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OPTICAL SYSTEM 249
and no provision for readily adjusting and correcting it. The error may vary appreciably from one projector to another and from one make to another; in general, machines with better sound optics are usually adjusted for azimuth more accurately than machines with poorer optics because increases in the high-frequency response of a projector markedly reveals azimuth deviation errors as harsh distortion. In recent years projector manufacturers have been quite conscious of this factor and measurable improvement has been made. The distortion resulting from azimuth deviation error in a projector is very annoying since it is harsh and raspy and there is very little that a projector user can do to overcome it except to return the offending machine to the manufacturer for proper adjustment.
Much study has been given to the relationship of the slit geometry to the film ; the relationship with respect to width is usually referred to as the aperture effect. The slit width for a 35-mm sound projector running at the standard speed of 90 ft. per minute has been established at 0.0013 in. For an equal aperture loss with 16-mm film running at its standard speed of 36 feet per minute, the slit width would be proportional to the film speed; the calculated width would be about 0.0005 in. Only Eastman Kodak and DeVry have supplied 16-mm projectors with a slit as fine as this ; all other manufacturers have supplied projectors with coarser slits and correspondingly poorer high-frequency response. The Bell and Howell projectors provide slits of about 0.0007-in. width and are correspondingly better than other machines that use still wider slits — some as wide as 0.001 in. Figure 61* and the following table show the theoretical output of a uniformly illuminated slit that is exactly normal (perpendicular) to the direction of film travel. The data is given in terms of the ratio of the wavelength on the film to the slit width. Figure 61A shows the approximate slit-loss characteristics for a 0.0005-in. slit (with 16-mm film running at the standard speed of 36 ft. per minute), and for a 0.00012-in. slit.
* Data for this curve is taken from the analysis of E. D. Cook which appeared in the JSMPE under the titles "The Aperture Effect (JSMPE, June 1930, p. 650) ; and "The Aperture Alignment Effect (JSMPE, Nov. 1933, p. 390). The full significance of this fundamental and profound analysis is only now beginning to be appreciated some two decades later. A modern interpretation was presented in 1949 at the Spring Convention of the SMPE in New York by J. A. Maurer; it is to be published later in the Journal. In the light of the recent demands for improved sound quality that are now being made, another paper by Cook should be of interest: "The Technical Aspects of the High-Fidelity Eeproducer" (JSMPE, Oct. 1935, p. 289). Despite the absence of fanfare in its presentation, Cook's work in motion pictures is worthy of close study.