PHOTOGRAPHIC LENSES
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to the horizontal causes vertical lines in the object to appear to converge in the image. The simplest method of dealing Math -such
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subjects when, as frequently happens, the desired view cannot be obtained with the axis of the lens intersecting the centre of the sensitive surface is to decenter the lens with respect to this surface, so that, for instance, in taking a view of a tall building from the ground the lens axis may be several inches above its normal position. The parallelism of vertical lines is thus preserved in the picture and the whole face of the building ia kept in satisfactory focus if the image plane is kept vertical. This method, in extreme cases, is, however, unsatisfactory on account of the poor quality of definition in the image in parts remote from the lens axis, and also on account of the extreme difference thus introduced in the effective exposures for different parts of the image. In these extreme cases it becomes necessary to use the lens in an inclined position, and to rectify the convergence of vertical lines by re-photographing the original record.
Referring again to the figure, which shows a section in the plane containing the axis and the projection of the axis on the object plane, let Cu be a line, parallel to the object plane PQ V, through C, the axial point of the common unit planes CV, meeting the image plane Vpq in v. Then v is the vanishing point in the record for the converging lines corresponding to the parallel vertical lines of the object. In copying this record so to correct this convergence it is evidently only necessary to ensure the parallelism of the line of greatest slope of the new image plane to Cv when pq is treated as the object. In order, however, to obtain a correct relation between horizontal and vertical distances in the final picture, the axis of the lens in this second operation must intersect the first record now serving as an object in the same point a as did the axis of the lens when the record was made. If the final record is to be increased in scale to in
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times that of the first record, the unit plane in the principal section must be made to meet the record in V where Vv = m.v.a. If now a circle is described on ~Va as diameter the axial point of the unit plane C must lie on the circumference, thus making VCa a right angle. It is evident from this construction that the only limitation which must be imposed on the lens used for the second operation is that its focal length shall not exceed $Va or %(l + m)va where m is considered positive. The original lens is, in such a case as has been considered, suitable for the purpose, but one of shorter focal length is frequently more convenient in order to reduce the axial separation between the object and image planes.
§ (7) THE POSSIBLE DEFECTS OF PHOTOGRAPHIC LENSES. — The matters that have been discussed hitherto relate to the correct use of a lens, and apply to all lenses alike, whether they are well or less well corrected for aberrations. The defects now to be considered depend upon the design of the lens, and are not under the control of the user.
(i.) Filming.—A defect which is not likely to be present in a new lens, but may develop in the course of time, is the filming of one or more of the glass surfaces. This is particularly likely to occur with extreme barium crown lenses of the types used in anastigmat lenses. When this filming is slight it is innocuous, but in some cases it causes the lens to appear of a decidedly brown tinge when examined by transmitted light, while iridescent colours may be seen by reflected light. In these more extreme examples the rapidity of the lens is seriously affected, and it is necessary to have the lens surfaces ro-polishecl. It is needless to say that this should only bo done by the maker of the lens.
Somewhat similar colours may be scon by reflected light when two surfaces which arc supposed to be cemented together have become partially or entirely uncemented. The most probable cause of this defect is a fall or violent shaking. In those oases also the lens ought to be returned to the maker for correction.
(ii.) Mounting Defects.—A lens that has been performing satisfactorily may be found unsatisfactory after cleaning or re-mounting. This is frequently experienced after a lens formerly carried in the maker's mount has been fitted into a between-lens shutter. The cause may be a scratch on the lens surface, which scatters light in all directions within the camera ; strain due to screwing up a Ions boo tightly in its cell; changes in the separations of components either through a lens being loose in its cell or to incorrect adjustment of the new mount; the insertion of a particular component the wrong way round, or to interchange of front and back com-
2D architectural subjects where the use of a lens with its axis inclinedsuch proportions that each lens he manufactures will be found sufficiently satisfactory by a large class of users.