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A METHOD FOR TESTING PHOTOGRAPHIC LENSES 83
method. It is a question, however, how much detail will remain in the halftone reproduction.
The test permits an interesting study of the effect of emulsion grain on the sharpness of the picture. The two circles reproduced in Fig. 3 tell the story. Of these two circles A was produced by a 50 mm. Raytar at full aperture and is in fact the center shot of lens 4 in Fig 2. Circle B was obtained in the following manner: A circular disc of white paper of the same diameter as circle A was mounted on a black background and stationed at the place where the photograph A was taken. In the place occupied by the test plate when A was taken a piece of panchromatic motion picture negative was located. The lens was stopped down until no spherical aberration could be detected and chromatic aberration was eliminated by means of a filter. Exposures were made of several different lengths and the best negative selected from the group. This was then printed on motion picture positive by contact printing. We now had a test plate that consisted of motion picture positive with a transparent spot in it whose diameter should have been exactly equal to the diameter of the hole in the test plate from which circle A was made. This positive print was substituted for the test plate, projected and photographed exactly in the same manner as before except that the lens aperture was reduced and a filter used to eliminate aberrations. The difference in sharpness of outline in the two circles is principally the result of emulsion grain.
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Fig. • 3 Shows defining power of lens as compared with that of film.
From this experiment the conclusion is inevitable that in the center of the picture the lack of perfect sharpness while it depends on both residual aberrations and emulsion grain, the emulsion grain effect is greater than the aberration effect in most of the lenses tested. Getting away from the center of the field the aberration effect rapidly grows larger. Here the superiority of the Raytar lens is evident.
Reference was made above to the possibility of using such a test also to investigate the performance of a lens on different kinds of photographic material and for different kinds of light. Light differs, photographically, in respect of distribution of energy with respect to wave length. This is generally but not always apparent as a difference in color. Quality differences in motion picture practice are limited to a relatively small range. Incandescent lighting is doubtless somewhat redder than the ordinary arc or daylight but it is a mistake