THE OPTICS OF PHOTOGRAPHIC LENSES 



35 



combination of vignetting and the cos* <t> law, will be made apparent, together with 

 the quality and sharpness of definition over the whole field. To detect inequality of 

 illumination most easily, the exposure should be on the short side, because then the 

 underexposure produced by vignetting becomes exaggerated. Enlarger lenses should 

 be tested with a cross-ruled glass screen in place of the negative, the image being 

 caught on a sheet of bromide paper. 



There have been many special lens-testing benches constructed which facilitate 

 the testing of photographic lenses, but the direct photographic test, accompanied by 

 tests for ghost images (see page 22), is likely to be as useful and as satisfactory as any 

 other for the actual photographer to perform 

 himself. A brief bibliography of lens tests is given 

 at the end of this chapter. 



Enlarging Lenses. — In general, the require- 

 ments to be satisfied by the lens on an enlarger are 

 not essentially different from those of a camera 

 lens. There are, however, a few minor points 

 which should be emphasized. Strictly, enlarging 

 lenses should be designed for the approximate 

 magnification under which they will be used, as a 

 change in object distance may seriously upset the 

 corrections of a photographic lens. For instance, 

 some good anastigmats which give a flat field with 

 a distant object project an image when used in an 

 enlarger which is backward curving, i.e., an image 

 in which the corners are too far from the lens. 

 Fortunately stopping down the enlarging lens will 

 help all aberrations except distortion and trans- 

 verse chromatic aberration. As regards these two 

 defects, it can easily be shown that, if the lens is 

 symmetrical about a central stop, they will be 

 automatically removed when used at imit magni- 



FiG. 32. — Diagram of a 

 simple view finder. 



-Diagram of the brilliant 

 view finder. 



fication. Hence, if good correction of these aberrations is important, a lens of 

 a symmetrical type is to be preferred for use on an enlarger. Incidentally, in 

 three-color work with color-separation negatives, chromatic difference of magnifica- 

 tion is a very serious defect and should be avoided wherever possible. For com- 

 mercial color work, apochromatic process lenses are made in which this aberration is 

 very well corrected for all colors of the spectrum. 



View Finders. — The simplest view finder is really a small camera with a simple 

 lens, a mirror, and a ground-glass screen (Fig. 32), and, since only a single mirror is 

 used, the picture seen is reversed from left to right. However, as ground glass is very 

 wasteful of light, it is more usual now to project the image into the plane of a viewing 

 lens, which is so chosen as to project an image of the first lens into the plane of the 

 observer's eyes (approximately) (Fig. 33). The observer then sees a brilliant picture 



