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306 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 at exactly equal distances, 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, removes some rear conjugate foci points farther from the lens than others, and since a difference of 1/64 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 picture will not be in sharp focus.
(61) After fifty hours of service which, particularly if heavy tension be used, may mean considerable wear of aperture plate tracks and tension shoes, test them, using a steel straightedge. Also carefully examine the lateral guides for possible grooving, which permits side movement of the film over the aperture. If there is visible wear, replace the parts with new ones immediately, else you may expect side movement of the screen image and/or intermittent out-of-focus effect proportionate to the amount of wear.
(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.