International projectionist (Jan-Dec 1958)

Projection CLINIC Heat on Film Cause of Focus Troubles UDGING FROM our mailbag, complaints anent CinemaScope print focus are J on the increase. A recent advice on this topic, typical among scores, is one from the State Theatre in Starksville, Miss., which refers to the Richard Mahoney (Wethersfield, Conn.) query in our "Letters" section for February last I p. 20). Our Mississippi friend writes, in part: "'Was most interested in the R. C. Ma honey bit about his difficulty in focusing CScope film, since I have about the same setup: we use Simplex X-L projectors, Peerless arclamps, and B. & L. lenses, pulling about 60 amps. Magoptical Prints Most Difficult "Most of my trouble is with 20th-Fox magoptical prints. At the changeover the print is wholly out of focus, with immediate adjustment of the lens a 'must'. Then during the running of the reel I must refocus four or five times by gradually letting the lens in toward the film. I have no such trouble with 'straight' optical prints. Could this trouble be caused by the magnetic tracks on the film? "We recently finished running "Sayonara" (Technicolor) which had the clearest photography I have ever seen on a CScope print. It was easy to focus and it stayed in focus, not being too much out even on changeovers. "I read recently that Todd-AO projectors have an automatic focusing device. True?" The causes of focus drift reside in the film and in the lens. As far as the lens is concerned, the absorption of heat by the glass causes it to expand. This changes the curvature of the lens surfaces by a very slight amount and ac A British Mitchell Fan Considering Robert A. Mitchell's Manual of Practical Projection, it is difficult to think of any aspects of projection and technical showmanship that is not dealt with in a thoroughly practical manner. This is ... no mere reprints of manufacturers' data . . . with a really excellent index. H. Howard Cricks Editor, Technical Section, The Ideal Kinema, London, England See back cover. cordingly alters the focal length. As a rule, the lens must be brought slightly closer to the aperture as the projection of a reel of film progresses. There is considerable evidence, however, that the film itself is the most important factor in focus drift. This has been especially true since the use of acetate film, for this type of film base is more readily warped by heat than the old inflammable nitrate film. In addition to having a greater rigidity than acetate "safety" film, nitrate base retains its rigidity up to the high temperature which is likely to ignite it. Acetate film, on the other hand, softens when warmed and buckles very readily. Magnetic Striping Contributory It is very likely that the presence of magnetic striping along the edges of acetate film on the base side results in an excessive absorption of heat by the film base by conduction from the hot tension pads of the gate. The absorbed heat causes the film to buckle; and the degree of buckling will be greatest near the end of the reel when the projector is the hottest. Since the gelatine emulsion produces a negative buckle, pulling the center of the frames toward the lamphouse, any increase in buckling caused by shrinkage of the edges of the film will necessarily assume a negative direction. The lens will then have to be moved nearer and nearer the aperture to follow the slow movement of the film in the direction of the lamphouse. Acetate vs. Nitrate Film It should be borne in mind that the gate tension pads may produce this kind of film-warping in acetate film even when they have a relatively low temperature. Temperatures too low to produce significant deformation of nitrate film will buckle acetate film severely, particularly if the absorption of heat is aided by a cemented layer of heat-conducting magnetic oxide. In the light of these factors, and with the film itself the main cause of the out-of-focus trouble, there would seem to be no present possibility of any "automatic" focusing device. Anti-Vibration Material From certain quarters have come endorsement of foam rubber as a supporting material under projectors. Informed opinion holds that it is not. Rubberand-plank beds for generators are excellent absorbers of vibration; but possible movement of a non-optical device is of no consequence. A layer of rubber between a machine and the rigid supporting floor kills vibration by (1) isolating and confining the vibration to the machine itself, and (2) dissipating vibration in the resilient material. The problem of vibration-proofing a projector is difficult because the machine must be anchored to the rigid floor to prevent vibratory movements of the projected images. In other words, the projector must rest upon the hard floor, or, at least on nothing more "bouncy" than cork. [This topic is discussed in detail in "'Machine Vibration and Image Steadiness" ( p. 5, et. seq.) .] Metal Screen "Corrugations" The increasing use of metallized screens in drive-in theatres has induced many questions from the field, probably the most common being that relating to the reason why such screens are "corrugated". Typical comment: "It would seem that these 'grooves' have no other purpose than to collect dirt." This notion is far wide of the mark. If the screen surface were smooth, it would act like a mirror, with all light being reflected in one direction only while the sides would remain dark. The corrugations produce the necessary lateral dispersion of the light. The Engineers, Too This is a practical, concise and very useful book . . . well organized, well cross-indexed in a comprehensive manner . . . with down-toearth suggestions to improve the daily projection routine. Ralph H. Heacock Writing in the Journal of the Society of Motion Picture and Television Engineers. See back cover. INTERNATIONAL PROJECTIONIST • MAY 1958 13