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Problems of Controlling Correct Photographic Peproduction
By DR. HERBERT MEYER, A. S. C.
PART IV
From the foregoing articles we might now conclude that the principal difficulties in rendering correct photographic reproduction are caused by the following reasons:
(a) Total visual contrast in the natural object is composed of three variables, brilliance contrast, color-hue contrast, and color saturation. Contrast in the photographic reproduction can only be rendered by brightness contrast.
(b) The color sensitivity curve of the eye is far different from that of any negative emulsion.
It is indeed remarkable that the professional cameraman, applying his sense of feeling and intuition, is able to overcome these difficulties regardless of the fact that he is not provided with instruments which would enable him to measure these variables quantitatively, and to adjust exposure and lighting conditions correspondingly. It might also be considered lamentable that the present day technic in this respect is comparable to that of the days when the "negative timer" was forced to judge the printing lights by visual examination of the negative without the aid of the Cinex tester. The question, therefore, arises as to what has been done and could be done to mechanize measurements of these existing differences by means of reliable instruments.
It has been mentioned before that correct photographic reproduction involves two different problems, that of exact reproduction of brightness distribution in the object which is referred to as "Orthochromatic reproduction," and that of deliberately distorted reproduction.
Orthochromatic Reproduction
Orthochromatic reproduction requires balancing of the color sensitivity curve of the negative film material used to that of the eye. This can be done by photographing through a compensating filter. The spectrophotometric transmission curve of such a filter is constructed by dividing the ordinates of the visibility curve by the corresponding ordinates of the photographic sensitivity curve of the negative material and by plotting the values thus obtained against wave length. A filter of this type, in combination with supersensitive Panchromatic emulsions, would be rather prohibitive for practical use due to the magnitude of the exposure factor necessary. For this reason a compromise must be considered and for practical Orthochromatic reproduction filters of a dark yellow or yellow-green color are used, such as Wratten G or Aero #1 and #2, which give sufficient approximation to the theoretical ideal in combination with supersensitive Panchromatic negative film.
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