International projectionist (Jan-Dec 1957)

outer zone of "fadeaway." The manufacturer's recommendations should be heeded when establishing working distance, although experimentation is not amiss in the case of inaccurate pressedglass mirrors which are not subsequently ground to the correct form. The magnification provided by any arc mirror should obviously be just great enough to cover the entire rectangular opening of the film aperture with the comparatively uniformly brilliant central portion of the HI crater image — the "spot." Too high a magnification (too large a spot) wastes light and overly heats the projector mechanism: too low a magnification increases total light transmission, but gives "hot-spot" projection— a bright spot of light at the center of the screen with dim, discolored edges and sides. Best LI Light: 3A Carbon Diameter It has been found that the average effective light source in low-intensity arcs is about 3/4 the diameter of the largest positive carbon intended to be used in the lamp. (Increasing the current in LI projection merely increases the diameter of the positive crater until it is about 4/5 the diameter of the carbon, when "spindling" occurs.) In high-intensity projection, however, strikingly different brilliancy-distribution characteristics of the crater require us to regard only the central part of the luminous gas ball held inside the crater by magnetic forces to be the effective light source. The dim, reddish light emanating from the incandescent shell of hard carbon surrounding the core is useless for projection, but very hot. Some designers base their mirror-curvature calculations upon the entire diameter of the positive carbon, a cause of unpleasing, film-blistering hot-spot screen illumination. Other designers regard the diagonal of the effective 35-mm film aperture as close to 30-mm (it is actually 25.4-mm = 1 inch) and take 1/2 the diameter of the HI positive as the diameter of the light source. For 9-mm carbons, therefore, mirror magnification should be 30/4.5 = 6.7; for 10-mm carbons, 30/5 = 6.0; and for 11-mm carbons, 30/5.5 = 5.5. Only when the carbon diameter exceeds 12-mm do we find magnifications of less than 5 required. Focus the Emulsion Layer A few of the fellows seem to think that either the background or the foreground of the picture can be focused sharply on the screen but not both. It seems to me that when one area of the picture is focused, all other areas will automatically be in the sharpest possible focus. YOU ARE RIGHT: the fellows who think that either background or foreground can be sharply focused, but not both at the same time, must be cameramen in disguise! The cameraman, unless the foreground object be beyond the Properly imaged c aperture "spot" ■Fz Mirror of "correct elliptical form";- — Working distance too short. Dotted line indicates curvature of mirror required for good *spot" imaging' \/ i — __ False focus Dotted curve indicates mirror curvature required for good "spot" imaging at this excessively long working distance. FIG. 2. True and false foci dependent on working distance. "infinity focus" of the lens he is using, can focus sharply only one or the other, and must make a choice. This is because foreground and background are at different distances from the camera. As a rule (as in a closeup), the foreground object is focused clearly, making the unimportant background more or less fuzzy on the finished film. In projection, however, both background and foreground are at the same distance from the lens because both lie in the same emulsion layer of the film. All that we projectionists really do, when we focus the picture, is bring the microscopic silver grains of the film emulsion into the sharpest possible focus. If the background is blurred, it's simply because it is out of focus on the film. You can't make the picture on the screen clearer than the picture on the film! The blurry background will be at its clearest when the sharply delineated foreground is in focus, but you can't sharpen a blurry picture when it has been photographed that way. The Necessary Blur Special camera lenses, as well as ordinary lenses "stopped down" to a small diameter, are used by the cameraman to bring both foreground and background into sharp focus simultaneously. Sometimes, however, it is considered desirable to blur the background detail in closeups by means of long-focus lenses used "wide open" to make the actor's head stand out more clearly from the relatively unimportant detail behind him. It is also interesting to note that longfocus lenses have a smaller depth of focus than short-focus lenses; and largeframe negative processes require the cameraman to use lenses of longer focal length. Backgrounds in pictures made from VistaVision, CinemaScope 55, and Technirama negatives are sometimes apt to be more blurry than in normally photographed 35-mm films. The desired extra sharpness of big-frame negatives is nullified by the camera lens! We have yet to see a picture made from a bigframe negative that the moviegoing public can distinguish from standard 35-mm photography. (And we also have yet to see the consistent knife-edge sharpness of image that characterized the early silent films, fhckery as they were. ) New Diffusing Screen A new projection screen, tradenamed "Uniglow," has been announced by Radiant Mfg. Corp. of Chicago. Although its reflective power is claimed to equal that of a glass-beaded surface its diffusing effect and wide-angle reflection permit use of a 40 per cent widerviewing angle. INTERNATIONAL PROJECTIONIST • JUNE 1957 21