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394 XII. PROCESSING AND RELEASE PRINTING
tamable and the most careful handling is desirable. Unfortunately, there is no simple, single, quantitative measure of picture quality or of picture quality degradation, and the same is true of sound quality and sound quality degradation. Since printers are responsible for quality degradation and quality variation, some of their characteristics that are responsible should be mentioned.
Loss of Resolution. One of the more obvious characteristics is loss of image resolution. This loss may arise from many causes. In a contact printer, for example, it may be due to the fact that the image-bearing film and the raw stock are not in good physical contact, or that they shift with respect to each other during exposure. The usual continuous contact printer is designed for a specific shrinkage condition of the image-bearing film and of the raw stock. With ideal conditions, the length of the outer surface of the image-bearing film (the emulsion surface) just matches the length of the inner surface of the raw stock (the emulsion surface). For film dimensions other than ideal, the raw stock is either too long or too short to fit the sprocket exactly, and the image-bearing film is likewise too short or too long. In either event the result is a slipping of the raw stock with respect to the image-bearing film during exposure. This slipping is evident as a "smearing" of the fine detail of the image. The magnitude of the smearing depends upon the differences between the films being printed and the films for which the machine was designed.
The effect can be measured for the picture area by printing a good test film bearing resolving power test charts. The results are evaluated by checking visually to determine the just-visible lines with a microscope having a magnification of 40 diameters. A resolution test film that has been recommended is described in "Specification for 16-Mm Motion Picture Kelease Prints," ASA Z52. 3-1944*; this was issued as a "War Standard by the American Standards Association. In the test leader recommended, the effect in sound printing is measured by observing visually the frequency at which the "smear" begins on a constant-amplitude variable-frequency test.
Loss of resolution occurs in all forms of printers. It may be checked in other printers by substantially the same method. In the testing of projection picture printers, it will be found that lens aberrations, particularly geometric distortion, will be quite common. The resolving power of a lens cannot be taken for granted ; when it is measured, illusions as to lens performance are likely to be shattered.
* The film described in this War Standard was not completed.