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THE PROJECTOR
sion, hence minimum wear of almost all parts of the
projector mechanism and of working of film
sprocket holes, is to reduce tension spring pressure until the screen image barely starts to become uns . fol
lowed by gradual pressure increase until the unsteadiness disappears. If drive motor speed can be varied, or the projector hand-cranked, so adjust Ion that
very slight unsteadiness in screen image appears when film speed is 100 feet per minute, which provides the minimum tension required at present standard ninety feet per minute projection speed. The projectionist should remember that excess tension imposes excess strain on the mechanism, hence excess repair and replacement costs which can easily be avoided by exercising ordinary care and judgment.
(58) Care must be exercised to prevent uneven tension where the tension is provided by two separate tension shoes, each with an independent spring. An ordinary, small postal scale will help to achieve the proper balance. First remove the letter pan. Arrange a block of wood under the two springs, so that each will strike it when pushed down a certain equal distance. Place the letter pan against the center of the shoe and press it down until it just misses contact with the wood. Note the scale reading and repeat the process on the other shoe. You will find this a reasonably accurate method of checking the tension of the two springs.
Flatness of Film Over Aperture
(59) If one conjugate foci point of the^ projection lens is the film and the other the screen surface, why must the film lie perfectly flat over the aperture?
One conjugate focus — that is, the optical center of the lens in relation to the film at the aperture — is very short as compared with the other — the optical center of the lens in relation to the screen — hence a very small change in its length will affect the front foci point considerably.
Suppose we now consider the surface of the film at the aperture as thousands of pinpoints, each of which