International projectionist (Jan 1963-June 1965)

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FIG. 2— Evolution of the B&H perf. Old~<style circular^ B&H negative perforation perf ovation use can accept film shrinkages up to 0.96% and 0.75%, respectively. If. however, there are variations in shrinkage, sometimes greater and sometimes less than the mean value, the picture will weave vertically, or "dance," in a random fashion. Variations in shrinkage result in corresponding irregularities in perforation pitch, and these, in turn, produce irregularities in frame registration even though the intermittent movement of the projector is functioning perfectly. Picture-Jump Tolerance It is ironic that severe demands of mechanical precision must be made of a material as pliant and dimensionally variable as plastic motion-picture film base. On the basis of accepted geneva-intermittent tolerances, the permissible limit of frame misregistration is 0.0004" (a vertical "jump" of 1/5 inch on a 36-foot screen). This is, of course, nearly three times the allowable error in film perforating (0.00015"), but often below the shrinkage error over the 3 to 5 frames of film between the intermittent sprocket and the projector aperture. To repeat: a uniform degree of shrinkage through substantially long lengths of film produces no visible effects on the screen. It does not cause the picture to be unsteady. The trouble is caused by frequent variations in shrinkage; and the longer the length of film between the intermittent sprocket and the aperture, the more pronounced is the weaving of the picture from this cause. How Weave is Caused The distance from a point in one frame of standard 35-mm film to the corresponding point in the succeeding frame is 4 times the perforation pitch. This amounts to a frame distance of 0.7480" in unshrunken long-pitch (0.1870") positive stock. Assuming that the shrinkage in a certain release print varies in an irregular manner from 0.2% to 0.4%, the frame distance will vary from 0.7465" to 0.7450". If there are 3 frames between the intermittent sprocket and the aperture, misregistration can be any amount up to the limiting value of 3 times the difference in frame distances, namely, 0.0045", fully 11 times the intermittentmovement jump tolerance. If sprocket and aperture are separated by 5 frames, the misregistration can have any value up to 0.0075", or nearly 19 times the jump tolerance. The 0.2% — 0.4% shrinkage variation employed here by way of illustration is not meant to suggest that variations of this magnitude are common. They are not. But it seems probable that smaller variations in shrinkage are frequently responsible for small-range picture weaving which cannot be accounted for by any known mechanical factors in the projector mechanism. The print, however, is not always the culprit. Similar 6 picture-weaving effects may be produced by printing from badly shrunken negatives. The effect of severe negative shrinkage is easily distinguished when the printer used was of the "continuous" type in which the negative and positive raw stock are pulled continuously (without intermittent motion ) past an exposure slit. In such a case the vertical dimension of the projected picture expands and contracts in an irregular manner to produce an unusual, but characteristic, weaving effect. A projector cannot give this peculiar effect. Ancient nitrate negatives used for printing reissued "screen classics" and TV "late show" releases are the most likely to give unsteady images on the screen and a tremendous flutter in the accompanying optical-track reproduction. Effect of Non-Uniform Conditions Because the original structure and composition of film stock is absolutely uniform, and because film is always manufactured under uniform conditions of temperature and relative humidity, the finished product may be expected to undergo its normal shrinkage in a uniform manner. That it does so even in spite of very non-uniform conditions of subsequent treatment, storage, and use is indicated by the extreme rocksteadiness of most professionally produced motion pictures. It is the exceptions that particularly interest us. As a matter of fact, the exceptional nature of shrinkage variations large enough to produce visible vertical dancing of the projected pictures leads us to look for differential effects in the conditions to which some, or perhaps most, theatre-release prints are subjected. It is well known that moisture and heat have the greatest effect upon the dimensions of cellulose-ester films, but release prints are also subjected to processing solutions, protective waxes or lacquers, film cleaning solvents, and the inevitable oil and grease which comes off the projectors. It is believed that the normal gradual shrinkage of film is caused by the slow loss of plastic solvents from the base material. The loss of these substances is accelerated by heat and moisture. Short-term exposure of film to excessively dry or moist air produces changes which are reversed, with some degree of restoration of the film to its original physical condition, by long-term storage under the ideal temperature and humidity conditions which prevailed during its manufacture. Inasmuch as weeks or months of such storage make it out of the question for heavily booked release prints (periodically subjected to intense heat) these films may shrink rather rapidly and possibly in an irregular manner. Projector Design Important Contrary to popular belief, prolonged exposure of film to excessively moist air hastens shrinkage. So long as the air remains humid, the film swells slightly and remains pliant and soft, and the gelatine emulsion is then especially liable to destruction by the growth of molds. But when taken into normally dry air, the effects of solvent loss, apparently hastened by the moisture to which the film was previously exposed, begin to show up. The film will be found to have shrunk more than films of similar age not subjected to high humidities. If variations in shrinkage cannot be entirely prevented by the most intelligent care which we are able to accord release prints, then it behooves the designers of motion picture projectors to position the intermittent sprocket as close below the film aperture as possible to minimize the shrinkage-variation weave of the projected pictures. A 2-frame sprocket-to-aperture film distance is entirely International Projectionist August, 7963