International projectionist (Jan 1961-Dec 1962)

from the handicap of sprocket teeth which do not fit the film perforations snugly, but are necessarily somewhat undersize. Unfortunately, this allows the film to slip, or "overshoot," on the face of the intermittent sprocket under certain conditions. When this happens, the picture jumps violently on the screen. This writer suspects that a relatively small degree of overshooting is rather common in normal operation when the sprocket teeth are abraded and the gate tension is wrongly distributed. The film comes to a stop in the gate the moment the intermittent sprocket stops turning only because of the frictional pressure of the gate tension pads on the margins of the film. The friction produced by the pads is the only thing that arrests the film, which would otherwise keep on traveling downward under its own momentum, slipping on the smooth face of the intermittent sprocket until the top edges of the film perforations bring up against the sprocket teeth. Even though the excess distance traveled by the film when it overshoots amounts to only about half a millimeter (almost .02 of an inch), this distance is approximately 50 times the maximum allowable registration error in a 33-mm projector. To appreciate the seriousness of complete overshooting, bear in mind that a Va-mm movement of the film at the aperture results in a movement of the image on a screen 40 feet wide of nearly one foot. This extreme degree of picturejump is observed on those rare occasions when the soft emulsion of a "green" print melts on the heated gate runners and allows the film to overshoot. All degrees of overshooting could be obviated by making use in 35-mm projectors of perforation-fitting registration pins which oscillate in and out of the perforations close to the aperture to insure perfect registration of the film each time the intermittent sprocket comes to rest. Registration pins are employed in cameras and printing machines for preparing step-printed master-positive, duplicate-negative, and optical effects films, but they are not practicable for projectors because they must be accurately dimensioned to fit one special size and shape of film perforation. Other methods must be resorted to in order to eliminate from 35-mm projection the rather spectacular picturejump of overshooting. Prevention of Overshooting The most effective remedy available to the American projectionist for the prevention of overshooting is adequate lubrication of the new prints. This is provided by the film laboratories and exchanges, and ordinarily consists of a coating of special wax along the perforation margins on the emulsion side of the film. If a "green" print gives trouble during its first showing — "chattering" in the gate, depositing bone-hard accumulations of emulsion on the gate runners, and overshooting to give a jumpy picture on the screen — it is advisable to oil the film by rubbing the edges of both sides of the film roll with a lightly oiled wad of cloth, following the convolutions of the film layers. Projector oil is satisfactory, but an extra-heavy white mineral oil, such as that sold by drugstores, is even better. We absolutely do not recommend the application of oil to the margins of the film as it runs through the projector. This method of oiling prints is messy, soils both film and projector, fogs the projection lens, and induces sideweave if the oil is applied first to one edge, then to the other edge, of the film. Most European projectionists have velvet-faced film Sprocket (^Moving) Just *"7 ".-■ jbater in the rest period FIG. 1 — How "overshooting" of the film takes place. The intermittent-sprocket teeth engaged in the film perforations pull one frame of film down in the gate. The sprocket then comes to a stop. But when gate tension is insufficient, or when softened emulsion from a "green" print is present on the film runners, the film continues to slide down on the face of the motionless intermittent sprocket until the top edges of the perforations bring up against the teeth, as shown in the bottom panel. The overshooting may amount to only about half a millimeter, but causes the picture to jump almost a foot on the screen! We avoid overshooting by using minimum pad pressure on new prints, oiling the edges of the film only when absolutely necessary. runners at their disposal for running unwaxed laboratory prints. Special runners made of plastic are sometimes used in studio projection rooms: and one American manufacturer (Motiograph) has successfully made use of bamboo runners for this purpose. A number of kinds of picture unsteadiness are known which can be separated from factors beyond the projectionist's control, such as worn prints (damaged or weakened perforations), faulty printing las from badly shrunken negatives), faulty functioning of the camera International Projectionist September 1962