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224 RICHARDSON'S BLUEBOOK OF PROJECTION
is sending its quota of rays to its corresponding point on the screen — as is actually the fact. If the film lies perfectly flat (we disregard the fact that to make this hold perfectly true the screen must present a spherical surface the center of which would be the optical center of the projection lens) then all these thousands of conjugate foci points would be of exactly equal length, hence all of them would focus equally well.
Suppose the film does not lie flat over the aperture. Instead it buckles in or out about l/64th of an inch. Such a condition, it is readily seen, makes some rear end conjugate foci points longer than others, and since a difference of l/64th of an inch in the short end will make quite a lot of difference as to where the front, or long end will focus, it is evident the screen image will not be in sharp focus over its entire area. If the center is focused, the sides will be "out," and vice versa.
(60) Naturally when the surface of the tension shoes and aperture plate tracks become worn, the film will pass the aperture in curved form, not perfectly flat as it should. Parts of the screen image will not be in sharp focus.
Because of the basic importance of perfectly flat aperture plate tracks and tension shoes wearing surfaces, it is recommended that (61) after each SO hours of projection service (260,000 feet of film) the surface tension shoes or aperture plate tracks (whichever is most convenient) be tested with a steel straight edge. If they do not present a perfectly straight, flat surface, then both tension shoes and aperture plate tracks should be replaced immediately.
(62) When installing new aperture plate tracks, or a new aperture plate, if plate and track form one unit, make very sure that the surface that is to receive them is perfectly clean. A small deposit of dirt, almost invisible, may throw them out of true, perhaps by several thousandths of an inch. When the installation is completed test the surfaces of both tracks with a steel straight-edge.