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84 CINEMATOGRAPHIC ANNUAL
to think that it has no blue in it and it is equally erroneous to think that arcs emit no red and yellow light.
At any rate, the test described above permits employing color differences as radical as may be desired. Filters running from red to blue may be introduced between the light source and the test plate, where they cannot influence the performance of the lens by virtue of any optical imperfections in the filter, and tests can be made that will indicate the performance of a lens for any desired color. At the bottom of Fig. 2 are two series of shots marked 4 R and 4 Blue. These were obtained with a 50 mm. Raytar lens focused with white light and exposed with a red and a blue filter respectively without refocusing. In spite of this very radical color difference the image quality is identical and the same as for white light. For this test an incandescent lamp was used and the filters employed were Eastman's Projection Red No. 24 and a C filter for the blue.
The result of the test was not surprising. The introduction of incandescent lighting and panchromatic emulsions into motion picture practice has led to some natural but generally unnecessary concern regarding color correction of objectives. This concern would have been well founded if low aperture lenses of 16 to 36 in. focus such as are used in commercial photography had been in question. The large aperture short focus lenses employed in motion picture photography are a different matter. For the former, the residual chromatic aberration of the colors other than those two for which chromatic correction is made, the so-called secondary spectrum, is of prime importance. In high aperture short focus lenses secondary spectrum becomes secondary in another sense. It becomes of less significance than other residual aberrations in the lens and also to mechanical considerations involving camera, film focusing, etc., hence the statement at the beginning of this paragraph.
Inasmuch as this subject has gone farther than speculation and conversation and has been discussed in the pages of the Amet'can Cinematographer it will not be inappropriate to look into the subject a little more in detail. We have no quarrel with any type of color correction that yields good results but we believe that it is misleading to claim there is only one type which is satisfactory. In spite of the fact that achromatic telescopes have been made since 1758 lens designers are not in complete accord as to just how achromatic telescope objectives can best be corrected for color. There is no convincing evidence that within the limits of the argument one type is not as good as another. For visual instruments of all kinds each lens designer prefers or is satisfied with color corrections possibly slightly different from those preferred by other designers and yet all of them may produce equally reputable instruments.
One reason for this, of course, lies in the fact that generally only two colors can be brought to a common focus. That m?ans that there is necessarily a multitude of other colors coming to all sorts or other foci. If we were dealing with light of only two wave lengths the problem would be relatively simple but we have to deal witn light of all the colors in the visible spectrum and the choice of which