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June, 1930] VARIABLE AREA SOUND FlLMS 643
range. The remainder of the paper will show how little they need to be modified in order to obtain good frequency range.
PHOTOGRAPHIC CHARACTERISTICS AND FREQUENCY RANGE
The questions to be decided by consideration of the frequency range are: What density above 1.0 is most desirable, and by what combination of exposure and development is it best to obtain this density?
Exposure depends, of course, on the speed of the film stock being used. But since we know that for best results we need a film of high resolving power and high working contrast, our choice is restricted to the various positive stocks or to emulsions which resemble them in general behavior. If we specify exposures by the densities to which they develop under average treatment the discussion will apply equally well to all of the films now in use.
Obtaining maximum response at high frequencies means working so as to obtain the maximum resolving power of the film. Now resolving power is an intricate subject in itself, and one which has been the subject of much research. This is hardly the time or place to review the results of this research in any detail. Those who are interested will find the most important papers in the Journal of the Optical Society for the years 1927, 1928, and 1929.
The results which are of immediate interest can be stated rather briefly. As applying to the positive film stocks used for variable area recording they are as follows:
Resolving power is not greatly affected by changes in the time of development or in the constitution of the developer. It does decrease somewhat with increasing development, and this decrease is most noticeable when the exposure has been heavy.
If we adopt an average development and give a series of different exposures we find that the resolving power reaches a maximum of about seventy lines per millimeter at a density in the neighborhood of 1.2 or 1.3. Over the range of densities from 1.0 to 1.6 the resolving power is high, sixty lines to the millimeter or better. Below a density of 1.0 or above 1.6 the resolution falls rapidly. The combination of exposure and development particularly to be avoided is one giving densities of 2.0 or greater, for under these conditions the resolving power is reduced to less than half its maximum value.
Resolving power is also dependent upon the contrast of the object being recorded on the film. If this contrast is less than about 10 (log contrast less than 1.0) the resolving power is poor; for object