edge
will be apparent m^the final print. It is not applicable
to miniature negatives, and if an enlarged negative is
made it is extremely easy to reduce the shadows lo-
cally in the positive and, if necessary, to follow this by
local intensification in the enlarged negative. This
treatment preserves, in some degree at least, the pro-
portionate densities of the shadow areas. The dye, on
the contrary, simply makes the area more opaque to
actinic light, and therefore does not carry through the
tonal proportionality.

Handwork.

The- negative often has a number of defects when
it is dry. Aside from the general density problem
which we have just discussed, there is almost always
a certain amount of mechanical injury. This includes
fine scratches, pinholes, dust spots, and similar imper 1
fections. There are times when, due to extreme soft-

452

-CHEMICAL AFTER-TREATMENT

ening of the emulsion or carelessness, a considerable
area of emulsion is totally removed. This often hap-
pens when the edge of a fingernail accidentally rubs
the soft emulsion. All these difficulties may be rem-
edied, but the degree of skill and the time involved are
such that it is far better to make a new negative when-
ever possible.

The handwork involved in removing the imper-
fections described involves the same kind of remedial
measures ordinarily used in portrait retouching a
process which involves a certain amount of alteration
in the actual appearance of the image. Indeed, this is
carried so far at times that features which are naturally
unattractive are totally altered into a presumably more
attractive form. As this involves a rather intimate
knowledge of the esthetics of portraiture, we will not
attempt to discuss retouching from that point of view.
Instead, we will confine ourselves to the use of re-
touching media for the removal or alleviation of defi-
nite imperfections.

Building Up or Reducing Densities.

Retouching consists of s.everal distinct operations.
Densities are built up by the use of a pencil. They
are reduced by superficial abrasion with a powder or
by gentle scraping with an extremely sharp knife. Thin
areas are built up by the use of charcoal or crayon on
a groundglass support, and occasionally dyes are used
in the manner which has already been described. The
equipment needed includes drawing pencils, two or
three fine sable brushes with sharp points, a pan of
black water color, and a stick of India ink. Other
items will be added from time to time as the necessity
arises.

The use of abrasives or the knife for the reduction
of density requires some training and constant prac-

453

THE PHOTOGRAPHIC NEGATIVED

tice. It was necessary for the photographer to acquire
a considerable amount of skill when retouching was
done on the large original camera negative. However,
as the miniature size makes it necessary to produce an
enlarged negative, and as the positive transparency is
an intermediate step in the production of this negative,
the use of any kind of mechanical abrasive is unnec-
essary. The use of the abrasive on the negative is al-
ways equivalent to the use of the pencil on the positive.
Therefore we shall consider only the process of build-
ing up the image and of obtaining full control by work-
ing on both the enlarged positive and the enlarged
negative.

Use of Glass Plates for Enlargements.

For reasons of convenience it is suggested that the
enlarged reproductions be made on glass plates. Many
professional photographers use films, but any one of
them will agree that it is more difficult to perform hand
operations on a film than on a plate. Any dealer can
supply you with process plates in any size you may
desire. When you buy your package of large plates il
would be wise to get a dozen 2j^ by 3}4 at the same
time to use for testing purposes.

The process emulsion is not widely different from
the emulsion on a conventional enlarging paper, al-
though one of the small plates should be tested by
allowing it to lie on the darkroom table for 10 minutes
while the safelight is turned on. One-half of the plate
is covered by three or four thicknesses of black paper.
After this exposure, develop the plate in your regular
developer. If the exposed half of the film shows any
darkening it will be advisable for you to change your
safelight to the next degree darker. Some process
plates are slightly orthochromatic, and cannot be han-
dled safely in the ordinary bromide paper workroom.

454

-CHEMICAL AFTER-TREATMENT

Process plates are ordinarily made for high con-
trast, and the formula recommended by the manufac-
turer is usually a rather hard-working one. That is,
it tends to build up extreme contrast. As you are
trying to duplicate a small negative you will want to
avoid contrast and thus retain all the tones in the
original. As a rule, a developer which is satisfactory
for making bromide prints or enlargements will be
quite satisfactory for developing the enlarged trans
parency and negative, if it is diluted with its own vol-
ume of water.

Retouching with a Brush.

Small scratches, dust spots, and similar imperfec-
tions in the original negative will be exaggerated in the
enlarged negative in a degree exactly proportional to
the degree of enlargement. If these defects are small
and sharp they may be removed either by pencil or
brush. The latter is preferable. Place two or three
drops of water in a white saucer or other china vessel
which has a smooth bottom. Place the end of the India
ink stick in this drop of water and rub the end of the
stick against the china, using firm pressure and a cir-
cular motion. The water will take tip the ink, and the
grinding should be continued until the water has ac-
quired a strong black color. The color must be heavy
enough so that when a brush is filled with it and drawn
across a sheet of paper, the line will be dark gray.

The ink is then placed where it will not collect
oust, and is permitted to dry upon the china. When
it is desired to use the ink, a small brush is moistened
in the mouth, touched to the dried ink", and worked
up on the china. Success in this work will depend
largely upon learning how to charge the brush. Wet
color is never deposited upon the negative in the way
that ink is placed upon paper. Instead, the brush- is

455

THE PHOTOGRAPHIC NEGATIVE-

half dry and ink is deposited only where the point of
the brush is placed with slight pressure. If the brush
is touched to the edge of the negative there should be
no appearance of a moist drop left behind. It is diffi-
cult to get the brush too dry, but very easy to get it
too wet.

The negative to be worked on is placed upon
a slanting support in which a hole has been cut so that
light may pass through the negative. If it is necessary
to rest the hand upon the negative a sheet of paper
should be laid over that portion of the negative to pre-
vent perspiration marks. Charge the brush as de-
scribed, and very carefully touch one of the open spaces
left by a speck of dust. The spot should take on a
little color. Repeated touches with a rapid staccato
dabbing movement will fill in the spot and gradually
bring it up to the density of the surrounding image.

Start with a freshly charged brush on those spots
in the heavier deposits, and as the brush gives up its
color work toward the lighter spots. This same pro-
cedure may be applied to larger spots and scratches.
It must be emphasized that only failure can be ex-
pected if the color is laid on as a uniform coating or
wash. Scratches, for example, are filled by making a
series of tiny dots along the scratch and bringing these
dots up to the density of the surrounding area. When
this is done the scratch will have taken on the appear-
ance of a line of dots something like a string of periods.
After this each open spot is treated just like any other
spot in the negative (see Fig. 83).

Large areas will fill in by making strips of dots
across the area to divide it into two smaller spots.
Each of these is then divided into two still smaller
ones, and this is continued until the area is entirely
filled. When the spot has not been entirely removed,
the process is repeated. The important thing to re-
member is that the tone is built up by a very large

456

-CHEMICAL AFTER-TREATMENT

Fig. 83. Pin holes and other small imperfections can be corrected
by spotting the negative, using a sharp-pointed sable brush.

457

THE PHOTOGRAPHIC NEGATIVE-

number of minute dots of color and never by brush
strokes. If too much color is put on a spot it is easily
removed by light rubbing with a small tuft of moist
cotton.

Beginners invariably object to moistening the
brush in their mouths. There is an excellent reason for
doing so. Saliva contains a ferment which attacks gel-
atin and makes it become liquid. This means that each
stroke of the brush is in a sense burning the color into
the gelatin. It will be found almost impossible to do
a smooth job if the brush is moistened with water, and
in some cases the color simply will not stick to the
gelatin. About the only substitute which can be used
and it is far from satisfactory is a solution of about
5 per cent ammonia in water.

There is a knack to spotting with the brush which
must be gained by practice. I have seen successful
teachers of retouching take students one after the other
through a course in this subject, but have never seen a
student learn spotting immediately. They see the en-
tire process, but it is only after several hours' practice
that the necessary touch is gained. Don't be discour-
aged if your first attempts at negative spotting are defi-
nitely crude.

Retouching with a Pencil.

Pencil work is even more difficult to master than
brush work. The best pencil is the so-called artist's
holder, which takes individual leads. It is advisable to
use pencils in the grades of H, 2H, 4H, and 6H, al-
though the 2H and 4H will meet most needs. A great
deal of disappointment can be avoided by having the
pencil sharpened correctly. The lead extends from
the holder about an inch and a half never less than
one inch. The point is sharpened on fine emery paper
by drawing the pencil across the paper with a rolling

458

THE PHOTOGRAPHIC NEGATIVE-

motion. The cone of the point is not less than one-
half inch long and preferably about three-quarters of
an inch. This gives a very long, slender point. It is
difficult to sharpen a retouching pencil because this
point must be absolutely needle-sharp and must be
kept that way by repeated strokes across the emerv
paper while the retouching is progressing (see Fig. 84).
The average so-called sharp pencil point is far too
broad for photographic retouching.

The face of the negative must be prepared to take
the pencil, and this is done by the application of re-
touching varnish, more popularly known as retouching
"dope." This is a sticky, resinous varnish which can be
made at home, but it is so inexpensive that it is cer-
tainly advisable to use the prepared product. It is not
necessary to apply this varnish over all the negative
only to those parts where it is desirable to add pen-
ciling. However, because so many amateurs have diffi-
culty in blending the edge of the varnished spot so it
will not show in the print, it may be advisable for the
beginner to cover the entire negative with varnish.

The amount of varnish to be used is determined
by experience. About 6 drops should be sufficient to
cover an 8 x 10 negative. The varnish is placed in the
center of the negative and is spread by a tuft of cotton.
Start spreading the dope from the center of the nega-
tive; use a circular motion and gradually enlarge this
until the entire surface of the negative is covered.
There must be enough varnish so that the entire sur-
face is covered, and there must be enough so that by
the time you reach the edges the center will be prac-
tically dry. It is essential that there be no ridges of
varnish any place on the negative. In fact, the nega-
tive will look just like it did before except that it will
have a slight sheen (see Figs. 85, 86).

Test the varnish with the pencil along the edge
of the negative. If too little varnish has been used

460

-CHEMICAL AFTER-TREATMENT

Hg. 85. The retouching varnish or "dope" can be applied to
the emulsion surface of the negative with the cork, as shown here.

461

THE PHOTOGRAPHIC NEGATIVE-

Rg. 86. After the "dope" has baan applied to the negative, it
is spread with a tuft of cotton over the area to be retouched.

462

-CHEMICAL AFTER-TREATMENT

the pencil will slip over the gelatin without leaving a
mark. If a little ball of varnish builds up on the pen-
cil point, it shows that too much varnish has been used.
When the negative is varnished correctly the pencil
will leave a mark similar to but lighter than one it
would leave on groundglass.

A support similar to that used for brush work is
necessary, and you might as well start by getting a
good professional retouching desk. They are inexpen-
sive, and no substitute is ever completely satisfactory.

The negative should be illuminated by light which
falls upon a sheet of white paper and is then reflected
through the negative. A light which shines directly
through the negative is so strong that you cannot see
the delicate tones for careful matching. Sit well back
from the negative, and do not try to see the spot too
distinctly. Carefully touch the point of the pencil to
the spot to be covered and then move it over a tiny
area, using a wiggling motion (see Fig. 87). Work
up to the edge of the spot not across it. If you can
see the pencil strokes they are too large. What you
should see is simply a gradual darkening of the spot
until it matches the surrounding tone. A similar treat-
ment will eliminate scratches, although unlike brush
work the scratch is penciled out with short, broken
strokes running in the direction of the scratch.

The pencil is superior to the brush when it is de-
sirable to build up the density of an area in which
there is a graded tone. Thus, outlines of light objects
against a dark background are sometimes too gray to
be satisfactory. The pencil will serve to build up the
edge and blend this tone imperceptibly into the orig-
inal negative tone. This work requires practice, but
once a method has been mastered it may be applied to
building up any kind of graded tone.

Remember that when you use a pencil on the pos-
itive transparency you are darkening the image, and

463

THE PHOTOGRAPHIC NEGATIVE-

the result will be exactly as you see it before you.
When you build up the negative it is exactly as though
you are working on the positive with a white pencil.
That is, each pencil stroke makes that particular area
lighter in the finished print.

Good pencil retouching is rarely visible on a con-
tact print. However, the pattern of the retouching is
gross compared with the grain structure of the image,
and only an expert can do work which is satisfactory
for enlarging more than about three diameters. In
working on an enlarged negative from which contact
prints will be made this is no great obstacle. If you
become deeply interested in this work it might be well
to consult some of the texts devoted exclusively to re-
touching. However, by following the instructions
given herein and practicing conscientiously you will
have the basis of pencil retouching which can be ex-
tended to suit almost any demand.

Retouching to Get Cloud Effects.

There are times when it is desired to work in
cloud effects this simply means the addition of den-
sities which have no definite edge. Groundglass is used
for this purpose, and a sheet of it is attached to the
enlarged negative or positive with the ground surface
away from the transparency. It is possible to separate
the two further by placing strips of cardboard between
edges. The distance which separates the ground sur-
face from the negative surface will determine the
amount of diffusion. The negative with the ground-
glass attached is laid face down on the retouching desk
or other illuminated surface, and the desired density
is added by sketching it on the ground surface. A fine
line is not very useful as it gives a broadened but very
weak density. For this reason groundglass work is
usually limited to comparatively broad masses which

464

-CHEMICAL AFTER-TREATMENT

Fig. 87. Pencil retouching Is employed to build up the density
of areas which print too dark. The work requires practice, but
can easily be mastered after a reasonable amount of experience.

THE PHOTOGRAPHIC NEGATIVE-

may be in the shape of definite directional strokes or
made to cover any desired area. The charcoal outline
is made somewhat smaller than the corresponding neg-
ative area and applied as evenly as possible. The pow-
der is blown from the groundglass and then, using the
ball of the little finger, the edge of the charpoal is
smeared or blended so that no sharp line will be vis-
ible even on the groundglass. Full appreciation of the
effect produced by two or three touches of charcoal
here and there cannot be gained until some exper-
iments are made.

The extent to which you indulge in handwork will
be governed by your patience, your skill, and your per-
sonal preference in this matter. There is no reason-
able objection to any amount of handwork which will
improve the final print. It need hardly be added that
where a great amount of handwork predominates over
the photographic character the altered picture cannot
sincerely be called an improvement over the original
This is, of course, a matter for you to decide, but a
comparatively small amount of sincere experimenta-
tion will demonstrate how e'asy it is to remedy many of
the negatives which ordinarily would be considered
useless.

466

VIII.

CHARACTERISTICS OF THE NEGATIVE

VIII.

Characteristics of the Negative

TOO often the amateur makes the mistake of think-
ing that all negatives are more or less identical.
This misconception is one of the greatest obstacles in
the path of successful negative making. The negative
has a full range of individual characteristics, the recog-
nition of which constitutes the somewhat unfamiliar
art of "reading a negative."

Negative reading includes not only the identifica-
tion of these characteristics, but also recognition of the
specific causative factors. An analysis based upon
these factors makes possible a complete critical analy-
sis which will reveal the source of any existent errors,
and be of material aid in the prevention of such errors
in the future. It is not necessary for ^the experienced
amateur to make prints from his negatives for the pur-
pose of criticism. In fact, it is easier to criticize the
negative than the print, because criticism of a print
must take into consideration any technical errors, in-
volved in the printing process itself.

469

THE PHOTOGRAPHIC NEGATIVE.

The photographic negative is primarily a "stencil"
which, instead of being cut out of thin cardboard, is
made up of a stratum of silver dust which has a con-
stantly varying density. Physically speaking, this is
the complete description of any photographic negative.
However, the way in which the silver dust is distrib-
uted throughout the gelatin of the film gives rise to a
variety of characteristics which have a definite effect
upon the subsequent print. The terms used to describe
these negatives are familiar to most amateurs, but
there is some confusion concerning the exact meaning
of the various terms. A negative may be thin, heavy,
hard, soft, sharj), diffuse. It may exhibit a considerable
amount of detail, or this may be almost entirely lack-
ing. It may have good gradation or it may be very
harsh. It has a definite color which may be gray,
brown, yellow, orange, blue, or some other color. It
also has characteristics not immediately apparent to
the eye.

Certain of these negative characteristics are
closely related, with the consequence that they fall
naturally into subdivisions. Tnf s sub-classification will
be apparent froni the description which follows.

It is unfortunate that description must be used
rather than illustration ; but no illustration subject to
reproduction has yet been made of a negative. The
quality of transparency which characterizes the nega-
tive cannot be reproduced upon paper, so that the very-
points which are to be stressed are invisible. This
makes reproductions of negatives so misleading that
it is debatable whether or not such reproductions do
not do more harm than good. The only method which
I have found satisfactory in dealing with students has
been actually to make the negative series. Obviously,
this is too costly for general distribution. Even dupli-
cation loses some of the quality, so our student nega-
tives are actual camera negatives. It would be an

470

.NEGATIVE CHARACTERISTICS

excellent plan to show all negative types, but it simply
is not possible when reproduction is involved.

Most of the characteristics to be described have
been mentioned in more or less detail in foregoing
chapters, but such repetition as may be involved is
justified by the opportunity of considering these vari-
ous factors in a sequence which will reveal their rela-
tionship, one to another, and which permits a study in
relation to their importance as negative characteristics.

Exposure and Density.

We have already seen that density refers to the
amount of silver contained in a unit area. We know
that the greater the amount of silver per unit area the
darker the film will be. This means that when the
negative is used for printing, a smaller amount of light
will pass through a dense portion of the negative than
through a light one. However, there is a fundamental
density which must be understood before discussing
the various densities found in a given negative. This
fundamental density is explained most easily by an
example which is rarely found outside the research lab-
oratory. A negative is made by exposing a film to the
light reflected from a uniform neutral gray surface.
This neutral surface represents the average reflection
from an average subject. The perfect monotone nega-
tive would contain exactly the same weight of silver
as a negative of the same size, perfectly exposed and
developed, which reproduces an average photographic
subject. The difference is that the picture negative
has the silver distributed in spots of various sizes and
density, while the test negative has uniform distribu-
tion.

If a series of such test negatives is made, each
negative receiving an exposure greater than the pre-
ceding one with the entire series advancing in geo-

471

THE PHOTOGRAPHIC NEGATIVE-

metric progression, and if these negatives are all given
identical development, the result will be a series of
negatives ranging from a very light, uniform deposit
to a very heavy, uniform deposit. This rather detailed
description is necessary to emphasize a fact which has
been stated repeatedly by many photographic research-
ers and which, as a rule, is totally disregarded by the
amateur. If you have visualized the series of experi-
mental negatives just described, and remember that
they all receive identical development, you will un-
doubtedly realize the importance of one of the funda-
mental principles of good photography namely, the
density of a negative is determined by the exposure
(see Figs. 88,89, 90).

One often hears arguments in camera clubs pur-
ported to demonstrate the fallacy of this statement.
Such arguments usually refer to the fact that increas-
ing the time of development will increase the density
of the negative, particularly in the highlights. Now of
course this is undoubtedly true. Not only is it true,
but it demonstrates beyond all question that the indi-
vidual advancing the argument has not the slightest
acquaintance with any scientific or research procedure.
Such arguments are on a par with any other half-
statement which, while it may be true as far as it goes,
gives a totally false impression.

The density the all-over density, the fundamental
density of a negative is determined by the exposure
and by the exposure alone. No matter what you do in
the way of processing to alter this density, the same
procedure applied to a negative which has received
greater exposure would result in obtaining greater
density. The one exception is the obvious one, namely,
that any procedure which tends to exhaust a greater
amount of silver than that in the emulsion will not be
differentiated from other processes which do the same
thing. The statement, however, does hold true for all

472

il

ii

I

H,W|, -i

X '. *> J^i.*TI

II

Mtf

-.**

I

c q

THE PHOTOGRAPHIC NEGATIVE-

densities less than the maximum possible density of
that particular film.

This is the reason for there being definite limits
to the extremes of exposure which will result in an
image. If a film is given a certain amount of extreme
overexposure, reversal will take place, as we have seen.
Short of this maximum it is usually possible to obtain
an image of sorts from any overexposure. Consider-
ing the other extreme, if the exposure has been so brief
that no image has been recorded, there is no agency
whatsoever which will cause that image to develop.
This is another of those very obvious facts which one
feels should really not be mentioned; but in spite of
its logic there is a widespread belief that a non-
existent latent image can be developed by prolonging
or "forcing" development.

As is usual in such erroneous beliefs, there is a
sound reason for this one. Sooner or later every ama-
teur will realize that he has exposed a roll of film and
that his exposures have been far under normal. From
his reference books or a fellow club member he will
obtain a formula for the development of underexposed
negatives, and using this formula he will obtain fairly
satisfactory negatives.

In too many cases the unsatisfactory consequence
is that the amateur proceeds, by reason of his wor-
ship of speed, to assign to his film his own private
speed rating which is four times greater than the facts
warrant. Such an amateur belongs to that school of
thought which considers an automobile in perfect con-
dition as long as it can be operated. It is true that
he uses the excessively high-sensitivity ratings and
gets negatives from which prints and enlargements
are made, and his answer to all arguments is the fact
that he is doing it. The reason that he is permitted
to win arguments in this way^ is that his fellow pho-
tographers are equally unfamiliar with the fundamental

-NEGATIVE CHARACTERISTICS

laws of the craft. Every experienced photographer
knows that these processes and formulas can be used,
and he also knows that they should be reserved for
emergency uses only. This is not because of tradition
or any other equally obscure motive, but simply be
cause he knows that this procedure will not give the
quality which every photographer has a right to expect
when he produces a negative.

It is appropriate to mention in this connection
that while the film manufacturer gives certain speed
ratings for his products, he makes the statement tha.
good usable negatives can be obtained by giving one-
half the exposure indicated by this rating. And in an
emergency, when the film is given the correct process-
ing after exposure, it may be possible in some cases to
obtain a usable negative with one-quarter the indi-
cated exposure. Now in the case of a film which is
rated at Weston 32, for example, this simply means
that a photographer who knows what he is doing can
use this film at an equivalent rating- of 128 and still
make a print. In such a case neither the photographer
nor the manufacturer would dream of following the
speed demon's example and use this fact as a reason
for assigning Weston 128 to this emulsion as its normal
rating. Such facts do not provide any proof they do
not even intimate that the density has been changed
by processing.

Let us make use of another homely example. Sup-
pose you bring home a quart of ice cream in a con-
tainer. Suppose, moreover, that the only utensil avail-
able for removing the ice cream would reach only half
way into the container. This would enable you to
remove one pint. If you follow the reasoning of the
usual amateur you will say that you did not get a quart
but that you got a pint of ice cream. Of course there
is only a pint that you can use, but that has nothing
in particular to do with the actual amount available.

THE PHOTOGRAPHIC NEGATIVE.

All you have to do is to get a longer spoon and dig the
rest of the ice cream from the container.

The average amateur photographer uses a devel-
oper which will develop only a portion of the latent im-
age, and if he is working with 35 millimeter film he
probably uses a developer which will obtain only a
small fraction of the available image. By using- a high
potential developer in fairly concentrated solution it is
possible to develop all the image. It is also possible
to develop almost all the image by a prolonged
action of a diluted developer. It must be remembered,
however, that there is no marked difference between
the exposed and the unexposed emulsion. Before all
the exposed silver grains are reduced, there will be
a considerable degree of reduction among the unex-
posed grains. It has not been possible to develop all
the latent image and none of the unexposed grains.

In normal photography we compromise by giving
an exposure which really is somewhat longer than is
theoretically necessary. Then we develop the film in
-such a way that we do get a satisfactorily large per-
centage of the exposed silver and a similarly satisfac-
torily small percentage of the unexposed grains. (This
development of the unexposed grains is, of course,
familiar to you as chemical fog.) This involves de-
velopment which is less than that required to complete
the chemical reaction. Speaking from a strictly chem-
ical point of view, this is underdevelopment.

It should be obvious, therefore, that special and
trick developing procedures based upon the use of ex-
tremely long penods of time, highly diluted develop-
ers, abnormally concentrated developers, and so on are
merely procedures with which every experienced pho-
tographer is thoroughly familiar. But he would not
dream of using them except in the specific emergency
when each one becomes more valuable than the normal
process.

478

-NEGATIVE CHARACTERISTICS

Development and Contrast.

Like the relationship which exists between ex-
posure and density, we have a similar relationship
existing between development and contrast. While
this is ordinarily said to be a factor of the length of
the development period, it is actually a factor of the
amount of development. Increasing the temperature
or increasing the concentration of developer will have
an effect upon contrast just as surely as will lengthen-
ing the period of time. Here again it is advisable to
establish pur limitations and state that all other con-
ditions being the same, the degree of contrast increases
with the length of the time of development.

Of course, any statement such as that in the pre-
ceding paragraph is subject to certain limitations and
exceptions. We all know that if development is indefi-
nitely prolonged there will be one period during which
the contrast increases steadily, and that following this
there will be another period in which the contrast de-
creases. There is a certain amount of actual loss of
contrast in the image, but the greater part of the effect
is caused by a steady growth of over-all fog which,
added to any given degree of contrast, must necessarily
reduce it.

Before becoming too deeply involved in a discus-
sion of contrast it might be advisable to arrive at a
definite understanding as to the exact meaning of the
term. For the present we wish to do this without
becoming involved in the intricacies of sensitometry.
Contrast in photography means exactly what it means
in any other connotation. That is, it means at differ-
ence more specifically, a difference between two un-
likes. Ordinarily you would hardly speak of the con-
trast between a pair of identical twins, but you might
aptly refer to the contrast between a young man in
his teens and his baby sister a couple of months old.

479

ThlE PHOTOGRAPHIC NEGATIVE.

It is quite correct to speak of the contrast between
two people of the same sex whose ages differ by about
ten years, even though the contrast might be slight.
Obviously, the contrast would be much greater be-
tween two persons of different sexes, one quite aged
and the other extremely young. So we have not only
the actual idea of difference expressed by contrast, but
we must also recognize the existence of degrees of
such difference.

In the photographic negative we have certain areas
which are more dense than other areas, and in any
photographic negative we may locate and identify that
specific area which has a greater density than any other
area in the negative. Likewise we may identify that
area which has less density than any other in the en-
tire negative. That is, two areas are the keys to the
contrast of that negative because the contrast of the
negative is the difference between the densities of these
two areas. This can be expressed numerically as the
actual ratio existing between the measured densities.
If the greater density is 3 times the lesser the nega-
tive has very low contrast, but if the denser area is
100 times as dense as the lighter then we have a nega-
tive of high contrast.

It is commonly thought that the numerical value of
the contrast of a negative is that value indicated by
gamma. This misconception is the basis of more mis-
understanding than any other single factor in sensi-
tometry. The contrast of the negative may be a re-
sult of photographing an original object in which the
contrast was extremely high. For example, it was
popular at one time to control such subjects by giving
a tremendously long exposure that is, the exposure
required for the deepest shadow and following this
exposure with a proportionately short development.
The result is a negative of rather high contrast but
extremely low gamma. As we shall see later, gamma

480

-NEGATIVE CHARACTERISTICS

is the numerical value of the contrast in a negative
which has been exposed to a carefully standardized
original whose scale of contrast is known and definite.
We shall go further into this in a later chapter, but
for the present it is enough to state that a negative
may not only be contrasty yet have a low gamma, but
it may also have a very high gamma and still be defi-
nitely of low contrast.

Defining the Terms.

A number of the terms used in photography once
had a logical meaning. Thus a negative was contrasty
if it had an extreme range of contrast. If this condi-
tion were combined with a loss of gradation, blocking
of the highlights, and empty shadows, the negative
was said to be harsh. With older photographic mate-
rials it was of course difficult to obtain a negative of
high contrast which did not have this harshness. Later
the word "harsh" gradually grew into hard. The re-
sult is that today a hard negative simply means one of
high contrast, and conversely a soft negative is one
of low contrast. This is unfortunate, because the ad-
jective "soft" is often understood to mean an absence
of image definition ; but this will be discussed later in
this chapter. In like manner both "hard** and "soft"
are very widely used to indicate the physical condition
of the emulsion before drying. It is advisable for the
amateur to stick to the older and perhaps less con-
venient terms of "harsh" or "contrasty" for that type
of negative, and "flat" as descriptive of the opposite
type.

Photography is peculiar in that it employs many
terms which have two antonyms, and the tw"o are rarely
synonymous. For example, we may say that a certain
negative is contrasty or has a high contrast. If the
contrast is not excessive but still greater than ordinary

481

I Sf^f-S-3
i^J^Ji

S. 'I* 1 Si*

C C-t, g O g f,

8 III-* lit
t I? 855 S S e

^";-g-ii*j
- ii-si-s sr -

. < t" J .a UJ

LE

THE PHOTOGRAPHIC NEGATIVE.

we may refer to the negative as brilliant. However,
all these terms refer definitely to a high order of
contrast. The condition which is described as "flat"
or "soft" indicates an absence of contrast, and there-
fore these words must be antonyms of contrasty. A
negative which does not have contrast usually presents
a wealth of detail. It surprises a good many amateurs,
some of them with a considerable amount of experi-
ence, to learn that "contrast" and "detail" are exactly
opposite terms. To have detail it is necessary that the
negative be soft or flat (but it is not to be assumed that
the soft negative necessarily has detail, i.e., when the
original had little detail).

There is another closely related factor that of
gradation. A negative which is contrasty to the point
of harshness cannot have good gradation. A negative
which is so soft that it presents the maximum amount
of detail cannot have a full range of gradation. Con-
sidering all these factors together, we may say that
"brilliant contrast" is at one end of the scale and "de-
tail" at the other, while "gradation" occupies the mid-
dle ground.

To be sure that this matter is not misunderstood
it is necessary for us to refer to the final print. A
contrasty or even a brilliant negative cannot be printed
on paper because there is no printing paper made which
has a reproduction scale great enough to reproduce all
the tones in the negative. A soft or detailed nega-
tive will produce a gray print. Now gray does not
mean muddy far from it. A gray print is simply one
in which pure white or pure black or both are limited
to extremely minute areas and serve only as accents.
The body of the image is made up of a range of tones
between light gray and dark gray. The prints of per-
fect gradation run the entire scale from the lightest to
the darkest tones which the paper is capable of pro-
ducing (see Figs. 91, 92).

484

i

.'f : isii&

THE PHOTOGRAPHIC NEGATIVE-

Having looked at these factors in their mutual
relationship, we will consider each one individually.

The Soft Negative.

Because contrast already has been discussed indi-
vidually, .we will give our first attention to the negative
of low contrast. The soft negative using the com-
mon term often has a maximum density range as
low as 7 or 8 to 1. If it goes much lower than this
it is rarely spoken of as anything but flat. This term
arises from the fact that a photograph made by arti-
ficial light, and where the light is placed as close as
possible to the lens, lacks any suggestion of plasticity.
Unfortunately such a negative exhibits the lowest
range of contrast ordinarily associated with extreme
softness. It will be seen, then, that while "soft" and
"flat" are not exactly synonymous, they are used to
describe closely related degrees of the same condition.

Detail.

Detail, as its name signifies, is a faithful reproduc-
tion of minute characteristics of the original. Recently
there was a considerable vogue in extremely sharp por-
traits. The success of the portrait was judged largely
by the extent to which the skin textures were repro-
duced. Enemies of this type of photography usually
referred to it as "pore" photography. Now it is ob-
vious that any extremely harsh lighting of the face
would obscure this skin texture by obliterating it in
the highlights and obscuring it in the shadow. Such
a result would leave the minute texture only in the few
half-tone regions between the extremes. For the rea-
son apparent in this example it is always considered
impossible for contrast and detail to exist simulta-
neously in one negative.

486

-NEGATIVE CHARACTERISTICS

Texture.

This has a very definite bearing upon the repro-
duction of texture. Texture is that quality in a pho-
tograph which reveals unmistakably the surface char-
acteristics of the object photographed. The most
elementary problems in texture are those concerned
with differentiation between a polished metal surface
and a surface such as a rug. This is the difference
which one expects to obtain automatically. As far as
the process of photography is concerned, it is just as
easy to differentiate between satin and sateen or be-
tween linen and cotton as between metal and cloth.
The difficulty lies in the usual absence of skill on the
part of the photographer (see Figs. 93 to 98).

Texture is reproduced through accuracy of minute
contrast and of minute tonal gradation. A skilled pho-
tographer has no difficulty in showing the difference
between living flesh and a fine wax model, for example.
Incidentally, this is done without a reproduction of the
somewhat objectionable skin texture reproduction of
"pore" photography. Because texture is so closely
bound up with gradation that is, subtlety of textural
reproduction we have to demonstrate the superiority
of photography over any other graphic medium. Or-
dinarily it is assumed that this superiority lies in the
extreme fidelity of tonal reproduction. This, of course,
is the most obvious superiority over other graphic
media which photography possesses. A well-made
photograph can and will faithfully reproduce differ-
ences of tone which the eye cannot perceive and which
are determined only by measurement. It may be
argued with some justification that a subtlety of tone
which cannot be appreciated by the eye is of little prac-
tical value. If the tone itself were the only point of
interest this would be perfectly true, but these same
nuances of tone are the very things which lend reality

487

THE PHOTOGRAPHIC NEGATIVE.

Rg. 93. By crossligMing fhis setup with two spotlights,

488

-NEGATIVE CHARACTERISTICS

*> I

both texture and pattern were obtained in the photograph.

489

THE PHOTOGRAPHIC NEGATIVE-

to texture. If photographic differences of tone were
limited to those which are visible as such, it would be
impossible to have the extremely faithful textural re-
production for which photography is famed.

This ability to reproduce texture is inherently
bound up with the ''continuous tone" photographic
quality. It is obvious that the more subtle textures can-
not be reproduced by any half-tone or other reproduc-
tion process, as the physical nature of the process
remoyes a certain amount of the delicate differences.
Gelatin printing perhaps most closely reproduces the
fine photographic quality, but it is costly and limited
in the number of reproductions possible from a given
plate. Therefore 'in practically all reference texts, the
illustrations fail to show what the context would seem
to indicate.

The ability to appreciate texture in a negative is
gained only after long experience. We are not familiar
with the actual elements of textural appearance, so that
when they are seen in negative form they are more or
less meaningless. However, the print will tell the
story provided the photographer has the ability to
make a good print. It may be added parenthetically
that the finest negative in the world is useless unless
the photographer who has made it is capable of making
a print worthy of the negative.

Color.

We have stated that there is a wide difference in
the color of the negative. Negatives fresh from proc-
essing may be black or neutral gray; they may have
a blue-black tone, a warm black, or even deep chocolate
color. They also may be almost any shade of yellow,
brown, orange, or tan-gray, while intensified negatives
are often a very definite yellow or orange. Negatives
which have been dyed to produce a finer grain struc-

490

-NEGATIVE CHARACTERISTICS

ture will of course have the color of the dye used, and
that may be anything within the choice of the pho-
tographer. Negatives which are blue-black or blue-
gray have different printing characteristics from those
which are yellow-orange or reddish in tone. The red
and yellow colors give a negative with a longer print-
ing time than its density would indicate, and have an
apparently greater contrast. Thus a hard negative
dye-toned violet and a soft negative dye-toned red will
both tend more toward normal than in their original
condition.

Among negatives of similar absolute density range
a blue or blue-black negative will not give as much
contrast as a brown or reddish-brown one. In actual
practice those negatives developed in a staining devel-
oper such as pyro have both greater density and greater
contrast than their appearance would indicate. For
this reason it is advisable to try to keep your negatives
as nearly uniform as possible in the matter of color.
This is not directly under your control, of course, but
any specific developer will usually produce negatives
of uniform color. If you continue to use one type of
developer, you will not have to worry about this prob-
lem. If you use three or four different types of de-
veloper and obtain negatives of corresponding color
variation, you will find it difficult to control printing
exposure unless you make systematic exposure tests

Resolving Power and Grain.

There is another factor of considerable importance
in negative quality, and that is resolving power. Re-
solving power is the ability of a negative to reproduce
extremely fine lines which are very close together. In
any photograph a sharp line is always represented by
a blended edge. Instead of being all black on one side
of a geometric line and all white on the other, there

491

THE PHOTOGRAPHIC NEGATIVE-

IS a certain intermediate boundary between the two
in which the deposit of silver grows steadily lighter.
This area may be comparatively narrow or it may be
extremely wide. For example, if you pour wet sand
on a board in parallel lines the sand will pile up in the
center of the ridge and the sides will flow only a small
amount. This leaves a number of high ridges with
deep valleys between them. If you do the same thing
with dry sand the sand will run, and the ridges will
be low and wide instead of high and narrow. This is
exactly the condition which exists in films of high re-
solving power and low resolving power respectively.

When a film has a low resolving power the fine
lines will tend to spread out until they join the spread
from the next line, and so the original which consists
of many fine lines will be reproduced as a uniform
gray area. An emulsion with a high resolving power
does not have this great spreading at the edges, and
the break between highlight and shadow is sufficiently
abrupt. Therefore, a much larger number of lines may
be photographed in the same negative area without
losing their identity. Stated in more practical terms,
a negative of high resolving power can be enlarged
satisfactorily to a much greater degree than one of
low resolving power. If you wish to consider the con-
ditions which actually exist rather than the visual ones,
the negative of high resolving power is actually sharper
that is, it has better definition than one of low re-
solving power.

As a rule, among negatives of a large general class
the finegrain negatives have a greater resolving power
than have the coarser-grained ones. It is also true
that negatives of high resolving power are of a fine-
grain nature. These facts, however, must not lead you
to the assumption that fine grain and resolving power
are identical. The resolving power is a characteristic
of a negative which has a fine grain, but other char-
acteristics are also necessary* Generally speaking, the

494

-NEGATIVE CHARACTERISTICS

emulsion which has a high resolving- power is slow, and
it is almost always true that the slower this type of
emulsion is made the higher its resolving power will
be. We know, of course, that extremely fast emulsions
usually have coarse grain and low resolving power.
Also, the emulsion which has a high resolving power
is inherently a contrasty emulsion We know that very
fast emulsions usually have an inherent lack of con-
trast. From the foregoing we would assume, and quite
correctly so, that the very fast emulsions suffer quite
definitely when measured on a basis of resolving power.

Practically it may be stated that there are emul-
sions which can be used at much greater degrees of
enlargement than any of the popularly used films, and
these emulsions are deliberately made to possess high
resolving power, contrast, and fine grain. Of course
they could be used for ordinary work, but they are
very slow. It is indeed difficult to hold the contrast
with such film to within those limits which are con-
sidered desirable for pictorial reproduction. A few
comparative tests made between films such as Micro-
File, Microcopy, and Mmipan, on the one hand, and
the super-highspeed films ordinarily used for amateur
photography on the other, give you a vivid example
of the extreme to which fine grain and definition can
be carried if you are only willing to sacrifice speed.

Of course a negative with a large-grain pattern
could not produce a highly resolved image. The grain
structure itself would produce the blending at the edge
of an area, which would preclude high resolving power.
To this extent there is an identity between fine grain
and high resolving power. Yet there are negatives
which have extremely finegrain structure yet which
form images whose boundaries are so diffusely blended
that the image shows considerably less resolution than
might be obtained from an emulsion of much larger
inherent grain pattern. This is the most striking dif-

495

THE PHOTOGRAPHIC NEGATIVE-

ference between fine grain and resolving power. A
practical aspect of these facts is encountered in using
miniature negatives which will break down in defini-
tion before the grain pattern becomes objectionable,
while other emulsions which do show an objectionable
grain pattern in the same degree of enlargement show
no indication whatsoever of breakdown.

Thin emulsions usually have greater resolving
power than heavy ones because irradiation in the thick
emulsion will break down the fine line. The thinner the
emulsion, the less will be this effect. Also, single
emulsions will usually display greater resolving power
than double-coated ones. In short, high resolving
power is related to any factors, positive or negative,
which tend to prevent the "spreading" of a sharp edge.

It is perfectly reasonable to say that an emulsion
of high resolving power will probably have high con-
trast and fine grain, but neither of these subordinate
factors will necessarily give an emulsion high resolving
power. It is obvious, therefore, that the subject of
grain must be considered frpin many points of view.
We have just considered its intimate relation with the
problems of resolving power. In an earlier part of the
discussion we saw that grain had no real physical
existence as such, but is an appearance produced by a
physical formation which is exactly opposite to it.
In short, if we take care of the holes in the negative,
the "grain" of the positive will take care of itself. We
have also seen that the condition which we must try
to prevent is the clinging together of actual grains into
clumps of such shape that the grain-producing aper-*
tures in the pattern will be prevented, or when present
they will be so uniformly distributed that the result is
an apparently uniform tone.

It is necessary, then, that we consider grain as a
characteristic of a negative which will have a certain
influence upon printing. Of course, we do not have

498

-NEGATIVE CHARACTERISTICS

to consider grain or resolving power in a negative
which is to be used for contact printing. When the
negative is to be projected to a considerable size, then
the grain pattern is important (see Figs. 99, 100, 101).

It has long been popular to illustrate the grain
distribution resulting from some specific developer by
making photomicrographs of an emulsion developed in
it. There are two objections to the use of the photo-
micrograph for this purpose. Every photographer is
familiar with the restriction imposed upon him by the
limitations of the depth of focus. When a photographer
works very close to his subject this limitation causes
the background to become diffused and out of focus.
This same thing applies to photomicrography, except
that in the case of the photomicrograph the planes of
the subject, both in front of and behind the plane of
principal focus, simply disappear. This effect, of course,
is variable and depends upon the technique used
Speaking generally, the photomicrograph does not give
the entire picture but is limited more or less to a sin-
gle plane. Therefore a photomicrograph will show the
true nature of the negative only when that negative
has an extremely thin deposit confined to its surface.
This effect, known as optical sectioning, is deliberately
used to make photographs of a single plane within a
comparatively thick material.

The other objection to the use of the photomicro-
graph for this purpose is the fact that the negative of
the photomicrograph presents a far more truthful pic-
ture of the actual conditions than does the positive
print. This is simply because the photomicrographic
negative has been made from a negative, and so in
relation to the problem we are investigating it is ac-
tually in the positive phase. If we want to know the
effect of a grain pattern we want to see the holes be-
tween the negative grains represented as dark with the

499

THE PHOTOGRAPHIC NEGATIVE-

Fig. 98. Good lighting, proper exposure, and correct develop-
ment combine to bring out texture of unpainted cypress plank.

500

-NEGATIVE CHARACTERISTICS

actual grain area light. The illustrations accompany-
ing Chapter V show direct projections from photo-
micrographs of a physically developed negative and a
chemically developed one, both in the same scale. The
significant point of this is that the only reliable test
for graininess of a negative is actual projection and in
microscopic examination. In the reproductions of the
projections, both of which were made at 40 diameters
enlargement, the physically developed negative will be
seen to have the greatest apparent freedom from grain,
while one would be inclined to choose the chemically
developed negative from the original photomicro-
graphs alone.

Negatives to Fit Your Needs.

You will want to know what kind of negative is
most suitable for your particular purposes. This, of
course, is something which cannot be stated without
knowing what your particular requirements may be.
In short, each experienced photographer has his own
standard from which he can obtain the best prints, and
for the establishment of most of these standards there
is no apparent reason. Once you have decided what
you prefer in the way of a negative, then it becomes
comparatively easy to make your negatives to this
standard.

As a rule, an enlarger which uses diffused light
works best with a negative of fairly thin density and
not too great contrast. An enlarger fitted with a con-
densing system will give satisfactory results from
negatives of moderate density and moderate to low
contrast, while for contact printing, negatives which
are definitely contrasty are often used. All these terms
are extremely vague; they are offered for use only
in comparing negative qualities. Many photographers
using diffusion enlargers work with much more con-
trasty negatives than others who use condenser en-

501

~^ s-rDTOS,A*ri c \=GAT.V _

largers. It 5s true, however, that there are negatives
entirely too soft for satisfactory Contact printing which
will give good enlargements. Likewise there are nega-
tives entirely too harJ for any projection which will
give quite satisfactory contact prints.

The ultimate print must be kept in mind at ail
times. A white object which is to appear as such in
the final print must have its image in the negative
sufficiently transparent for the light to pass through
in quantity great enough to produce the detail which
prevents the object from appearing as a pure white
silhouette. This imposes a very stringent limitation
upon the maximum density of the negative. Maximum
exposure is that which will barely suffice to produce a
completely black area when passed through the clear
film base. This is the exposure which will give the
rich shadows essential in a good print. The strongest
highlight must therefore be limited to a density which,
in this same period of exposure, will permit the paper
to be exposed sufficiently to produce the delicate shad-
ing which makes the highlight detail.

If the exposure is greatet than this, shadow de-
tail will be burned out. If the exposure is less than
this, highlight detail will not be visible. We have a
leeway or latitude from only two sources. If the light-
ing of the original subject has been sufficiently low
in contrast so that the practical scale of the negative
has not been used entirely, we will have a slight lee-
way in exposure without detriment. The other lati-
tude is obtained by the various grades of contrast pro-
vided by the paper used. However, it must always be
remembered that the necessity for using hard or soft
paper is in a measure a confession of insufficient scale
to produce a perfect negative.

It is the dream of every photographer, amateur
and professional, to master photography to such an ex-
tent that all his negatives will be printed on the

502

-NEGATIVE CHARACTERISTICS

same grade of paper using the same time *f exposure
It must be understood that this is a goal to be achieved
and not a condition which could become practical fnr
anything other than straight record and laboratnrx
photography. It is practical to work toward the timr
when this, procedure will reproduce effectively the true
appearance of the original. This still leaves us full
freedom for the exercise of our personal judgment and
choice in determining the pictorial value of any pho-
tograph which we make.

Most of the discussion up to the present point has
been concerned with the method of obtaining the vari-
ous specific characteristics which have been described.
Therefore, when by personal experiment you have de-
termined the quality of negative which you prefer, the
production of this negative will be easy by simply fol-
lowing the instructions already presented.

For example, if you find that your negatives are
so contrasty that you cannot print them, not even by
using the softest paper and a special soft positive de-
veloper, you have the solution immediately at hand ;
but first check the results. Under the conditions just
mentioned, your print will show highlights of pure
white without any detail when the shadows are printed
to their fullest depth. When detail is printed into the
highlights all the shadows, even those of intermedi-
ate depth, become massed into a more or less uni-
form black splotch. If you attempt to compromise,
your shadows will be too dark and your highlights too
white. If one of these three conditions does not exist
in your print, you have mistaken the diagnosis. For
example, if your highlights not only print out in full
detail but begin to get a little gray, and your shadows
are a light gray instead of being a rich black, you are
working in the wrong direction. Such a print indicates
an absence of, rather than too much, contrast. How-
ever, to return to the first assumption that the negative

503

ha* i<jf rr.'ieh contract, you will recognize this a< a sig-
r.ai for \ 01: :. > re 'luce the time of negative development.

If your relative is so dark and hea\ y that an ex-
p *>vre 'if 20 :*ee'i:dr= or more is required to produce
e%en a light deposit in the shadows, then you will rec-
ogni/e overexp< >sure. In this case yu can arbitrarily
a^yign a ser.aitixity rating to that particular emulsion
higher than the nnc generally accepted. If you like
an extremely thin negative but somewhat full scale,
you will find it advisable to increase the speed rating
still further. This, of course, is the same as giving a
shorter exposure.

It is possible to use two or three exposures with-
out greatly affecting the resulting print, provided each
exposure "is followed by its complementary develop-
ment technique. For example, if \ou decrease devel-
opment following a normal exposure to get a very thin
negative, the negative will not have the desirable
amount of contrast. It is sometimes possible to de-
crease the exposure as much as 50 per cent without
seriously affecting the printing 1 result. The reason is
not difficult to understand. If you have a full-strength
negative, the shadows must be heavy enough to pre-
vent clogging during the comparatively long exposure
necessary with good rich highlight deposits. If the
exposure" is cut in half, the negative image will be con-
siderably thinner in the shadows so thin that they
would not hold detail if the accompanying highlights
were extremely black. However, with a thin and deli-
cate highlight the over-all density is so slight that a
comparatively brief exposure will cut through the high-
light sufficiently, and this brief exposure will permit the
retention of shadow detail where the shadow deposit is
of extreme delicacy. This adjustment of exposure to
produce a given contrast with a given development
does not indicate contrast control by exposure. The
apparent paradox is explained when one considers the

504

-NEGATIVE CHARACTERISTICS

Rg. 99. Above, photomicrograph of medium, uniform tone area
from a negative which had been developed In p-diamine-glycin.
The original negative was then placed in the enlarger and a
I75x positive projection made from the same tone area (below).

505

-MJ r-C'"C'S ; 'A :: -* C * i: 3 <* ""!'/ __ __. _.

proportionate air-ount.^ >f light which pass simultan-
eously through various areas of the negative.

This fact :> not to "he confused with film latitude.
Film latitude is? a quality which must be limited to a
given processing. It RUIV be defined as that charac-
teristic of an emulsion which will result in satisfactory
negatives from a certain \ariation of exposure both
greater and less than normal when the film is developed
by a specified procedure.

This basis for individual ratings of sensitivity has
received little attention, but some consideration will
show that the man who demands a rich, heavy nega-
tive must of necessity use greater exposure than one
who use? a negative so delicate that the image is hardly
visible.

Thin vs. Heavy Negatives.

There is very little choice between the thin nega-
tive and the heavy one as long as the proportionate
differences in tones are maintained throughout. It is
much easier to preserve delicate gradation in a heavy
negative than in a light one. On the contrary, it is
more difficult to produce a good print from a heavy
negative than from a light one. These apparent con-
tradictory statements can be reconciled by any pho-
tographer who will do a little experimenting in his
own darkroom. He will find the difficulties are those
of maintaining infinitesimal variations in the thin im-
age and in keeping^ the extreme densities of his
heavy negative within the reproduction scale of the
paper used. For this reason most amateurs of today
pursue an intermediate course and make use of nega-
tives which twenty years ago would have been con-
sidered extremely soft as well as thin, but which today
are simply average.

There are undeniable advantages to be gained by

506

.NEGATIVE CHARACTERISTICS

using a very thin negative. The advantages which He
with the heavy negative are just as undeniable. At
we have had occasion to observe before, photography
is essentially a craft of compromise in which wise ama-
teurs carefully balance advantages and disadvantages
and seek to maintain that average balance which gives
them the greatest general advantage. At the same
time they never hesitate to make use of the extreme
advantages of any technical factor when circumstances
warrant their paying the price of other advantages lost.
Much of the failure which discourages even amateurs
of considerable experience when trying out a new tech-
nique arises from the fact that the attendant technique
is a highly specialized one usually imitating some
individual who, after long experience', has mastered the
use of an unusual negative type.

Experimenting with New Techniques.

I remember a specific example of this kind. A
certain young man an ardent experimenter with an
educational background which warranted him in mak-
ing extensive experiments had developed a technique
whereby he produced prints of beautiful quality from
negatives so thin that they could only be studied in
detail by laying them upon a piece of white paper.
His prints were so good that many neighborhood ama-
teurs tried to imitate his technique. Among all the
aspiring imitators only one achieved success. He was
the only one in the group who paid sufficient atten-
tion to the process to note several contributing factors.
First, exposure was followed by complementary de-
velopment. Second, the enlarger used had been con-
verted to a very low-intensity indirect lighting type.
Probably most significant of all was the fact that this
particular young man limited himself to subjects of
extremely delicate tone and ordinarily high key. The

so;

g. 100. Above, photomicrograph of uniform tone area developed
D-76. Below, 1 75x enlargement from same area. Note fine gram.

508

-NEGATIVE CHARACTERISTICS

fig. 101. Above, photomicrograph of r overdeveloped in D-76.
Befow, I75x blow-up of same area. Compare grain with Rg. 100.

509

THE PHOTOGRAPHIC NEGATIVE-

tonal range of his prints, although they never looked
flat or muddy, rarely exceeded one-half the scale be-
tween pure black and pure white.

When you are introduced to some unknown tech-
nique it would be .well for you to investigate every
factor in the technique very carefully if you wish to
imitate it successfully. Never be guilty of asking
someone what average exposure he gives when en-
larging. This, by the very nature of photography, must
be governed by many factors, all of which you would
have 'to know before you could make intelligent use of
the information requested. For example, you would
have to know both the color and the intensity of the
light used. This is governed by, but is not necessarily
identical with, or even directly proportional to, the in-
tensity and color of the lamp itself. The influential
factor is the intensity and color of the light which falls
upon the paper. Many enlargers have condensing
lenses which are so green that the color temperature of
the light at the paper is effectively higher than at the
source.

You must also know the degree of enlargement,
the lens aperture used, the particular type and grade of
paper, and the negative density involved. Assuming all
the working conditions to be identical, some ama-
teurs prefer a negative so heavy that the exposure
may run from 1 to 5 minutes, while another prefers
negatives so light that exposures in the same enlarger
and under the same conditions would be from 5 to 10
seconds. Yet both of these photographers, if suffi-
ciently skilled, would turn out prints of highly satis-
factory quality and conceivably would produce prints
which were indistinguishable when exhibited.

The problem of negative quality or more strictly
speaking, of negative individuality is a serious one.
This is not because any one type is so much to be pre-
ferred over another, but more often because the aver-

510

-NEGATIVE CHARACTERISTICS

age amateur habitually produces negatives of such an
extreme variety of types that he has no opportunity to
master any one of them. Consequently he fails to pro-
duce a high-quality print from any one of them.

You have undoubtedly surmised the conclusion to-
ward which these remarks have been directed. If you
wish to achieve the highest possible success in photog-
raphy as considered from a purely technical point of
view, experiment with the various types of negatives,
limiting yourself to those which are capable of produc-
ing a satisfactory print. From these you can determine
by actual printing experiment that type with which
your personal technique appears to be most compatible.
When you have determined which type gives you the
best results, devote your attention to producing this
type of negative exclusively. Of course you cannot
make all your negatives alike, as the different sub-
jects which you photograph will demand a certain
variation within limits. You can, however, so establish
these limits that they will not form any impediment in
your work and yet will be sufficiently close to throw all
your negatives into a specific type class. Then learn
how to get the best possible print from this particular
type of negative, and don't worry if you cannot produce
a superb print from negatives with other general
characteristics. The successful salon exhibitor, the suc-
cessful commercial or portrait photographer, and the
successful amateur are specialists. Primarily they are
specialists in subject matter. Rarely, indeed, do you find
one photographer who excels in still life, landscape,
figure, and portraiture. It is quite logical that the
choice of subject matter will in a measure influence the
choice of a negative type. This interrelationship is not
stringent; in fact there is no relationship involved
which is absolute. But ordinarily speaking, the type of
subject chosen for specialization will at least indicate
a possible preference in the negative type.

511

THE PHOTOGRAPHIC NEGATIVE

More About the Emulsion.

A photographic emulsion is necessarily very un-
stable and is affected to a considerable degree by many
agencies other than light. Pressure upon a sensitive
emulsion particularly pressure along a narrow line
such as might be caused by drawing a highly polished
point across the film will result in as definite a deposit
of silver as will exposure to light. There are many
chemical fumes which will produce a general reduction
of the silver in the form of fog. As film is usually
packed either in sheets piled closely together or in
strips tightly rolled up, fog^of this nature is usually
considerably heavier along the edges than in the center.
In fact, one way in which this fog may be detected is
by its high degree of uniformity along the edge and its
gradual decrease in density toward the center of the
film. Fog which is caused by light striking the edge
of the film is usually much heavier in some spots than
in others.

When the film is exposed to dampness as well
as to chemical fumes, the dampness penetrates more
easily by reason of the absorption of the packing paper.
It must be realized that in speaking of chemical fumes
we are not limited to heavy or noticeable fumes ; the
term must cover any constituent of the atmosphere
which has a harmful effect upon the emulsion. Film
stored in a room where illuminating gas is used will
become fogged in a comparatively short time. Any
kind of sulfur gas in the air is dangerous. Vapors in-
cluding very slight amounts of mercury, oil of mustard,
or chlorine will react with the film. The best way to
preserve film before use is to keep it in some storage
place which is relatively dry and if possible where the
temperature does not at any time exceed 65 F.
The warmer and damper the atmosphere surround-
ing the film, the shorter will be its useful life.

512

-NEGATIVE CHARACTERISTICS

Under ordinary conditions the standard package
which contains the film is proof against deterioration
for a period of a year or more. There are certain
regions, however, particularly in the tropics and sub-
tropics, where there is a combination of high tempera-
ture and high humidity which makes it difficult to keep
film in good condition for any extended period of time.
In the Gulf States, where for a considerable part of the
year temperatures above 90 F. and humidity greater
than 85 are almost the normal condition, the standard
packing has proven entirely satisfactory. I have used
film two years old which has been stored under these
conditions, and have found that except for a slight loss
of sensitivity the films were fairly satisfactory. The
increase in general fog was noticeable, but so uniform
that the result was limited to a loss of contrast and
the other losses incident to a heavier-than-normal fog
level.

Where conditions are so extreme that film is seri-
ously damaged in a comparatively short time, or where
film has to be carried under variable conditions for a
considerable period of time (such as is the case in ex-
ploring parties), a special packing commonly known as
tropical packing can be obtained. This packing pro-
vides an outer shield of tin, lead, or similar soft metal
in which the film is hermetically sealed. It must be
remembered that when a film is removed from this
packing the protection of the packing is completely
destroyed. As a matter of fact, it is often advisable to
remove the film from the packing a few hours before
it is to be used. Otherwise, if the dry film is placed
directly in the camera, there may be a condensation of
moisture upon the dry surface. Films removed from
such a package should be exposed and processed as
soon as is reasonably practical.

There are certain locations in which the climatic
conditions are such that the latent image will fade

THE PHOTOGRAPHIC NEGATIVE-

slightly and in time disappear completely. It has been
reported that in the Congo valley film may be ex-
posed, left untouched for a week, and then re-exposed.
Immediate developing after the second exposure re-
veals the second image without a trace of the first.
Conditions like this are extreme, but it is only reason-
able to suppose that a degree of protection which is
efficient under such extreme conditions would be pro-
portionately even more efficient under less abnormal
conditions. Of course there is no particular reason for
obtaining tropical packing for any normal purpose
within the limits of the continental United States.

There is another factor which we have not con-
sidered concerning the interval between exposure and
development. E. R. Bullock has contributed some
valuable information on this subject in communica-
tion No. 425 from the Kodak Research Laboratories
(Volume XV, 1931-32). The fact that there is a growth
of the latent image to a certain extent corresponding
to an actual increase in exposure has been established
beyond question. However, the fact has been made
the basis of many rather bizarre statements, including
one to the eff ect that simply permitting exposed film to
remain undeveloped would be equivalent to a two- or
threefold increase in exposure.

As a matter of fact, we might compare the impact
of the light upon the emulsion to the fall of an acrobat
into a net. The original impact carries the net down
and this in turn causes an extension of springs or ropes
which support the net, and so the force is felt for an
appreciable interval after the original impact. Under
actual laboratory test conditions it has been found
that the maximum effect is observed after an interval
as short as five minutes following exposure, and that
after an interval of five or six hours further increase
becomes almost negligible. Influencing factors are the
type of emulsion used, the intensity of the exposure,

514

-NEGATIVE CHARACTERISTICS

and the temperature and humidity of the air.

The color of the light seems to have little or no
effect, nor does prefogging or any of the other solari-
zation effects sometimes resorted to for the purpose of
influencing the subsequent exposure. The increase in
density under such conditions is on the order of 5 to 7
per cent. This is great enough to be extremely interest-
ing to the laboratory researcher; but when practical
photography demands an increase in density of at least
25 per cent to make an appreciable change, the slight
density gained is certainly of greater theoretical than
practical interest.

Color Sensitivity.

There is another attribute of the original emulsion
which has a very great etfect upon the finished nega-
tive. This is the range of color sensitivity of the emul-
sion itself. As we have already stated, we do not have
definite information regarding the specific action of
color sensitization. S. E. Sheppard, in the Journal of
the Franklin Institute, 1930, published a most inter-
esting paper on the optical sensitizing of silver alloys
by colloidal silver. ^ The basis for the experimental
work in this direction is the discovery of Becquerel,
who found that a preliminary exposure of a silver
chloride emulsion to blue-violet (or white) light fol-
lowed by a fresh exposure of yellow or green light
resulted in an image of greater density than the pre-
liminary exposure alone would produce. Liippo-Cramer
demonstrated in 1909 that this increased sensitivity
was due to optical sensitizing by colloidal silver. He
combined colloidal silver in gelatin, and after careful
preparation obtained an emulsion which was sensitive
to ultraviolet, to the entire visible spectrum, and even
into the infrared. It is interesting to note that here an
effect is obtained which might logically be produced

515

THE PHOTOGRAPHIC NEGATIVE-

by the change in the size of the penetrating particle,
and one which is in many ways similar to the effect
obtained with dye.

It is hardly within our province to go deeply into
the subject matter of this particular report, as it is
largely concerned with more recent developments in
atomic physics. But it is interesting to note that one
authority attributes the sensitizing action to the pres-
ence of "a bromide ion surrounded on all sides by silver
ions in the lattice. This will be strongly deformed,
since the field of the silver ion is not screened by
neighboring bromide ions."

Among the various theories presented, perhaps
the most interesting is the one based upon the known
photoelectric activity of silver and other materials. If
this is indeed the case, the sensitization by colloidal
silver is roughly analogous to the use of intensifying
screens in radiography. In other words, the original
light induces fluorescence by the particles themselves
that is, the particles of colloidal silver and this in
turn reacts with the silver halide in a manner similar to
that of the shorter wavelengths of light. Whether this
is a true fluorescence a re-emanation of a radiation
within the visible band or whether, as is most likely,
the disassociating effect is directly electronic is not im-
portant. The mere fact that the colloidal silver acted
upon by a long wavelength such as visible red or infra-
red is stimulated to reradiate energy of any form which
will produce the desired effect is significant in forming
our conception of color sensitizing.

We should be perfectly willing to accept the fact
that the reaction of the emulsion to specific colors is
only incidentally achieved by the use of dyes which are
themselves coloring agents. There is definitely a co-
incidence here, because the evidence shows that the
color of the dye as such has little if any direct relation
to the color-sensitizing of the emulsion. All the evi-

516

-NEGATIVE CHARACTERISTICS

dence points toward a rather complex ionic reaction
which conceivably could be produced by a colorless ma-
terial, namely, colloidal silver.

This discussion, however, leads us again into the
fascinating field of conjecture where theory is not al-
ways identical with fact, and where certainly neither
theory nor fact has any great influence upon the prac-
tical application of the material in question to the work
of the photographer. Of course we must realize that
the problems of the research laboratory and of the
manufacturer of sensitive materials, while having no
direct and practical bearing upon our own work, are
doubly interesting to the inquiring amateur. The ma-
terial which he uses today is a direct result of the
research of yesterday, and the insight which a super-
ficial acquaintance with these processes brings to the
amateur gives him a certain familiarity with his sensi-
tive materials. This, in turn, is often of value not only
in interpreting the results obtained but in indicating
probable procedures to be followed for the most satis-
factory results.

The color-sensitizing of photographic emulsions
has long since passed out of the experimental stage, al-
though research in this direction is never abandoned
Dr even allowed to slow up. The one significant factor
concerning color-sensitization is that the film manu-
facturers have never yet produced an emulsion with
characteristics satisfactory for commercial manufac-
ture yet which was definitely blind to the blue end^ of
the spectrum. Almost any combination in sensitivity
in the other two-thirds of the spectrum can be pro-
duced at will in a commercially satisfactory emulsion.

It must be remembered, of course, that sensitivity
or lack of sensitivity is always based upon practical
application. Suppose an emulsion has a sensitivity of
such nature that it will be fully exposed by the red, and
during that same interval of time the green will not

517

I

<''' '?'

H

'*? '.ifr': 1 .

1 ' / . .;''.,'. ,,

E
m

-g

"2"Q-g*
|2 *

-gJi

5T-

3 "8

l|*l

-f:lj5
8"St*

THE PHOTOGRAPHIC NEGATIVE-

produce an exposure sufficient to result in a develop-
able image. That emulsion may truthfully be said to
be green-blind, even though a prolonged exposure to
green alone would produce a developable image. So
when we speak of such a green-blind emulsion it must
be remembered that "green-blind" is only a relative
term, and that all other similar designations are like-
wise relative.

It is assumed that orthochromatic film is sensitive
to blue and green but not to red. However, it is a
known fact that some of our fast orthochromatic films
can be used with a red-orange filter and a satisfactory
negative obtained in a reasonably short time. Under
the same conditions of apertuf e and light, the exposure
necessary for the red would be sufficient to burn the
image out completely if an unfiltered exposure were
given. The foregoing statement, of course, takes into
consideration the influence of the filter factor. Even
when this is taken into consideration the stated result
will be obtained.

We have already seen the extreme vagueness of
the nomenclature of sensitivity. With an infinite va-
riety of sensitivity types possible, and some dozen or
more distinct types actually available, the accepted tri-
part film subdivision is only one step better than no
classification at all. It is true that the advanced ama-
teur has been taught to think in terms of six classifica-
tions. These are color-blind, ortho, high green ortho,
and panchromatic types A, B, and C. Perhaps these
six subdivisions are sufficient for practical purposes,
but every darkroom worker knows that his color-blind
enlarging paper is quite distinct in its color sensitivity
from the "process" or "commercial" negative emulsions
which are placed in the same sensitivity group. Inas-
much as no specific names have been given beyond the
six mentioned, we will as a matter of convenience guide
our discussion by these six definitions (see Fig. 102).

520

-NEGATIVE CHARACTERISTICS.

Basic Nature of the Spectrum.

At the risk of some repetition the basic nature of
the spectrum should be reviewed. Visible light can
easily be broken up into its constituent elements.
These elements consecutively placed and visually ex-
amined are revealed as a band of color starting with
pale violet, running through blue, green, orange, and
ending with deep red. This arrangement of color, seen
in the rainbow, is commonly called the visible spec-
trum. Its length is not given definitely because it can-
not be; it differs for each individual.

The spectrum can be divided arbitrarily into any
number of subdivisions as it is a continuous band of
color. Because of the mystic significance of the num-
ber "seven," the spectrum was considered for a long
time to consist of seven colors. Scientifically as ap-
plied to three-color vision, photography, and reproduc-
tion the spectrum is divided into three approximately
equal divisions. When the spectrum has been sub-
divided, each division can be recombined to provide a
uniform mean color. The result is a blue of a some-
what soft and rich quality, a clear green, and a red
which is on the order of a rich vermilion. The- names
given to the three bands on the basis of their composi-
tion rather than a too literal interpretation of their
actual apparent color are blue-violet, green, and red-
orange.

It is significant that while yellow is a color of the
greatest importance in photography, it is not a part
of the spectrum. There is a very narrow spectral band
to which the name "yellow" has been assigned, but it
is not a yellow in the sense that it would be recognized
as such visually. When light reflected from an open
summer sky is broken up by a spectroscope and all
the spectrum obscured except this so-called yellow line,
and a number of individuals are requested to identify

521

THE PHOTOGRAPHIC NEGATIVE..

this color, the term "yellow" is always modified. One
of the most striking results of this experiment is that
some people will call the color a greenish or pale
lemon-yellow, while others will state that the same
color is a slightly reddish or orange-yellow.

All this is interesting to us because the yellow with
which we are concerned in photography is actually a
mixture of red and green. That is, two-thirds of the
spectrum taken together produces the yellow with
which we are familiar. It is obvious, therefore, that
any emulsion which is sensitive to green must also be
sensitive to yellow. It is a curious fact that ortho-
chromatic film was originally advocated as a yellow-
sensitive emulsion rather than what it really was a
green-sensitive one.

Our three general types of emulsions, as we have
stated previously, correspond to the three arbitrary
divisions of the spectrum which have been mentioned.
If the spectrum were divided into four by subdividing
the red into two equal portions, we would find that
type A and type C panchromatic represent, respec-
tively, the corresponding sensitivity groups. It is not
true, as so often stated, that this existing fact (three
groups of sensitivity corresponding to three arbitrary
subdivisions of the spectrum) proves that the tri-part
subdivision is a natural one. It only proves that the in-
genuity of the researcher has gradually Extended the
sensitivity of the film by what were really impercep-
tible degrees.

Types of Color-Blind Emulsions.

Among the color-blind emulsions the ones com-
monly used for negative making include that group
which is commercially designed for positive reproduc-
tion. These are the positive motion-picture films, dia-
positive and transparency films and plates, and lantern-

522

.NEGATIVE CHARACTERISTICS

slide plates. Most of these emulsions are strictly blue-
sensitive, but so limited in their reaction that they
might with a little stretching of the truth be called
violet-sensitive. These emulsions are most often used
for the reproduction of black-and-white copy. In this
sense "black-and-white" means copy without half-
tones, or as it is perhaps more commonly known, u line
copy." The most common form is letterpress that is,
printed pages of books, newspapers, etc. It also in-
cludes mechanical drawings, uncolored maps, plates,
and similar material. This choice is dictated by the
fact that these emulsions are not only color-blind a
characteristic which is of extremely slight importance
but because they are ordinarily slow, fine-grained and
capable of reproducing extreme contrast.

The fallacy of believing that the color sensitivity
has anything to do with the line-copy reproduction is
proven, even to the unthinking photographer, by the
fact that recently a new type of panchromatic material
has been introduced expressly for photographing fine
detail. It actually surpasses the color-blind material
in making small-scale copies of large black-and-white
originals. This emulsion, used on the narrow-width
films (usually 35 mm) for copying books and manu-
scripts, is marketed under such 'names as Micropan,
Micro-File, and Microcopy. It is made as a panchro-
matic emulsion so that any filter combination may be
used which the color or condition of the original may
dictate. This will be discussed later.

Another type of color-blind material is the ordi-
nary "commercial" emulsion. This is used for routine
commercial work such as real-estate publicity, cata-
logue illustrations, and so forth. It is not used as
widely now as it once was because the public has be-
come accustomed to panchromatic rendition, and the
false color values obtained with color-blind material are

523

THE PHOTOGRAPHIC NEGATIVE.

not acceptable. Strangely enough, color-blind material
is sometimes used to obtain pictorial effects through
the exaggeration of haze and smoke effects incident to
photography by a blue-violet light.

The conception of color sensitivity, particularly
as complicated by the use of filters, remains a mystery
to many amateurs. It is basically a specific application
of colored light. If you could use a gigantic filter placed
between your subject and the sun, or placed over your
artificial lights so that the light itself is colored, the
effect would to all intents and purposes be identical
with that which you obtain by placing a comparatively
small filter over the lens. As a matter of fact, in
photomicrography it is standard practice to place a
filter between the light and the object rather than be-
tween the object and the camera. The use of a color-
blind film, therefore, is equivalent to illuminating your
subject with a pure blue light; and by extension the
same thing applies to the other conditions of specific
sensitivity and the use of filters.

The average amateur, however, will ordinarily re-
strict his use of color-blind materials to the reproduc-
tion of line copy insofar as negative making is con-
cerned.

Types of Orthochromatic Emulsions.

In the orthochromatic class we have two sub-
divisions, the normal orthochromatic which has a sensi-
tivity to the green ordinarily somewhat less than its
sensitivity to blue, and the orthochromatic in which
the green sensitivity is as great as that of the blue, or
even slightly higher. Naturally there are intermediate
grades, so that we have almost a continuous scale of
emulsion sensitivity. The ordinary or original type of
orthochromatic emulsion is now regarded by many
people as a sort of glorified color-blind emulsion. The

524

-NEGATIVE CHARACTERISTICS

most widely used form of this emulsion is the popular
"chrome" type of roll film, while the definitely nigh-
green-sensitive film is widely used in commercial and
portrait photography.

For^the casual amateur, for the individual whose
camera is a source of souvenir photographs, for the
specialist in outdoor and landscape photography, and
for the portrait photographer, the orthochromatic emul-
sion offers many attractions. The advantage in land-
scape photography should be obvious inasmuch as the
role of the emulsion is to reproduce an infinite number
of tonal gradations, all of which may be generically
labeled "green." The orthochromatic film tends to give
a more brilliant negative, particularly with outdoor sub-
jects, than does the panchromatic type. We are, of
course, now speaking about the use of the film without
a filter.

In portrait work the orthochromatic emulsion
gives a greater degree of contrast. This, in turn, exag-
gerates the tonal differences of the face and produces
a result which, while less faithful to the original than
a panchromatic reproduction, is more attractive be-
cause of the same exaggeration of features produced by
skillfully applied makeup. Without going into any de-
bate upon the question of morality or taste involved in
the use of makeup, it cannot be denied that skillfully
applied makeup does give an accentuation to the fea-
tures which makes the feminine face more attractive
than it is without. It is well known among photogra-
phers that when working without filters it is necessary
to use a heavier makeup on a model photographed with
panchromatic than when an orthochromatic emulsion
is used. It is suggested that the beginner whose work
is largely restricted to outdoor subjects, particularly
landscapes or other scenes where foliage is predomi-
nant, make orthochromatic film his routine material.

This is not in contradiction of the fact that pan-

525

THE PHOTOGRAPHIC NEGATIVE-

chromatic film is the most versatile and most valuable
sensitive material we have, but because its capabilities
cannot be used to advantage until the user has thor-
oughly mastered the proper application of filters. If
you are willing to take the time to learn the truth about
filters, to learn how to arrive at a choice of filters
through an analysis of the subject rather than by the
rule of thumb, then you should make a type B pan-
chromatic film your routine medium.

Types of Panchromatic Emulsions.

The panchromatic emulsions are designated ds
types A, B, and C. Type A is typical of the earlier
panchromatic emulsions in which green sensitivity is
sacrificed for red. The region of least sensitivity is
usually about wavelength 500 or between 500 and 520
millimicrons. The deficiency continues to about 560,
which is in the region of the distinct yellow or lemon-
green. The peak sensitivity is usually at about wave-
length 600, which is practically the location of the
so-called yellow line which is specifically denoted as
the sodium line. It must be understood that these
characteristics are not the definite characteristics of
any existing, specific emulsion, but simply serve to
indicate the general characteristics which all films of
this type approximate.

This is a good emulsion for technical purposes, and
may be obtained in very low sensitivity factors includ-
ing some emulsions which are rather contrasty and
extremely finegrained. This original panchromatic
type is primarily an emulsion to be used with contrast
filters that is, filters which are chosen for some spe-
cific effect usually quite different from the so-called nat-
ural correction. When a high degree of green sensi-
tivity is necessary in work of this kind it is usual to
employ an ortho rather than pan emulsion.

526

-NEGATIVE CHARACTERISTICS

Photo by Franklin H.

Orthoehromatic film offers many attractions for outdoor work.
This fine picture was made on one of the "chrome" type films.

527

THE PHOTOGRAPHIC NEGATIVE-

For the time being we will pass over type B to con-
sider type C. Type C panchromatic is made for one
specific purpose, and that purpose is making photo-
graphs at high speed by artificial illumination. Be-
cause of the inherent low intensity of blue in any in-
candescent light, films of type C show a red reaction
as much as ten times as great in the deep red as in the
blue. By daylight the conditions are almost reversed,
although not to quite the same degree. There are
certain extreme speed films (such as Tri-X and Super-
pan Press) which have emulsions of this type. It is
interesting to note that the equivalent tungsten speed
is usually about four-fifths of the daylight speed,
whereas the type B is more often in the neighborhood
of five-eighths to three-quarters. Therefore type C is
fundamentally a film to be used with artificial light
when short exposures are necessary.

Type B is the nearest practical approach to the
ideal sensitivity range. The sensitivity of this type of
emulsion, which is represented to a greater or lesser de-
gree by most popular modern panchromatic emulsions,
approaches the average visual sensitivity about as
closely as we can expect. The exact visual correction
cannot be reproduced because the visual curve is not
a fixed characteristic. Through mental compensation
we have learned to transpose the curve from daylight
to artificial light conditions with very little apparent
change.

The type B panchromatic emulsion shows its maxi-
mum sensitivity in the blue range under daylight condi-
tions and in the red range under artificial light condi-
tions, while the eye maintains its maximum sensitivity
in the yellow-green region under both types of light.
It is interesting to note, however, that most of these
type B emulsions have an almost uniform response to
the average green under both light conditions. Inci-
dentally, this green sensitivity is almost as great as in

528

-NEGATIVE CHARACTERISTICS

an ordinary type of orthpchromatic film, thus over-
coming the gravest objection to the use of a panchro-
matic emulsion.

This type of emulsion, which has quite properly
been called the orthopanchromatic type, is the nearest
approach we have to a genuinely universal emulsion.
It can be obtained in a sufficient variety of speeds and
grain structures to meet most demands, and through
the use of color filters it can be made to yield a truly
astonishing variety of tonal balances and degrees of
contrast.

The statement has often been made that a pan-
chromatic emulsion is the only emulsion needed by any
photographer because it can be made to reproduce the
effect of any conventional emulsion. This statement is
absolutely true, but as stated it is highly deceptive. No
single panchromatic emulsion will do this because cer-
tain types of "work require a different inherent con-
trast than others. If the photographer is permitted
every kind of panchromatic emulsion, he can imitate
with satisfactory accuracy the results obtained from
in ortho or a color-blind emulsion. However, if he is
going to have such a selection of emulsions available,
there is no logical reason why the orthochromatic
should not be added.

To leave the realm of theory and return to the prac-
tical aspects of applied color sensitivity, it is suggested
that the more serious amateur make panchromatic his
basic emulsion. In pursuance of this idea it is further
suggested that experimental work be done with filters
to provide a competent knowledge of their application.
If for any reason this cannot be done, it is best to make
an orthochromatic emulsion the basic one.

The choice of panchromatic type will depend upon
the individual conditions, although ordinarily a type B
film will be used. If picture taking is to be done
largely by artificial light or under other poor light

529

THE PHOTOGRAPHIC NEGATIVE-

conditions, one of the extremely fast films, which are
usually type C, should be used.

For cameras using 2j4 inch rollfilm and smaller,
a finegrain emulsion should be chosen, but for sizes
larger than this there is no particular need for it. For
the best all-around results, therefore, a type B pan-
chromatic film of moderate speed and fine grain will
serve practically every purpose.

IX

SENSITOMETRY

IX.

Sensitometry

QENSITOMETRY is a study of the mathematical
i^ relationship which exists between exposure and
the density of the developed image. It can be, and is,
applied to the study of both negative and positive im-
ages on transparent and opaque materials.

The entire process of photography depends upon
obtaining a physical image as the result of exposure.
The image can be silver, as in the ordinary photo-
graph; but it can also be produced by a differential
hardening of the gelatin. In the latter case, plain
gelatin will provide a matrix for dye printing. Gelatin
impregnated with pigment will produce a visible image
(dependent upon the density of the pigment layer),
as in carbon and carbro processes. An image can be
produced as the result of a differentiation in the water
content of an emulsion; this is the basis of the oil
and bromoil processes.

The exposure, too, may vary widely both in degree
and in kind. The degree of exposure depends upon

THE PHOTOGRAPHIC NEGATIVE-

the quality of the radiation used. Thus, the radiation
used for the exposure may be X-ray, ultraviolet, vis-
ible light, or infrared. The variation in kind may be
explained by the following comparison : Exposure used
with ordinary photography is an exposure to radia-
tion (light), while in processes such as carbro the ex-
posure is a chemical one in which light or other radia-
tion plays no part.

Yet, no matter what the exposure, no matter what
the character of the resulting image, some variation
of sensitometry may be applied to its study. The
significant point to be remembered is that the sen-
sitometric procedure used should, insofar as may be
practical, duplicate that used in the routine processes
which the sensitometric study shall control. For ex-
ample, sensitometric study of exposure to ultraviolet
would provide little information of value regarding
exposure to visible light.

Sensitometry is as nearly subjective as any phys-
ical process can be. The original object photographed
is intangible, being simply reflected light. The images
are two-dimensional with no definite characteristics
other than density. Here we do not deal with images
of rocks, trees, houses, and human beings; here we
have no simulation of solidity and three dimensions.
Therefore, it will be necessary for us to reshape our
present conception of some familiar photographic
factors.

It might be argued that if sensitometry is so far
removed from practical photography its value is ques-
tionable. That assumption is both right and wrong.
It is right because there are hundreds of competent
photographers (in the sense of being successful in
making pictures with a camera) who know nothing
beyond the bare essentials of photometry. It is wrong
in that all modern research into photographic prac-
tice research in the field of emulsions, in the field

534-

-.SENSITOMETRY

of exposure, in short, most photographic research
is based upon sensitometry. Moreover, those branches
of photography which are more scientific than aesthet-
ic such as sound recording for motion pictures, the
practice of color photography, the photographic pho-
tometry of astronomy can hardly be carried on suc-
cessfully without the aid of sensitometry.

Some Definitions.

In order to understand more fully the factors in-
volved in the study, let's first give our attention to a
few definitions. These will cover the processes, fac-
tors, and instruments which are to be discussed in this
chapter.

Exposure. The exposure is of the utmost im-
portance in sensitometry. In gauging exposure we
must know the intensity of the light, the time during
which it is permitted to act, and its spectral character-
istics. We can gauge intensity with a reliable foot-
candle meter; the time of exposure is under mechan-
ical control ; the spectral characteristics or "color value"
of the light may be assumed to be constant for any
reliable, standardized light source. The light should
have the characteristics of normal daylight, average
normal incandescent light, or some region between
the two. This, of course, is not true when the sensi-
tometric investigation concerns the action of light of
some specific color. The same thing holds true if the
investigation concerns any of the photographically
active "invisible radiations." Therefore, in this as in
other factors, general statements must be recognized
as being subject to exception, and the general state-
ment recognized as applicable only to the typical case.

The duration of the exposure must be considered
in two parts. First we have the exposure of a given
time duration. This means that the exposure is started,

THE PHOTOGRAPHIC NEGATIVE-

allowed to continue and when the time period has
elapsed, the exposure is stopped. The second case is
when the exposure has a value equivalent to a given
time duration. In this case the unit of exposure may
be very brief, and this brief exposure repeated many
times until the sum of the short exposures equals the
stated total period. It is a recognized fact that a
continuous exposure and a similar intermittent expo-
sure will produce different results even though their
total lumen-second factor is identical.

Exposure also varies with the type of control
used. The intensity of the exposing light may be
maintained at a uniform level, and variations in ex-
posure obtained by variation in the time of exposure.
This is the time-base exposure which is ordinarily used
in the more exacting type of research ; control sensi-
tometry in commercial application usually is based
upon exposure controlled by means of intensity dif-
ferences. In practical application the first method is
similar to the use of the shutter to control the time
of exposure, an operation familiar to every amateur.
The other is just as familiar because it employs a
series of screens which resemble somewhat an or-
dinary negative. Thus, printing through a screen
which has a number of areas of different density will
produce a shaded print just as a negative produces a
print made up of various tones.

Sensitometer. Exposure usually is not made in
a camera, but in an instrument or device known
as a sensitometer. This is often a long box,
not unlike a camera, with a holder for film or plate
at one end. In front of the box a disk containing
cutout sectors revolves to make the exposures in a
series of circular, concentric stripes or segments of
such stripes. A variation is a flat panel in front of
which a sliding panel, also containing cutout stripes,
is slowly raised or lowered. Still another variation

536

THE PHOTOGRAPHIC NEGATIVE-

IS a cutout cylinder with the light near its center and
the film exposed in a corresponding curved position.
All these variations of time-base sensitometers have
their corresponding types in which the cutout mask
is motionless and the plate or film is moved past these
openings. It will be seen that any uniform system
of exposure must follow one or the other basic pat-
terns of straight motion in. a plane, revolving motion
in a plane, or rotating motion in a cylindrical path.

The intensity type sensitometer also has three
variations, although two of them are rarely used. In
the first the exposures are all uniform in time, but the
intensity of the light is altered between exposures.
This necessitates making the exposures successively
instead of simultaneously, which is a disadvantage.
The light may be dimmed by moving it farther from
the film and utilizing the inverse square law of in-
tensity; or it may be dimmed by a rheostat in the
circuit. The rheostat control is impractical because
with each change of intensity there is also a change
of color, so that the net result is one of intensity plus
color instead of change in intensity alone.

An intermediate type is one in which successive
exposures are made to the same light, but a difference
is produced by the introduction of neutral density
screens between the light and the sensitive material.
This is hardly a practical instrument.

The third type is one in which the various neutral
density screens are all combined into one artificial
"negative." That is, a sheet of transparent material
such as glass or celluloid bears a series of increasing
densities. This type of sensitometer is commonly used
by amateurs; it is placed between the picture nega-
tive and printing paper to indicate the best exposure
for printing. Development will reveal a series of
tones which are not unlike those obtained with the
time-base sensitometer (see Figs. 103, 108, 109).

538

-SENSITOMETRY

The time-base sensitometer, so constructed that
the slotted disk turns once to make the exposure, is
the best type for experimental use. It is not difficult
to construct. Using an electric clock motor, such a
device an be made at home, and will be quite accurate
enough even for advanced work. Using black card-
board, several disks can be made to give a series of
exposures based upon any system of progression de-
sired. For example, one disk can have sectors ad-
vancing in geometric progression that is, 1, 2, 4, 8,
16, etc.; another can produce advances on the basis
of the square root of two ; another can be divided into
five-second intervals; and so forth. The density type
of sensitometer is not as accurate, and it is almost
impossible for the amateur to make one which will
be at all satisfactory. Remember that sensitometry
cannot be practiced unless the exact value of each
exposure step is definitely known!

No, you can't make sentitometric measurements
on your pictorial and portrait negatives. You can
measure the densities of these negatives, but you can't
use this information in sensitometric work unless you
also know the value of the exposure which resulted
in these densities.

Development. There has been a great deal of
discussion concerning standard developers for sen-
sitometry. This is a problem which needs only the
application of common sense for its solution. The
developer to be used is the developer you normally
use ! There are standard developers for sensitometry,
but these are used when the sensitometric control is
exerted over the manufacture of emulsions. Here
again we have a division of interest. Sensitometry
will show what happens in making the negative. We
know quite well that the negative image is a product
of exposure plus emulsion plus developer. The alter-
ation of any one will seriously affect the other two.

539

THE PHOTOGRAPHIC NEGATIVE-

As we use a differential exposure as a measuring stick
it is obvious that we have two possible combinations :

1. Standard emulsion and unknown developer, the
developer being the factor under investigation.

2. Standard developer and unknown emulsion, the
emulsion being the factor under investigation.

As few of us are concerned with experimental
emulsions, we may disregard the standard developer.
On the other hand, as we have a most fertile field open
in the investigation of both exposure and development,
we do need a standard emulsion. This would offer a
serious problem if we were engaged in exacting re-
search. But for our purposes, for the control of prac-
tical photographic processes,, we may assume that any
film from a reputable maker is a "standard" emulsion,
providing that films for both the test and the practical
application be taken from the same original package
or from packages bearing the same emulsion number.
Sensitometric control using films of different emulsion
batches can, and often does, do more harm than good.

Developer Control. Carefully prepared sensito-
metric control indicates the time and temperature
of development which will give the result desired
from an exposure, and also the concentration of the
developer to be used. As a secondary factor, rela-
tively unimportant, it will indicate the printing ex-
posure which will give the most faithful reproduction
of the original tones.

We often hear or read the expression "develop to
a gamma of 0.7," or something like that. We know
that contrast does increase with development, so with
a control it is of course entirely practical to continue
development to some exact degree wanted and to stop
there. That is the essence of sensitometric control in
the photographic workshop. A practical application
is that of keeping the three separation negatives of a
color set within the contrast limits necessary for good

540

-SENSITOMETRY

reproduction. Nor is this process limited to negative
making. A good densitometer is arranged for reading
print densities as well as negative densities.

Densitometer. The densitometer is an instru-
ment used to measure the density of the developed
image. It may have any of several different forms,
many of them based upon entirely different principles.
Some of them are visual, some electric ; some are com-
parison instruments, some are inverse-square-law in-
struments, some make use of polarized light for dim-
ming, and some are straight photometers. The in-
verse-square type is the simplest instrument for home
construction.

Photometers are instruments used to measure
light. In the broader sense all exposure meters are
photometers. However, there is one instrument which,
to use its full name, is known as the Pulfrich Gradation
Photometer. This instrument resembles a comparison
microscope having two tubes and lenses, but with a
single eyepiece. The field seen is divided by a hair-
line, and one half of the field comes from each objective
lens. If a constant light (which need have no standard
value) is used to illuminate both lenses and a negative
is placed before one lens, the corresponding half field
will be darkened. When this occurs, the diaphragm of
the other tube is slowly closed until the field is again
uniform. The reading of the control wheel now indi-
cates the relative transmission of the negative as com-
pared with open light. Reference to a graph gives the
density which corresponds to this percentage of trans-
mission. If the instrument mirror be replaced by a
special holder and a print placed on one side with a
standard white reflector on the other, the same com-
parison of reflected intensity is obtained.

Although the gradation photometer is an instru-
ment of wide laboratory value, it has some shortcom-
ings as a densitometer. These are primarily in the

541

THE PHOTOGRAPHIC NEGATIVE-

translation of the readings and in the comparatively
small size of stages for both negative and positive
types. However, density calibration may be obtained
on special order. One advantage is that by the use
of an auxiliary ocular the negative image may be seen
and any desired area selected. Then upon return to
the usual optical system the diffuse field reading is
observed. Any desired area may be selected by the
use of auxiliary stage stops.

This instrument is in one sense the prototype of
the densitometer, and represents the basic principle of
density measurement. It is a far more elaborate and
costly instrument than the densitometer, and as its
additional uses will probably not appeal to the pho-
tographer we shall discuss those instruments made for
the specific purpose of measuring photographic density.
However, before doing this we must leave the discus-
sion long enough to consider the factors involved in
density measurements.

Transmission. This is the factor which rep-
resents the amount of light that passes through a more
or less transparent substance. It is given a numerical
value corresponding to percentage. Thus if 90% of
the light passes through the substance, the transmis-
sion value is 90; if only one per cent passes, the value
is 1 ; if only 1/1000 (1/10 of 1%) of the light gets
through the film, the transmission is 0.1 ; and so forth.

This practice has not been standardized. Many
workers hold the opinion that the transmission factor
should be represented by the normal decimal value, a
contention which is certainly sound. This means that
the 90% factor would be 0.90, the one per cent as 0.01,
and the 1/1000 would be 0.001. Because this is a
logical procedure, it is the one we shall use in this dis-
cussion. Another reason for using the decimal values
is that this is the only system which will give the cor-
rect opacity values by a normal computation.

542

-SENSITOMETRY

Opacity. Opacity is the opposite of transmis-
sion. When light falls upon a partially transparent
substance the amount which passes through represents
the transmission, and the part which does not pass
through represents the opacity. Both together must
equal the whole, and if opacity were valued in per-
centage, it would always equal 100 minus the trans-
mission. However, instead of negative percentages,
we give to opacity a value which is the reciprocal of the
transmission value.

As the reciprocal of a number is that sum by which
the number must be multiplied to equal one, and as
the transmission is always less than one itself, it fol-
lows that the opacity must always be greater than one.
(In the first mentioned system, the reciprocal is taken
as that sum by which the number must be multiplied
to equal 100, thus involving an indirect computation.)
Using our previous transmission values of 0.90, 0.01,
and 0,001, the opacity values corresponding would be
expressed as l.llll+> 100, and 1000 respectively.

Density. Transparency is a quality which re-
sults from the negative deposit which is not there, so
it is a negative quantity. Opacity is a measure of the
effect of the negative deposit which is there, so it is a
positive factor, more desirable to work with than the
transparency. However, we do not use the opacity
value in photography. Instead we make use of an-
other factor, density. Density i the logarithm of the
opacity. This has the advantage of advancing at a
rate of progression more nearly approaching the rate
of building of the photographic image.

Before leaving the discussion of these three fac-
tors, there is one conception of opacity which is in-
teresting. Opacity may be said to be a direct measure
of the number of units of light which must fall upon a
given surface in order that one unit may pass through.
While the three-factors are distinctly different in their

543

t

O O O O IH <H TH IH i-j N CNI N N N f*5 W) eo fO eo ^ * ^J ^ -^ 10 10
N N CS N W N* CS CN N W C*4 CS CS C^J W N W* O$ C*S CN W c4 CS M* W

^| ^( ^ ^H ^| ^ ^| ,H ^H iH tH ^H TH ^-H CS CS <N <N CSl CN <N N C

8Om
iH

^ ^ if) 10

CM CM CM CM CM CM CM CM CM~

8888888888g8| ^ , _

*li!

~ r > I, o>

k ^fc j* ^

, ,, ^vOOOOCM^vOOOOCM^vOOOC**^-**^""^

?ioinsONONqsosqr*r*^^^oqoooqoqooc
CM" CM" CM" CM' CM* CM" CM* CM" CM' CM CM' CM* CM* CM' CM" CM' CM" CM CM" CM CM" CM" CM" to

t*. to O 00 *^ *^ ** O\ VM if) O vO CO T-4 *H CM O OS * CM w CM O Q

* "" 1 ' ~? ON ^ *O *O t* O CM l/) t* O P^ ^ Os CM to ON CO t^ ^H 10 O

CMCMCMCMCMCMCMCMCM^HT-lrHHiHHr-lrH^H*- ,_.,

qqqqqqqoqqqoqqopqpqqqooq - w ^

-5 Jc.!2E
*J!.= -w^=

HCM"
^j vq oq ON IH eo 10 1> O CM ( to r q ro so os CM 10 ON co *>; TM v) q

dill

^VOOOOCM^iVOOOOCM^lsboOOCMTHsOOOOCMTHsOOOO

io o o so vo sq so o t-|^i>r>t^oq oq oq oq oq ON ON os ON ON q

THE PHOTOGRAPHIC NESATIVE-

real characteristics, it will be seen that there is a defi-
nite value for any two of these factors which must al-
ways correspond to a definite value for the third.
Thus, transmission 0.144 and opacity 6.92 must always
correspond to each other, and both must correspond to
density 0.84. A list of such related values is given
in Fig. 104 on the preceding pages.

The Densitometer.

From the foregoing it is obvious that the densi-
tometer may be calibrated in any one of the three fac-
tors and still be a thoroughly practical instrument.
Moreover, it is equally obvious that an instrument
scale can be changed from one set of factors to another
without altering any part of the instrument other than
the scales themselves. However, it is common prac-
tice for all photographic instruments to be calibrated
directly in terms of density.

Most of the densitometers in use are of the visual
type. A divided field is observed and brought to a
condition where the two parts of the field are made
equally bright, the position of balance being deter-
mined by visual examination of the divided field. This
type is subdivided into several models, depending upon
the mechanism used to dim the open light to make it
conform to the negative transmission.

There are many types of electric densitometers,
the principal difference being that some make use of
true photoelectric cells while others utilize the self-
generating barrier layer cell ordinarily used in ex-
posure meters. The true photoelectric cell must have
a current applied to it from a battery or electric outlet.
The amount of electricity which flows through the cell
depends on the amount of light which falls upon it.
This type of cell is extremely sensitive to minute
changes in light intensity, and so is adapted to making
density measurements with a low-power light which

546

-SENSITOMETRY

cannot injure the negative. However, it requires an
auxiliary circuit, and also requires some skill on the
part of the operator to obtain accurate results.

Densitometers using the barrier layer cell, which
actually generates electricity when light falls upon it,
are now being developed. Originally this type of cell
was too low in sensitivity for accurate response. Cer-
tainly those devices which are made for use with ex-
posure meters as the sensitive element are not satis-
factory for the most accurate work. One manufac-
turer is said to be making such an instrument of the
highest accuracy which, in a perfected form, will be
ideal for general use. It will avoid the natural human
error all eyes grow tired and some people can never
learn to match fields accurately. At the same time it
will not have the delicacy and instability of the photo-
electric valve cell.

It might be thought that extreme delicacy is de-
sirable, but those who have had experience with a ra-
dio set which howls every time a control is touched
know this is not always true. The photoelectric valve
cell instrument is often so delicate that stimuli which
have nothing to do with the exciting light will change
the reading. Moreover, the operator must know its
vagaries and how to set it to zero point and how to
keep it in adjustment ; otherwise the readings will wan-
der all over the map. The photronic instrument, with
its familiar stability and constant reading, would in-
deed be a boon to advanced amateur photographers.

The visual densitometer is a photometer adapted
for photographic density measurements. We have al-
ready described the laboratory photometer as the basic
type. Now we shall give our attention to other visual
instruments.

As the earliest photometers were of the inverse
square type, so was this type 'of construction evolved
early in the development of the densitometer. The

547

M

iss;^

-3"o = -

-SENSITOMETRY

inverse square law states that the intensity of light
changes in inverse proportion to the square of the dis-
tance of the light source. Thus if a light at one inch
has an intensity of 100 candles, at 10 inches it has an
intensity of only one candle. As it is possible to work
to dimensions such as these, it will be seen that a full-
range densitometer with calibration from 0.00 to 3.0
would require an intensity change of 1 to 1000. ' The
square root of 1000 is only 31.62+, and a design which
allows the nearest position of the light to be 1/32 of its
farthest will permit density readings up to 3.0. How-
ever, this type of densitometer is usually the "box"
type in which mirrors are arranged to reflect the light
so that it goes around the "box" and finally emerges.
The light moves between the two base mirrors, above
one of which the negative is placed. It is evident that
if the light is so arranged that the field is balanced
when the light is most distant from the negative mir-
ror, any movement of the light toward that mirror will
increase the intensity of the light upon the negative ac-
cording to the inverse square law, and that the in-
tensity of the other part of the field will be diminished
(see Fig. 10S).

For example, suppose the "box" is laid out in a
straight line and the light is placed midway between
two measuring instruments. 1 We shall assume that the
light is 10 inches away from each meter, and that each
meter registers one unit of light that is, A = 1 and
B = 1. Now if we move the light to a position five
inches from A, we shall have the condition A = 4, and
B = 0.44, or a ratio of approximately 9:1. Now if the
light is moved to within 1 inch of A, we have A = 100
and B = .25 (approximately), or a ratio of 400:1. This
would include a range of somewhat more than density
to 2.6, so that if the "box" has 10-inch sides a per-
fectly practical instrument can be made. It must be

549

THE PHOTOGRAPHIC NEGATIVE-

remembered that the short end arms of the box are
also included in the measurement.

The next type of densitometer also makes use of
the reflection box that is, the four-directional travel
of the light beam but the light does not move except
to balance the field to the zero point. , A prepared gray
wedge moves through the open field beam and thus
serves to bring the intensity down to that of the nega-
tive area. The Eastman densitometer (Fig. 106), a
standard laboratory instrument, is of this type, the
wedge being-circular in shape and bearing 1 a circular
scale of densities both for reflection and transmission.

A similar densitometer is made in England, per-
haps the only thoroughly satisfactory instrument ever
to be sold at a low price. This instrument is simply a
box containing two small mirrors, and with two holes
cut in the top. A slot at one end takes a linear gray
scale, and within its limitations of density it has proven
a thoroughly efficient and reliable instrument. It was
originally marketed at about ten dollars, but is prob-
ably not available now (see Fig. 116).

Another type of visual instrument uses a polariza-
tion photometer as the eyepiece of the "box." In this
case the light from the negative passes unimpeded to
the eye, while the open light is dimmed by rotation of
a polarizer. As this instrument alone sells for more
than the usual densitometer, it was never popular ex-
cept in large commercial laboratories. The polarizing
elements used in these instruments are the conven-
tional, costly prisms chosen for their perfect optical
quality and high light transmission.

The Eastman densitometer using the circular
wedge has a disk almost 9 inches in diameter and bear-
ing density calibration in almost uniform divisions,
while the polarization disk is usually 3 inches or less
in diameter, with the full range limited to a quarter
rotation and with the effective dimming of the field

550

THE PHOTOGRAPHIC NEGATIVE-

Hmited to about 30 degrees of arc or less. This is due
to the non-uniform extinction of the polarizer. For
this reason the wedge type instrument has proven more
popular.

Accessories. In order to work out problems
in sensitometry the student should supply himself with
a table of logarithms (obtainable at any bookstore
which handles high school and college textbooks),
some sheets of graph paper (either regular or semi-
log divisions), a drawing 'board, triangle, straight edge,
metric scale, protractor, and pencils. It is true that
sensitometry can be handled purely as a mathematical
problem, but it has been found that more usable in-
formation is obtained if the characteristic curve is
drawn and the desired information derived from that.
Moreover, it has also been found that while the mathe-
matical computation of gamma gets many students
hopelessly mired, the graphic method can be used
with just as much accuracy, much greater $ase, and far
more rapidity. The student whose mathematics is
limited to arithmetic can, if he has log tables, do all of
the necessary computation of sensitometry by the
graphic method and arrive at conclusions just as accu-
rate as those of the mathematical wizard. For that
reason we shall stress the graphic method of computa-
tion.

The Characteristic Curve.

The result of a sensitometric analysis is computed
as a curve, referred to as the characteristic curve, or
often as the H&D curve after its originators, Ferdinand
Hurter and Charles Driffield. You will remember
from school days that a "curve" in graphic work may
be a straight line, but it happens that the photographic
curve takes on the shape of a more or less flattened
letter "S" (see Figs. 10 to 12, Ch. II). The paper used
is divided into spaces by lines running both vertically

332

-SENSITOMETRY

and horizontally. The horizontal lines indicate the
values of the abscissas, while vertical lines indicate the
values of the ordinates. Usually the abscissas are given
density values and the ordinates are given exposure
values.

If the paper is divided into regular squares, the
ascending 1 values along the left-hand vertical edge are
the density values. Remember that density is a log-
arithmic value. The lower, horizontal edge may be
evenly divided, or the divisions may be crowded to-
gether at the left. If the former is the case, the ex-
posures are listed along the bottom in regular loga-
rithmic sequence. In the latter case the paper is di-
vided into uneven spaces of logarithmic value, so that
entering the normal exposure values in sequence will
automatically space them on a logarithmic base. It is
always simpler to use the table of logs and convert the
exposure values to logarithms, and then use ordinary
cross-section paper. In fact such paper may be made
by drawing horizontal and vertical lines at regular in-
tervals, say J4 Y*> or 1 inch apart. Suppose our ex-
posures were 1, 2, 4, 8, 16, 32, and 64 seconds. If we
refer to the log tables we shall find the following
values :

Number Logarithm

1 0.000

2 0.301
4 0.602
8 * 0.903

16 1.204

32 1.505

64 1.806

Now suppose that your divisions are spaced one
inch apart. If you used the natural system of measure-
ment, and if your last exposure resulted in a density
of 3,0, your graph paper would be 3 inches high and
64 inches long. By using logs, the same graph can be

553

THE PHOTOGRAPHIC NEGATIVE-

made on a sheet measuring 1.8 x 3 inches or any multi-
ple thereof. In the first case the curve would rise too
gradually to show anything, while in the second the
curve is sufficiently abrupt to reveal the characteristics
of the negative. Thus the reason for using log ex-
posures.

As a rule, the space occupied by exposure 1 on the
horizontal scale is made equal to density 1 on the ver-
tical scale, but this is not at all necessary. It is only
important that you establish a definite system for your-
self and then stick to that system so that the curves
will all bear a definite relationship to each other. If
you vary the values of your abscissas and ordinates you
will end with confusion much worse than no result
at all.

If you expose the sensitive material with a disk
sensitometer, the disk can be cut from a piece of card-
board according to the following instructions (see Fig.
107). Read this and the next paragraph carefully be-
fore starting to work. First cut a 10-inch disk from
the cardboard. Then, starting with a center circle 4
inches in diameter, draw 10 additional concentric
circles at J4-inch intervals. This makes a band 2^4
inches wide and composed of 10 circular stripes, and
leaves an outside supporting band of J^ inch. Now
draw 6 radii spaced 60 degrees apart, just as you would
cut a pie into 6 equal pieces, and construct "spokes"
along these radii making each 10 degrees wide. These
spokes use up 60 degrees of your disk. Using the pro-
tractor, mark a radial slit j degree wide on the first
or outer J^-inch circular stripe and adjacent to one of
the spokes. The outer band is used for the j4-degree
slit because it is the smallest used, and will be con-
siderably wider on the outer circle than it would be
on the small inner circle. Cut out the slit, and extend
the cut next to the spoke toward the center of the disk
until it crosses all ten of the circular 54-inch bands or

554

-SENSITOMETRY

OUTSIDE

SUPPORTING

BAND

CONTROL NOTCH

10* SPOKE

Fig. 107* Exposure disk for sensitometer. Arrangement of sec-
tors Is preferable to that shown in Pig. 103; the >/2 slit will
be wider on outer band and therefore easier to cut accurately.
Control notch can be cut where desired (see Rgs. 108 and 109).

stripes. Now in the second J4-inch circle (second from
the outside) mark off one full degree, then continue
marking off sectors from the same starting point (the
cut along the edge of the spoke) according to the fol-
lowing table :

Circle
Degrees

I

&

Since the first or J^-degree slit will represent an ar-
bitrary exposure value of "one," each of the follow-
ing slits has a value equal to just twice the number

555

THE PHOTOGRAPHIC NEGATIVE-

Fig. 108. Wiring diagram of disk sensitometer shown in Figs. 103
and 109. The device is designed to operate on 1 10-v. 60-cycle AC
current. Self-starting clock motor CM is actuated by pushbutton
PB; as disk D turns, the contact ridge SC on timing spring SP
leaves timing notch N, closing contact between adjusting screw
AS and the contact block C. This keeps disk turning after the
button is released. B is an insulating block, and EE are leads
to the enlarger which is used simply as a light source. After
disk makes one complete revolution, SC drops into N, breaking
the contact. This stops the motor and turns off the enlarger.

556

-SENSITOMETRY

of degrees. Therefore exposure values for the disk
range from 1 to 512.

As the length of the slits increases they will include
the "spokes," which are not to be cut out. So for each
spoke included you must add ten degrees to the total
length of the slit. Thus, when the sectors are cut out,
the cutout portion of any band represents the total
number of degrees called for, and the finished disk will
correspond to Fig. 107. If this disk is now laid over a
sheet of sensitive film or paper and rotated it will in
a single revolution give 10 exposures ranging from
1 to 512, or a log range of 0.00 to 2.70. It must be re-
membered that these are only relative exposures. In
practice the light intensity is so low that exposure 1
is barely sufficient to make a perceptible deposit upon
the film. Thus emulsions as slow as contact paper or
as fast as the most ultra-rapid modern film can be ex-
posed correctly for later study.

While it is out of place here to give details for the
construction of instruments, Figs. 108 and 109 show a
fairly simple disk sensitometer which was put together
in a few hours and functions quite satisfactorily.
Assuming that the sensitometer disk is driven by a
clock motor and makes one revolution in one minute,
this is equivalent to 360 degrees in 60 seconds or 6
degrees per second. As the unit slot is J4 degree
wide, the film beneath it receives an exposure of 1/12
second per revolution. The other exposures are:

Circle 1 23456789 10

Exposure 1/12 1/6 1/3 2/3 1.33 2.66 5.32 10.6 21.2 42.4

Expose a piece of film with the light at any de-
sired distance. For films use a 10-watt bulb or less
at 3 or more feet. If the innermost circle is as black
as the outermost, you have at least 512 times too much

557

THE PHOTOGRAPHIC NEGATIVE.

exposure. Interpose a sheet or two of paper to dim
it and try again.

Assume that this time circles 5 to 10 are solid
black, but there is some differentiation among the
others. This indicates that circle 5 received the maxi-
mum exposure, so the light must once more be re-
duced, this time to 1/16 its previous value. However,
assume that the first test shows nothing in circles 1 to 4,
but that 5 shows a slight tint. Then that is the ex-
posure for circle 1, and the light must be increased 16
times. Once you become familiar with the instrument
you will find it possible to judge the light closely
enough so that a single test will give you the informa-
tion necessary to make a good exposure.

The film is developed in your standard working
developer and when dry, the density of each stripe is
read on the densitometer. It is better to buy your
densitometer than to try to make it, because it is al-
most impossible to calibrate the instrument without
a reference wedge. As the wedge is the most costly
part of a densitometer, it actually would be cheaper to
buy a commercial densitometer of the non-wedge type
than to buy a conventional calibrated wedge. The
densities of the various stripes are read, and each
reading is entered on the graph paper. This is simple
for those who have done graph work, but if you
haven't, a word of explanation will be given. Suppose
we have made our readings and that they are as fol-
lows:

Circle Exposure Log Exposure Density

1 1 0.00 0.1

2 2 0.30 0.12

3 4 0.60 0.24

4 8 0.90 0.50

5 16 1.20 0.80

6 32 1.50 1.12

7 64 1.80 1.45

8 128 2.10 1.77

9 256 2.40 2.00
10 512 2.70 2.10

558

-SENSITOMETRY

Rg. 109. Above, side view of svnsitometer. Below, closeup of
wiring, lettered the same as Rg. 108. Here the adjusting screw
AS Is a part of the contact C instead of working against it, and
the sliding contact SC is at the end of the arm instead of the
middle. The construction shown in the diagram allows a greater
opening between contact and spring, and facilitates adjustment.

559

fO

o

(0

a:

X

01

O

_J

IO

o

-

1

-

-

i

-

1

i

~rr

-

"-

I

-

_LL

--

-

l_l_

L

-

\

-

\

---

-

; 1

-

-

-

-

-

-

-

-

-

1 1 *

1

|_|_.

i
I

-.

"

-

-

-

-

-

--

-

-

-

--

j

,

-

_

i i

~M~

-

-

-

-

-

--

_

_

-

1

! 1

-

--

--

-

!

_

-

~

-

-

-

-

!

1

a 2 2 2

AISN3Q

ti

C O

s*
^

28.

IE S

I I

in
cvi

q

ftt

in q

A1ISN3Q

THE PHOTOGRAPHIC NEGATIVE-

Starting with the first log exposure and the first
density proceed as follows : Run up the extreme left-
hand vertical edge of the graph which represents log
exposure 0.00, until "you reach the first 1/10 subdivi-
sion of the first major division. This represents den-
sity 0.1. Make a dot here. Now move to the vertical
line which represents log exposure 0.30 and on it make
a dot just above the 0.1 line. On the vertical line rep-
resenting log exposure 0.60 make a dot about midway
between the 0.2 and 0.3 density lines. On the log 0.90
vertical line make a dot at the 0.5 density line, and so
forth, until on the log 2.70 vertical line you make a dot
on the 0.7 line above the major horizontal division line
numbered 2.5. Reference to Fig. 110 will make this
clear.

When this is done, draw straight lines connecting
each adjacent pair of dots. Then lightly sketch in a
line which will convert the series of short lines into a
continuous curve (Figs. Ill, 112). After the dots have
been connected by short lines, the straight-line portion
may have a more or less zig-zag appearance, represent-
ing small errors which may be disregarded when the
continuous curve is drawn. It is astonishing how the
dots will fall into an almost perfect "curve" with toe,
straight-line section, and shoulder after a little ex-
perience in reading the densitometer. While small er-
rors can be disregarded, gross errors (where a dot may
fall way out of line) call for a re-reading of the film.
If an error is consistent, examine the exposure disk.
You rnay find that one of the sectors is off 2 or 3 de-
grees. Remember that the disk must be cut accurately,
or errors are bound to appear in your work.

There are several sources of minor errors which
tend to throw the survey points off slightly, but they
may be ignored unless the displacement of the dots is
considerable.

1. Errors in reading the densitometer. This may

562

-

-

-

-

1
1
1

0.5 1.0 1.5 2.0 2.5 3.0 3.5 1
LOG EXPOSURE |

-

--\

i

-

-

T

-

-

-

-

-

-

-

_.

-

-

--

__

..

-

-

-

i

-

*

-

-.

Tf

-

-

-

"

--

\

\

-

"

-

\

~

__

-

s

-

~

-

-

-

-

-

s

s,

^

__

:

.

.

_

.L

r

-

s

s

s

\

-

-

v

s

-

-

-

-

-

_

-

-

:.

"

:

-

-

-

-

Ss

s

Ss

s

s

-

_

-

:

.

'.
_

-

-

-

-

s

s

s

\

-

-

-

-

-

-

:

-

1

;1

-

~

,.

-

-

-

_

-

-

_

"

-

-

-

a a a 2 g

A1ISN3Q

f

II.O 2.0 3.O 4.O 5.0 6.0 7.0 1

LOG EXPOSURE |

Fig. f!3. Extreme overexposure results in decreasing densities
and image of reversed values, shown by descending shoulder curve.

1

-

1

^*

-^

i JXT

I/I

...

7

--

v

-

-

\

-

\

--

s

^

~

\

Sj

\

-

^

-

\

-

s

Vj

-

-

--

-

s

^

-

\

\

\

._

v

s,

-

S

S^

-

s

-

N

-

\

i P P in o

i N - - a

A1ISN3Q

-SENSITOMETRY

be caused by an instrument which is inaccurate or out
of adjustment. A dust spot on the negative can affect
the over-all reading. In every case a slight error in
observation is expected, so that density readings are
made by taking the average of three or more readings
for each exposure.

2. Emulsions are not always uniform in their sil-
ver distribution, nor is the effect of development con-
stant. These errors are too slight to affect picture
taking, but the densitometer reveals them.

3. Variable drying and dust particles on the nega-
tive will affect the reading somewhat. It must be re-
membered that while the densitometer aperture is
small, it integrates all densities within that area.

Reading the Curve.

You will find that your curve is made up of a cen-
tral portion which is a straight line. At the left (lower)
end of this line there is an upward curve, the toe; at
the right (upper) end there is a downward curve, the
shoulder. If the exposure is carried to extremes, this
shoulder will continue its relative downward direction,
and the densities will actually grow less with increas-
ing exposure ; but this requires an excessive amount of
exposure (see Fig. 113).

The toe is curved because equal increases in ex-
posure produce relatively small increases in density,
although the amount of increase is greater for each
exposure step. This is the region of underexposure.
Likewise at the shoulder the increase in density is
small as compared with increase in exposure, and the
amount of increase grows smaller with each succeed-
ing exposure. This is just the reverse in procedure
of the toe, but the ultimate result is strikingly similar.
This is the region of overexposure.

In the center lies the straight line, and along this

565

THE PHOTOGRAPHIC NEGATIVE.

line any increase in exposure will result in^a uniform
increase in density. Thus, if the exposure is doubled,
the density will be doubled, and so forth. It must be
remembered that this has nothing to do with the actual
density. The increases in log exposure may be in the
progression 1, 2, 3, etc., and the density increase may
be 0.25, 0.50, 0.75; or it may be 1.25, 2.50, and 3.75.
However, when the curve is such that log exposure 1
gives density 1, log exposure 2 gives density 2, and log
exposure 3 gives density 3, we have a condition in
which uniform increases in exposure give uniform and
proportional increases in density. In this condition
the relative intensities of the original light will be re-
produced as a series of negative densities bearing the
same relationship. In short, we have a faithful tonal
reproduction. In the uniformly divided graph paper
this straight line rises at an angle of 45 degrees.

The negative in which this proportional increase
occurs is one which has a gamma of unity. This brings
us to a discussion of a much-talked-of and, for practical
purposes, a superlatively unimportant factor in sen-
sitometry, namely, gamma.

Gamma.

Gamma is a number which represents the rela-
tionship existing between the range of sensitometric
exposure and the extreme range of the negative den-
sity. Thus if you have an extremely contrasty subject
and condense the tonal values byoverexposure and
underdevelopment, you have a gamma less than unity
because each degree of exposure is representd by a de-
gree of density correspondingly smaller. If you have
a flat subject and build up contrast by overdevelop-
ment, you will have a gamma of more than unity be-
cause comparatively small degrees of exposure are
represented by comparatively large degrees of density.

566

ex-

of determining gamma, as
mma of curve shown to be 0.9

hod

.21

H

i:

ii

-i

" c

.W

IE 0.

A1ISN30

THE PHOTOGRAPHIC NEGATIVE.

This is true even though the first photograph may be
contrasty and the latter one flat.

For those who like the figures in the case, let us
represent density by D. Then Dl will be the least
density in a sensitometrically exposed negative, and
D2 will be the greatest density. You cannot make
gamma readings on ordinary negatives for you must
know the log exposure for each tonal area. Now then
D2 minus Dl gives the difference or density range. If
we let IE equal log exposure, then we shall have 1E2
minus 1E1 equals the exposure range. Having these
factors from controlled exposure and from densitom-
eter readings we also have :

D2 Dl
Gamma a

1E2 1EI

However, there is a much simpler way to do it,
and this is the graphic method in which gamma is de-
termined by utilizing an otherwise wasted corner of
your graph sheet (see Fig. 114). On the base line of
the graph, which includes log E 1.0, 2.0, 3.0, make a
dot where any one of these four vertical lines intersects
the base line. Now using a parallel ruler (or a triangle
if care is taken) draw a line from this dot and con-
tinue it until it intersects the next major vertical line ;
and make it parallel to the straight line portion of the
curve. Where this line intersects the next major divi-
sion read the density value of the intersection, and
this reading will be the gamma of the negative. Thus,
if you start at 1.0 read at the intersection of 2.0; or if
you started at 2.0 read at the intersection of 3.0, etc.

Gamma simply tells you whether the density dif-
ferences are relatively smaller or greater than the cor-
responding light intensity differences of the original,
and how much smaller or greater they are. True,

568

-SENSITOMETRY

gamma is of the utmost value in certain work, such as
color printing, or in motion-picture work where auto-
matic and semi-automatic methods must be held to a
standard. But it is open to serious question whether
or not the ability to determine gamma, or to work to a
certain gamma, has ever made or will ever make a re-
nowned pictorialist from a mediocre sna'p-shooter !
Personally I have never known of any amateur who has
been aided in his printing and enlarging by working
under sensitometric control. The results simply are
not worth the investment of time and money unless you
are working in some special methods where differences
too slight for the eye to appreciate will affect the re-
sult in some unwanted manner. However, it's good
to have an understanding of the real meaning of
gamma.

Gamma Infinity.

This is the greatest gamma which can be obtained.
While gamma is largely a function of development
liberally affected by the inherent emulsion quality,
gamma infinity is largely a matter of emulsion charac-
teristic, although decidedly affected by the developer
used. It is known that the longer development con-
tinues the greater will be the contrast. It is also known
that forced development invites fog. Therefore, when
that point is reached where fog is growing so rapildy
that the contrast actually decreases with further de-
velopment, then that turning point marks gamma in-
finity.

Because this end point is difficult to determine, be-
cause the high densities involved are difficult to read,
and because the presence of the fog makes* reading
difficult, gamma infinity is usually determined by com-
putation. Two strips are exposed exactly alike in the
sensitometer (it is better to expose a single strip and

569

THE PHOTOGRAPHIC NEGAT1VE-

cut it in half if the sensitometric design permits).
These two strips are developed so that one is developed
for exactly twice the time of the other. These are
known as tl and t2. Then the formula is :

Gamma Infinity

(2z gamma tl) ftg qr| iT'a t2

Gamma in Practice.

As we have said, gamma is highly important to
;he research worker, to the controller of mechanized
processing, to the developer of sound tracks, to the
color worker ; but the photographic picture maker will
make just as good pictures if he has never heard of
gamma. A remark similar to that once brought out
this question at a lecture: "Then you mean that we
should pay no attention to negative contrast just go
ahead and print everything on any old grade of pa-
per?" No, that is exactly what is not meant. If you
are going to base printing on gamma, then be sure you
understand what it means; do not confuse it with
negative contrast, for it is not the same thing at all !

For example, you can have a negative which is so
flat that it is hardly more than a monotone and it may
still have a gamma in excess of one. On the contrary,
you may have a negative so hard that no paper is soft
enough to print it, and yet that hard negative may have
a gamma of 0.5 or 0.6. Do not make this mistake!
No one can look at a negative and even guess at its
gamma unless he is familiar with the original !

With any given emulsion, gamma is a function of
development. If you have a developer which will de-
velop a negative to gamma 1 in 12 minutes at 65,
then any emulsion identical with the test emulsion will
also be developed to a gamma of one in that developer
in 12 minutes at 65. This is in accord with photo-

570

-SENSITOMETRY

graphic law. Now if you run tests and find that this
developer will develop to gamma 0.5 in 5 minutes, to
gamma 1.0 in 12 minutes, and to gamma 1.3 in 20 min-
utes, you are ready for the final test.

Expose two negatives. For one, stretch a gray
woolen blanket where even light will fall upon it, and
make a photograph which will include nothing but the
cloth. Then make a square tube of cardboard or wood
about a foot square and 2 feet long. Line it with black
velvet except for the extreme front edges ; paint these
edges white. Now make a head-on photograph using
the exposure shown to be correct for a pearl gray card
held 3 inches back in the mouth of the tube. Of course
the card is used only for the meter reading and is not
included in the picture.

Develop the blanket photograph 20 minutes and
the tube negative for 5 minutes. The blanket nega-
tive will be a gray monotone with no contrast range
whatsoever, although its gamma is 1.3. The tube will
present the extreme range of white and black as a
white too opaque to print and a black with no vestige
of silver deposit. Here you have contrast far too ex-
treme to be printed correctly, although you know the
negative gamma is only 0.5 !

No, the relationship between gamma and negative
contrast range is a vague one at best. As a governing
factor it might just as well not exist at all. True, these
examples are extreme, but they show the danger of
accepting as fact a principle which works out approxi-
mately under normal circumstances. It is true that
with normal light and normal exposure and normal
subject, the relationship between gamma and negative
contrast range become apparently closer. But the fact
that extreme examples such as the one just described
will shatter any evidence of normal relationship is
conclusive proof that lesser differences are only
changes of degree and not of kind.

571

THE PHOTOGRAPHIC NEGATIVE

Exposure Range.

Now, either factor of gamma is vitally important
to every photographer, but he learns their importance a
long, long time before he starts to worry about gamma.
We refer to the exposure range E2 - El and the den-
sity range D2 Dl. Exposure range is not often
thought of in those terms, but instead we speak of
"brightness range," although no one knows why. The
brightness is not the point, but the exposure is. So that
brings up another point which the student of sensi-
tometry knows full well but which other photographers
seem to ignore, and that is that in most normal photo-
graphs the exposure ranges from extreme underex-
posure to extreme overexposure all in the same nega-
tive. Shadows must be underexposed or they will not
look like shadows ; highlights must be overexposed or
they will be gray ! So our so-called "brightness range"
is known to student photographers by its true name,
exposure range the exposure caused by the bright-
ness is all we are concerned with.

Exposure range is important because subsequent
development must be controlled to correspond. If the
exposure range is extreme, development must be cur-
tailed ; but if it is low, then development must be ex-
tended. Now I expect this will raise some questions.
Are we not controlling gamma by this manipulation
of development ? Certainly we are, and this brings us
to the critical point of this whole discussion of gamma.

Automatic time-and-temperature development will
result in negatives which are all of approximately the
same gamma. This is the supposed ideal condition
but is it? It is if you are after fidelity of tonal repro-
duction, but is that to be desired ? We think not. This
means that dull subjects will be flat, and brilliant sub-
jects will be too hard to print. It may be acknowl-
edged that if the photographic process could reproduce

572

-SENSITOMETRY

faithfully the full range of the average, normal, out-
door scene then such a condition would be desirable,
and a constant gamma would be advantageous. How-
ever, photography is crude ; its limitations are closely
set, and only the abnormally dull object can be faith-
fully reproduced.

In view of the severe limitations of photography
it is usually (but not always by any means) desirable
to utilize the fullest possible tonal range in reproduc-
tion, regardless of the character of the original subject.
Therefore, the careful amateur will endeavor so to
handle his material that D2 Dl will always be a con-
stant and that means gamma will be varied in each
negative.

It is comparatively easy to maintain a relatively
stable gamma. Just use a developer kept up to stand-
ard, and use a rigid time-and-temperature system. In
short, the average, normal technique does just this ; and
the result is that the amateur learns to avoid many sub-
jects which would give him exquisite pictures, just
because experience has taught him that such subjects
are outside the limits of his gamma fixation (yes, he
learns to evaluate his subjects in this way even though
he never thinks of gamma) .

However, when density range (D2 Dl) is made
the standard, and the amateur learns how to vary the
time of development so as to approximate this stand-
ard, then there are few things indeed which the eye
can see which cannot be photographed and photo-
graphed well. Yes, gamma is like fire. It is an indis-
pensable servant when kept in its place, but a terrible
master when it breaks its bonds. It has already proven
a veritable "Old Man of the Sea," and has made a good
start toward ruining the success of many promising
amateurs who have elected to dabble in the laboratory
end instead of putting cameras to their destined pur-
pose that of making pictures.

573

THE PHOTOGRAPHIC NEGATIVE _

H & D Value.

If you expose a half-dozen films identically in the
sensitometer, give them increasing times of develop-
ment, then draw all the curves upon the same paper,
you will see that each increase in development makes
the straight-line portion become more nearly perpen-
dicular. However, if you disregard the toe and extend
these straight-line portions, you will find that all these
extensions eventually meet at a common point (see
Fig. 115). This may or may not lie on the base line.
(This is not always strictly true, but in simple emul-
sions was true often enough to be recognized as signif-
icant in determining H & D value.) If there is any
free bromide in the developer, either added or dis-
solved from the emulsion, then there is a shift of the
intersection toward the right. It might also be men-
tioned that in some of the newer super-speed emul-
sions, continued development seems to shift the whole
curve to the left, creating the photographic paradox
of increasing the sensitivity of the emulsion by over-
development.

However, ordinary emulsions do work quite regu-
larly according to the law. This point of intersection
is known as the inertia point. The value indicated by
this point of intersection is used as a divisor for a
factor (H & D factor 34) which gives the H & D num-
ber of that emulsion. For example, if the inertia is
0.02 we have the H & D number :

34

- H&D 1700.

0.02

It will be noticed from this equation that high-speed
emulsions have low inertia values while slow emulsions

574

0.5 I.O 1.5 2.0 2.5 3.0 3.5 1

LOG EXPOSURE |

Rg. 115. The longer the development, the more nearly vertical the straight-line por-
tion, and the higher the gamma. Various developing times with a given emulsion
produce curves as shown. Extensions of the straight line portions meet at "inertia point."

_

\

-

-

~

-

-

i

\

-

l

V

V

\

\

\

-

\

y

V

-

^

\

\

y

\

..

-

-

-

\

s

\

^

\

i

-

-

-

s

\

\

s

_

s

v

\

"

-

^

.

y

\

s

1

>

\

y

\

-

-

-

-

\

^

V

s

\

\

s

V

\

-

s

Sy

y

-

-

s

t

^

s

x

"

-

v

^

V

^

\

V

--

\

^

N

^

-

-

-

s s

^
s^

b

-

J

L \

"VI

&

\BL

-

-

.

_

NT

-

-

T

io o in o jn o

CM oi - -

AIISN3Q

THE PHOTOGRAPHIC NEGATIVE-

have high inertia values. For years the H & D number
was used as a speed rating, and as originally developed
it was a good one as it indicated an exposure between
the threshold and full exposures. (The minimum ex-
posure that will yield a perceptible darkening of the
emulsion is known as the threshold exposure.) How-
ever, the manipulation to which the H & D system has
been subjected may be recognized by the fact that
emulsions of similar sensitivity but made by different
manufacturers may be rated as far apart as four to
one! Today, the H & D value has 110 value as a
measure of sensitivity.

Some Further Considerations of Sensitometry.

So far we have covered the headlines of this fas-
cinating subject ; now suppose we explore a few of the
byways. There is always something new in sensi-
tometry, but there are some almost fundamental facts
to which attention should be given after the basic
principles have been grasped. These things are by
way of elaboration of the basic discussion.

As sensitometry is concerned with the physical
characteristics of the photographic image, that is, the
physical properties of the silver embedded in the gela-
tin, and as the principal property of the image is that
of obstructing light, we should give some attention to
the formation and structure of that image. This
applies not only to the normal image, but to one which
is formed under what may be termed abnormal condi-
tions, such as with the use of a filter.

The photographic image is a very real thing; it
has length, breadth, and also thickness. It may prop-
erly be considered to consist of a great many layers
of silver grains laid one upon the other, so that density
is not so much a matter of a closer grouping of the
particles, as a matter of the depth to which additional

576

-SENSITOMETRY

layers are deposited. This is true of both the negative
and positive images. In fact there is, sensitometri-
cally speaking, no differentiation of negative and posi-
tive. Both are photographic images.

The instruments used in large research labora-
tories are elaborate and costly. Those used in smaller
laboratories are also costly if they are of high accuracy.
It is true that inexpensive densitometers are said to be
in process of development and soon to be offered to the
public ; but at present a high-grade densitometer runs
above one hundred dollars and any instrument worth
' using will cost at least a few dollars less, than fifty.
Therefore, let us first turn our attention to home-made
devices which, although in no way substitutes for con-
ventional instruments, will prove of value and permit
a certain type of seiisitometric control to be exerted.
By giving up actual numerical valuations and working
on a comparison basis, such devices may be made at
home.

The Comparison Scale.

Obtain a section liner from a dealer in artists' and
draughtsmens' supplies. This is a device which moves
a straightedge over a sheet of paper at regular inter-
vals so that uniform shading may be drawn. Set this
device so that each movement of the straightedge will
advance it about J4 inch or more. Make tests with
your enlarger, so that a 1 -second exposure will pro-
duce a barely perceptible tint upon a sensitized film
(use process film or similar slow material). You will
probably have to close the lens diaphragm to its
smallest aperture and insert one or two pieces of white
paper in the negative carrier. If paper is used, throw
the lens out of focus to overcome the grain pattern of
the paper.

Lay out a piece of film. This need not be more

577

THE PHOTOGRAPHIC NEGATIVE.

than 2 inches wide and 10 inches long. Set the section
liner so that the straightedge will move down along
this strip. Now with masking tape attach a sheet of
black paper to the leading edge of the straightedge,
and bring it down to cover the film with about J4 inch
of the paper extending beyond the end of the film.

Turn on the light, and using either a sweep clock
or audible timer, move the arm once each second.
The paper will be drawn back J4 inch each second and
you will then expose the film in single-second intervals
for 32 seconds. As soon as the black paper gets to
within J^ inch of the end of the film, extinguish the
light and develop the film. If the contrast range is
not sufficient, you can make another strip using 2-
second intervals. If you want to make a log exposure
strip, with the exposures running 1, 2, 4, 8, 16, 32, 64,
128, 256, and S12, start with an exposure of 256 seconds ;
then move the paper and expose for 128 seconds, and
so on down the line until you get to the end of the
series. Then when you have exposed for one second",
you move the mask and again make a 1 -second
exposure. Leave one unexposed section on the end.
This gives you the desired series as can be seen :

Total

Single Total ***?

No. Exposure Exposure Strip

1 256 256 512

2 128 256 + 128 - 384 256

3 64 $84+64-448 128

4 32 448+32-480 64

5 16 480 + 16 - 496 32

6 8 496+8-504 16

7
8
9
10
11

504+4-508 8

508+2-510 4

510 + 1 - 511 2

511 + 1 - 512 1

578

-SENSITOMETRY

Fig. 116. An amateur densltometer, wherein step wedge ii pulled
through by hand. Negative lies on top of box irndtr whtt* window;
comparison field is seen within by looking into an aperture at
the end. Object in the foreground fi a simple reflection scale.

579

THE PHOTOGRAPHIC NEGATIVE-

With care and a few trials, a strip can be obtained
which has these definite relationships. When the
light intensity required to produce a fully graded strip
under such conditions has been obtained, it is sug-
gested that a full 8x10 inch film be exposed. After
it is dry it can be cut into 6 or 8 identical strips. This
strip forms the basis of your comparison densitometer.
Of course the disk sensitometer is more convenient
than the section liner for making this density wedge,
but it is also more difficult to construct. In their most
elementary form the tone wedge and the section liner
with which it was exposed constitute your "densitom-
eter" and "sensitometer." We shall refer to the density
wedge as a "comparison scale."

If you have access to a densitometer you can read
the values of your scale steps, but this isn't necessary
for working upon the comparison basis. Such a com-
parison instrument has been used quite successfully
in color control, and while it lacks a great deal of the
advantage of a good densitometer, it is actually to be
preferred to the cheap but inaccurate meter.

The Gray Scale.

For the best results it is advisable to make use of
a gray scale in making your exposures, just as is done
in color photography. These may be purchased for a
few cents, but as the commercial scales have tints in
regular squares, it is used only as a standard.

Expose several sheets of matte paper to varying
light and try to obtain uniform tones over the entire
sheet. Try to make not less than 30 degrees of gray
between black and white. Fix out an undeveloped
sheet for the white, and expose a sheet to a 100-watt
lamp for 2 or 3 seconds to produce the black step.
When these are dry, compare them with the com-
mercial scale and try to select matching tints. Num-

580

-SENSITOMETRY

ber these on the back of the sheet and make your own
scales from them. Make the individual tints about
3 x 8 in one scale, 2 x 6 in another, 1x3 and J^ x 1J4
in two more. The idea of the various sizes is to obtain
a readable size tint along the edge of your negative
regardless of the size of the original object. Include
this scale when making exposures, and place it so that
the ends of the individual tints extend slightly outside
the picture area. Now when the negative is developed,
the safe edge may be trimmed off the side where the
gray scale appears, leaving the tint running right off
the edge of the film.

Assume that the tints are Ji inch wide in the nega-
tive. Cut a length of black cardboard slightly larger
than your comparison scale, and cut square notches
into one edge, Ji inch deep and % inch wide, spaced
so that one notch is centered upon each single step in
your scale. A preferred construction is a similar but
wider cardboard with rectangular openings y& x J4
inch. The comparison scale is attached permanently
or temporarily to this mask, and the black border
prevents visual confusion. The negatives are held
against the edge of the comparison film, and the scale
steps in the negative can be matched with those of the
comparison scale, or they can be assigned intermediate
values. You can interpolate intermediate values in
making the original scale to get 21 steps instead of 11.

It may be said in passing that a, series of paper
tints each with a small hole punched in it provides an
excellent comparison gauge for reading the reflection
values of paper prints. Such a gauge can also be
calibrated by comparison with a reflection densitom-
eter, but in such case it is advisable to convert the
reflection densities to reflection percentages.

And now that we are so familiar with sensitometry
that we make our own instruments and set our own
standards, what is it all about ?

581

THE PHOTOGRAPHIC NEGATIVE _

The Photographic Image.

Let us assume that we have a series of neutral
density screens which transmit just half of the incident
light. While these could be stacked up without seri-
ously impairing the experiment, let us place them one
upon the other with a space of an inch or so between
each adjacent pair. Then let us see what happens to
100% of incident light.

Screen
Incident
Transmitted
Osdty
Density

1
100%
50%
2.0
0.30

2
50%
25%
4.0
0.60

3
25%
12.5%
8.0
0.90

4
12.5%
6.25%
16.0
1.20

5

6.25%
3.125%
32.0
1.50

The negative is formed by the deposition of silver
in the gelatin, deposited in a certain ratio to the amount
of light to which it is exposed. If we ignore the toe
and shoulder curves for the time being we may assume
that if one unit of light will produce a unit deposit of
silver, 2 units will deposit 2 units, and so on. Now
if we consider 5 successive exposures, each of which
results in a silver film of 50% transmission quality,
these 5 exposures may be substituted for the 5 neutral
density screens, and the screen numbers at the top
of the table may be now referred to units of light in
the exposure. This reveals some interesting facts.

If 5 units of exposure gave a deposit 5 times as dark
as that following a single unit exposure, then exposure
5 would produce a 10% transmission, an opacity of 10,
and a density of 1.0. However, the light-obstructing
power grows by steps of density 0.30, and as density
has a logarithmic progression, we see that the normal
growth of photographic density is also a logarithmic
progression. This is thfe fundamental reason for using

582

SENSITOMETRY

logarithms instead of ordinary values simply because
it reflects the natural progression of photographic
image growth.

Film-Speed Ratings.

One of the most widely used applications of sen-
sitornetry is that of determining the specific sensitivity
or "speed" of an emulsion* We have already shown
how the H & D values are obtained. Other systems
also* are based upon sensitometry, but their origins
and methods of development are widely different.
There are some systems which take the least amount
of light which will produce a visible density as the
fundamental index. This is hardly advisable. Two
emulsions might have the same threshold exposure
that is, required the same amount of light for the first
visible density to be produced. Yet if the first emul-
sion has a long, sloping toe and the other a short one
rising abruptly, the second emulsion would have the
greater sensitivity.

Other systems base their indexes upon the ex-
posure which is necessary to produce a negative den-
sity of 1.0. This is a much better system, for density
1.0 lies more or less midway in the tonal range of a
good average negative, and so avoids the possible
error of starting with the tip of the toe. As we have
seen, the H & D system uses neither of these but in-
stead an extension of the straight-line portion of the
curve.

All systems have their advantages and disadvan-
tages ; but those systems which aim at half-tones rather
than either extreme are likely to be more satisfactory.
Incidentally, it should be noted that although the
inertia value is found at the base of the graph, it is a
midway system because it is governed by the central
portion of the curve.

583

THE PHOTOGRAPHIC NEGATIVE

The Eder-Hecht Sensitometer.

At one time it was popular for amateurs to make
their own determinations of relative sensitivity, and
among the simpler sensitometers for this purpose was
the Eder-Hecht type. This is nothing more than a
printing frame with spring covers over both faces.
Enclosed is an artificial step negative in which each
tenth step is numbered. Along the sides are strips of
gelatin colored blue, green, yellow, and red, with the
graded steps running 1 beneath them. The film to be
tested is exposed behind this negative and developed
(see Figs. 117, 118).

Naturally such a device is useless unless there is
some predetermined exposure control, because with
fixed and numbered steps, a correspondingly fixed
exposure is necessary. To meet this demand the outfit
includes a needle set in a wooden handle, and a box
containing several small pieces of magnesium ribbon.
The exposure is made by picking up a piece of mag-
nesium, supporting it at a distance of one meter from
the frame, and igniting the magnesium.

This system is very simple, almost crude, but the
results obtained with it are strikingly uniform, and
for certain purposes this sensitometer finds use even
today in the laboratory. For example, the tricolor
filters give the whole story of sensitivity to the three
separation colors on a single film. This information
may be translated directly into filter factors. The
only correction is that of differences between filters.
To overcome this, the original filters are removed
from the negative and replaced by the ones in actual
use. Or one side may contain the A-B-C set of tricolor
filters and the other side the filters used for making
separations from Kodachrome. For this purpose alone
the sensitometer is worth more than its modest cost.
The Sanger-Sheppard sensitometer is similar except

584

-SENSITOMETRY

u

Fig. 117. Film exposed under the Ed er-Hecht tablet. Note color
stripes R, red; G, yellow; GR, green; B, blue. Film tests E-H 90.

THE PHOTOGRAPHIC NEGATIVE.

that the densities, instead of being arranged in ladder
formation, occupy rectangular spaces. It is not quite
as convenient as the Eder-Hecht type, but is highly
satisfactory.

This description of the Eder-Hecht sensitometer
has been given because it is typical of those measure-
ments of sensitivity which base the film-speed rating
upon the exposure producing the least perceptible
deposit. In the discussion of H & D values, the prob-
lem of sensitivity rating was mentioned; but as the
H & D characteristics have more importance than
mere sensitivity rating, the subject was not elaborated
there.

The Eder-Hecht sensitometric tablet already de-
scribed is the actual basis for speed rating according
to the Eder-Hecht rating system, the last visible num-
ber being the speed number of the film. It can be
understood that wedges turned out in quantity will
probably vary, and this is true; so we encounter the
first source of possible error. However, this is minor
when we consider the low possibility of determining
the "least perceptible deposit," and the great likelihood
of its being obscured by fog. We have already pointed
out that the threshold exposure does not take into
account the shape of the curve, and threshold ratings
are to be regarded with deep suspicion.

Scheiner Film-Speed Ratings.

It may surprise many amateurs to know that the
Scheiner film-speed ratings are also threshold ratings.
They differ from Eder-Hecht ratings only in the
method of primary determination and the values of
the numbers used. In both cases a series of numbers
in arithmetic progression indicates logarithmic in-
creases. Thus, every three degrees Scheiner represent
a doubling in sensitivity. This same thing is true of

586

THE PHOTOGRAPHIC NEGATIVE.

DIN as well as of Eder-Hecht. The Scheiner exposure
is made on a logarithmic base with the ratios of ex-
posure extending from 1 to 100. The resulting in-
creasing exposure is read by means of an arbitrary
scale. Thus we have the series :

Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Relative

Exposure 1248 16 32 64 128 256

and so forth. The Scheiner is belter than the Eder-
Hecht only in that the determinative exposure is made
through a time-base sector wheel, and to that extent
has less liklihood of error. All of the fundamental
objections to the threshold system still hold.

Other Systems of Sensitivity Rating.

Some years ago the DIN system was given to the
world as the ultimate in sensitivity rating. It was
foolproof; it was unalterable; it was the universal
standard. Now what is the DIN system? Why, noth-
ing whatsoever but our old friend Scheiner except
that the exposure required to produce density 0.1 is
taken rather than the vague threshold exposure. Den-
sity 0.1 is still so far down the toe that the objections
to the threshold index still hold insofar as the curve
shape is concerned; it is still hard to determine and
it is easily masked by fog, but it offers the advantage
of a firm basis, a known, definable, and measurable
point of departure. In actual practice DIN offers no
real advantages over Scheiner, and it is subject to the
same manipulation as is the Scheiner system. In recent
years the Scheiner rating has had to be identified as
being European, German, or American, because a film
rated at Scheiner 23 in America was rated 26 in Ger-

588

-SENSITOMETRY

many and 29 in other parts of Europe. The German
film of equal rating was one-half the American speed,
and other European films of equal rating were only
one-fourth the American speed. Naturally this re-
sulted in great confusion, particularly when Ameri-
cans went to Europe and suddenly found themselves
losing pictures through gross underexposure.

The Watkins and Wynne systems of rating were
based upon the inertia point and differed from H & D
largely in their numerical values. They have been
little used in this country and are now obsolete, so we
will not discuss them.

We now encounter two modern systems, one the
minimum useful gradient, the other the D-log-E base.
The minimum useful gradient is based upon that ex-
posure which falls upon the part of the curve where
the density increase per unit of exposure is a definite
fraction of what it is on the straight-line part of the
curve. In short, it lies at a definite point in the upper
part of the toe curve, where trie chosen fraction is great
enough for good results. In practice it is that exposure
which will produce a print satisfactory to the average
amateur. This sounds rather vague, and it is a vague
system until the important fraction has been assigned
a definite value. The system is being used by East-
man, who have determined the value of the fraction as
suited to their needs.

An exposure which is based upon the characteristic
of the relationship of density and log exposure for a
definite gamma is probably the most sensible approach
to the problem. The point is located in the straight-
line portion of the curve, and permits the inclusion
factor for gamma, so that the speed rating for a process
film recognizes the fact that it will be developed to a
high gamma, while the factor for a portrait film recog-
nizes the fact that the gamma will be low. Such a
system is the Weston method of rating. The fact that

589

THE PHOTOGRAPHIC NEGATIVE-

this system gives good results in all parts of the world
and that even beginners are usually successful when
using it suggests that, if not the ideal speed rating, it
is at least a substantial approach to the ideal.

A truly ideal rating cannot be reached, because
there is no "correct" exposure, no absolute sensitivity
rating. The sensitivity of a film depends upon the
nature of the light, the color, texture, and general form
of the object, and above all upon the aesthetic effect
desired. Therefore, if we have a system which will
indicate for us the exposure which would be satis-
factory in a sensitometric test, we can let experience
guide us in the degree of departure necessary to cope
with the details of the individual picture to be made.
It must always be remembered that strict sensitometry
and aesthetics mix about as well as oil and water. You
can emulsify them, but it is a difficult task.

Curves and Their Interpretation.

The characteristic curve as drawn from sen-
sitometric data provides a wealth of information re-
garding the developed image, as well as the inherent
characteristics of the emulsion and the developer. But
there are other curves just as valuable as the H & D
curve, and as these also provide information which
may be included within the scope of sensitometry, we
should give some attention to them. Those relating
to specific spectral sensitivity are perhaps the most im-
portant, because they reveal the exact color response
of the emulsion. Quite similar curves show the char-
acteristics of filters, and by combining an emulsion
curve and a filter curve you see graphically the actual
effect of using a combination of specific filter and
specific emulsion.

Visible color is made up of a series of wave forms
whose lengths vary from about 380 millimicrons in the

590

-SENSITOMETRY

violet to about 750 millimicrons in the red (see Fig. 3,
Ch. I). We say "about" because the exact limit varies
with different individuals. The radiation immediately
adjoining the violet that is shorter than the shortest
visible violet marks the beginning of the ultraviolet,
while that adjoining the deepest visible red marks the
infrared. The only separation is in the vision of the
beholder, because the wave train goes right along in
unbroken sequence. It is common knowledge that
some people cannot hear a shrill sound while others
are able to hear it. It is the same with light ; but when-
ever the boundary is passed, for that individual, either
ultraviolet or infrared starts. For that reason the
laboratory curve usually includes considerable of both
without any demarcation. This is logical because the
emulsion is not subjected to the same limitation as is
the human eye, and as long as we have emulsions sensi-
tive to wavelengths in the ultraviolet (in the neighbor-
hood of 250 to 300 millimicrons) and in the infrared
(in the neighborhood of 1000 millimicrons), our curves
should be based upon that scale.

However, as a rough guide keep in mind the divi-
sions 400, 500, 600, and 750. For approximations you
can regard these as marking : 400, beginning of visible
violet ; 500, boundary between blue and green ; 600,
boundary between green and red; and 750, the lower
limit of visible red.

Therefore the base line of your graph is divided
into sections representing wavelength 250 to 1000 for
the total spectrum, or 380 to /SO for the visible
spectrum, A convenient division is one space for every
25 units ; start at 375 and let consecutive divisions equal
400, 425, 450, 475, 500, and so on. For the benefit of
those familiar with the non-uniform spectrum of a
prism, it may be said that the spectrum produced by
the diffraction grating is of uniform linear value, so
that the location of any two points provides a standard

591

CO

CD
O

O

LU

0.
CO

O

I

O

Z

O
O

O
I

<

2

O

X
O
O

o
i
<

2
O

I
O

g O

IS

THE PHOTOGRAPHIC NEGATIVE-

for the accurate determination of wavelength value. Of
course in a graphic outline, the spacing is uniform by
reason of construction.

As a rule the vertical scale is logarithmic, the first
line representing density 1, the second 2, the third 3,
and so on. Thus, in actual light intensity the first
line has a value of 10, the second of 100, the third of
1000, and so on. This is the reason spectral photo-
graphs published by film manufacturers have such
slight differences from point to point.

Wedge Spectrograms.

If light is passed through a prism, or preferably
through a fine grating, it will be spread out into a
colored band in which the wavelengths are side by side
instead of superimposed. If this band is projected
upon an emulsion surface, an automatic record of the
color sensitivity is obtained. However, as the reaction
is recorded simply as varying densities, special instru-
ments are required to convert this record into a graph.
A gray wedge which varies in density from 0.0 to 4.0 is
placed in the path of the beam. ' Under such restraint,
the brighter parts of the band pass through heavier
portions of the wedge, while weaker colors can pass
only through the thinner portions of the band. But if
the light is of uniform strength its intensity grades
from bottom to top, and the colors to which the film is
more sensitive will be recorded as higher peaks than
others. Thus automatically a record is made which
indicates the sensitivity of the emulsion to that color
by the height of. the line (see Fig. 119).

These wedge spectrograms are supplied by most
film manufacturers, and may be converted directly into
curves by direct copying, as the spectrogram is a true
graphic curve in itself. Thus by taking the wavelength
divisions and the density divisions from the spectro-

594

ULTRA-
VIOLET

VISIBLE SPECTRUM

INFRA-
RED

VIOLET) BLUE) GREEN | ORANGE |

RED

Ik^SSSSSSaSSSS

B

CC^VCiVCsWNVCSNV
**CaK<C3tt*CC*N!COO

ssssssiPiifi^ss

Fig. 120. A, color curv* showing response of panchromatic film
to daylight. B, the transmission curve of a deep yellow filter. C,
curve* superimposed show net result of the combination. The
reason for large filter factors with deep filters It obvious.

THE PHOTOGRAPHIC NEGATIVE.

gram and transferring them to your graph paper, you
will have a true graph of the color sensitivity of the film.

The Filter Curve.

The filter curve presents greater difficulty. If it is
made photographically, the resultant curve is neces-
sarily one which gives the net result of emulsion color
sensitivity plus filter curve. For example, if it is a red
filter and the emulsion used is ortho, there will be no
result. This means the filter is opaque, as it is to an
ortho emulsion j but that is far from the true char-
acteristic of the filter. Commercial testing laboratories
will make spectrophotometric records of a filter, or you
can make a good guess with an ordinary spectroscope ;
but generally you will find it easier to depend upon the
curves supplied by the manufacturer.

If you redraw the filter curve upon the same type
of graph blank used for the emulsion curve, but drawn
upon transparent paper, you can superimpose the filter
curve upon that of the emulsion, then that portion of
the curve which is common to both curves is the net
usable light. To make such readings easier, it is sug-
gested that instead of a single line curve, the portion
of the graph outside the line be blackened. This is
done on both curves, then the usable light for any com-
bination is represented by the amount of white paper
visible. This white area may then be identified as to
wavelength (color) and intensity (density), so that you
will have a very good idea of the actual photographic
result. The accompanying diagram (Fig. 120) will
make this more clear.

Most other graphs used in photography are of the
usual type; that is, they represent time against tem-
perature, concentration against vigor of action, or other
common factors, and are read exactly like the stock-
market and production graphs which are found in the

596

-SENSITOMETRY

daily press. The curves already discussed, i. e., those
which involve logarithmic values, are more significant,
and are the ones most commonly used.

Thus far we have skimmed the highlights of sen-
sitometry as applied to the negative. No considera-
tion has been given to the subject of the control of the
positive through sensitometric analysis, nor have we
space for such a discussion. Hovs&ever, this is a method
of control which, like the exposure meter, gives us the
technically correct exposure from which we may depart
in any direction and to any degree desired for the pro-
duction of some specific effect.

We have purposely avoided the more advanced
mathematics of sensitometry and have limited theoreti-
cal discussion as far as possible. Sensitometry is a
complex subject, one which has not been thoroughly
explored. To treat of it fully would not 'only require
far more space than we have available, but such a
discussion would undoubtedly prove unendurably bor-
ing. However, it is hoped that the treatment of the
subject as presented will enable you to utilize the
fundamentals of this branch of photographic science,
and that it has presented clearly the fundamentals of
the measurement of negative characteristics. , There
are two facts so vital to sensitometry that we venture
to, close this chapter by repeating them.

Sensitometry is based upon these two funda-
mentals :

1. A series of graded exposures of known value.

2. The accurate measurement of the densities re-
sulting from this series of known exposures.

597