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BLACK-AND-WHITE REVERSAL MATERIALS 65
When universal film is to be used as a reversal, the exposure is increased (usually 2x) compared with the exposure it would be given as a negative. Fortunately, a convenient system of speed ratings known as the ASA (American Standards Association) Exposure Index* simplifies the manner of expressing photographic film speed.
Since the advent of World War II, the importance of the universal type of reversal film has grown at a very rapid rate. For special purposes, such as the gun-sight-aiming-point (GSAP) cameras that were used to record the firing of machine guns and other weapons of an airplane, there are occasions when the developing of film as a negative had decided advantages. Interchangeability is effected and warehousing problems greatly simplified if the universal type of reversal replaces regular reversal and negative types in such applications.
Generally speaking, reversal materials of the smallest exposure index practicable should be used as orginal materials, because they are the least grainy. Even with the very best processing at all steps, graininess in prints is always a factor that must be kept to an absolute minimum. Graininess in an original is always aggravated by the release printing operation ; for this reason materials with a reversal exposure index of the order of 25 or lower are definitely preferred. Materials of higher index should not be used if it is at all possible to avoid them since their graininess is usually noticeable in the original and becomes quite objectionable in the release prints produced from such originals.
Simple graininess comparisons can readily be made with a conventional high-quality projector, with a 750-watt lamp projecting test pictures upon a good, clean matte screen approximately four feet wide. All tests viewed should be observed from the center of the screen at a distance equal to 2x screen width. Any new commercial projector with a good gate and a good lens is suitable. Such graininess tests should never be conducted with production film, because of the scratching and other damage that would be certain to result from handling. Tests should be original photographs of identical subject matter, properly exposed and developed, and projected for comparison at a single showing. Better graininess comparisons can of course be made with more elaborate arrangements, such as two interconnected identical machines projecting images side by side on the same screen ; for most purposes the more elaborate arrangements are not justified.
*ASA Z38.2.1— 1947, "Photographic Speed and Exposure Index, Method for Determining. ' '