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BLACK-AND-WHITE REVERSAL MATERIALS 63
Consistency in the manner of use depends upon the training and habits of the photographer. Generally speaking, if the photographer consistently measures highlight areas of the picture and keeps the shadow illumination contrast not greater than 4 to 1 with respect to the highlight illumination, the result should be acceptable, and with a little experience, should be good. In practice, the measurement should be made of light reflected from a reference test object which may be a piece of new bristol board free of dust, smudges, etc. Consistency in the judgment used in transferring data so obtained into the aperture setting is probably the largest potential source of error. Corrections are required for changes in film speed, in camera speed, and in differences between the actual light transmission of the lens and the theoretical light transmission calculated from the "/" stop markings on it, and other factors. In many cases in which results are inconsistent, flagrant causes are usually to be found in lens transmission differences and in errors in iris adjustment as well as in camera speed differences and deviations.
Processing Consistency. Processing accounts for another potentially serious source of variation. The processing of Kodachrome is probably the most consistent processing service available. Despite its excellent control, noticeable differences will be apparent in the side-by-side projection of pieces of the same film developed in different laboratories or in the same laboratory at different times. Fortunately, side-by-side comparisons are rarely required. The processing of reversal films is not so consistent; too often factors other than sensitometric control (such as excessive hypo and poor film drying, etc.) complicate matters and further encourage a decided preference for the color film.
Black-and-White Reversal Materials
Present-day reversal materials still give the impression that they are intended for the amateur, who, according to a current fallacy, likes his film "as fast as possible and as hard as nails. " In original reversal materials today there is still a big need for a low-contrast, long-reproductionscale material, since there is no such thing on the photographic market and we have been struggling along without it for ten years or more. Ansco did manufacture a film called "Old Type Superpan," which was a long-scale material of beautifully low contrast, but it was unfortunately withdrawn from the market when the faster emulsions of the ' ' Supreme ' ' type apeared. The film manufacturer who supplies such material and incorporates in it the new emulsion improvements of recent years as %g