American cinematographer (Jan-Dec 1952)

down to position the left eye image. Thus there are periods of flicker that occur at different times for each eye. If we break this sequence of events down, we find that the first light period has a value of 12.5% of the complete picture cycle. The flicker blade on the projector shutter (considering a twobladed shutter) gives a dark period last¬ ing 12.5% to be followed by a light period of the same, then a long dark period consuming 62.5% for pull-down and eclipse to permit the other eye to see its image. If standard sound-film speed of 24 frames a second is used, the resulting flicker is very annoying. Stepping up the projection to 48 frames a second in¬ creases flicker frequency twice, but it still is noticeable. There is a physiological effect that is likely to become disagreeably apparent — usually headache or nausea — after a few minutes of viewing pictures pro¬ jected in this way. A complete period of darkness for one eye, while light reaches the other, will probably always result in visual fatigue, if not in nausea, no matter how high, within workable limits, the flicker frequency is brought. Flicker of low frequency calls for traction on the control muscles of the irises when bright light enters one or both eyes. The rapid occurrence of the transmission of stimuli, first from one eye, then from the other, and the motor messages from the brain to the muscles, delivered in rapid sequence, probably accounts, in part, for the visual discom¬ fort experienced by most people when viewing “eclipse” stereo movies. Perception of flicker depends upon the intensity of the interrupted light, as well as the flicker frequency. The more intense the light, the higher the fre¬ quency must go before flicker fusion is attained. Also, the larger the angular field over which flicker is distributed, the greater the consciousness of flicker. Hence the dimmer the picture and the smaller it is, the lower becomes the flicker fusion frequency. There are two ways to project and view eclipse stereograms. One is by using rotating or vibrating shutter de¬ vices held up in front of the eyes. These are synchronized electrically with the projector. The other method is to em¬ ploy a rotating polarizer in front of the projector lens and polarizing spectacles for the viewer. In one position the polarizer delivers light through the left spectacle filter, in the other through the right filter. Alternate frame, or eclipse, projec¬ tion must have twice the number of frames required for conventional films. That means doubling the length and providing for faster projection speed. If the alternate frames are photo Model M52 SENSITESTER • IMMEDIATE DELIVERY to the lab owner interested in — QUALITY EFFICIENCY ECONOMY ® DUAL MACHINE 1 — Sensitometer 2 — Scene Tester • HIGHLY ACCURATE Electronic timer unaffected by climatic changes. • NEW TYPE LONG-LIFE Cold Light Exposure Unit • WITHOUT ADDITIONAL EXPENSE will match any printer • $1675.00 35mm Model • $1875.00 35mm1 6mm Combination Model F.O.B. Hollywood, Calif. AIIT kEEVES MOTION DICTUM EQUIP. 7512 Santa Monica Blvd. Hollywood 46 California SALES • SERVICE • RENTALS 35 mm. • 16 mm. C AMER AS*M O VIOL AS*DOLL V S Complete Line of Equipment for Production Available for Rental Mitchell: Standard Hi-Speed NC BNC 16 mm. Bell & Howell: Standard Shiftover Eyemos Maurer: 16 mm. Cameras Moviola: Editing Machines Synchronizers SPECIALISTS IN ALL TYPES OF CAMERA REPAIR WORK. LENSES MOUNTED EVER Y T H I N G PHOTOGRAPHIC AND CINEMATIC FOR PROFESSIONAL AND AMATEUR The World’s Largest Variety of Cameras and Pro¬ jectors. Studio and Laboratory Equipment with Latest Improvements as Used in the Hollywood Studios. New and Used . . . BARGAINS HOLLYWOOD CAMERA EXCHANGE 1600 Cahuenga Boulevard HO-3651 • Hollywood, Calif. • Cable Hocamex f&i, X March, 1952 American Cinematographer 131