American cinematographer (Jan-Dec 1959)

advance of the film, the light coming through the lens must be cut off momentarily, and this is effected by the shutter. In most 16mm cameras the shutter is of the rotary disctype. Part of the disc is cut away to permit the passage of light to the film for the exposure. The disc shutter rotates continuously as the camera is operated. Obviously, the larger the opening of the shutter, the more light reaches each frame of film and consequently the greater is the period of exposure. But there are some defi¬ nitely limiting factors. Most important of these is the mechanical problem of moving the film. During the period between the exposure of two successive frames ( that is, the time period during which the shutter is “closed”), the film must be started, moved, then stopped dead. Clearly, if the open part of the shutter is large, the film must accelerate, move and decelerate very quickly. If the open sector of the shutter is smaller, the film can be moved more slowly and, accordingly, more gently. But we pay for this less strenuous movement by getting less light for the exposure. What has all this to do with ordinary camerawork? you may ask, remembering, of course, that changing speeds and shutter openings are possible only with a few cine cameras. Well to explain further, suppose we have an ordinary still camera and the established exposure for a given shot is f/8 at 1/25 second. If we shorten the exposure time to 1/50 second, we will have to open up the lens a correspond¬ ing amount — to f/5.6 — in order to secure the same ex¬ posure. It’s the same in motion picture photography. Suppose we VARIABLE SHUTTERS are now incorporated in some of the high-speed motion picture cameras used by armed forces to record the performance of missiles and aircraft. are using one of the popular cine cameras which has a shutter opening of 201° and gives an exposure interval of 1/27 second at 16 f.p.s. If we shoot a scene with this cam¬ era and find that f/8 is the right stop to use, the resultant exposure will be different from what another photographer would secure with a camera having a smaller shutter open¬ ing. Let’s say the other photographer’s camera has a shut¬ ter giving a 1/48 second exposure at 16 f.p.s. If he is to match our exposure on the scene, he will have to shoot it at f/6.7 (or lens stop nearest this figure, i.e. f/6.3). If we, with our 1/27 sec. shutter, are shooting at f/2.5, the other filmer will have to open up to f/1.9 to get comparable re¬ sults; and if we are shooting at f/1.9, the other fellow with the faster shutter (giving less exposure per interval) won’t be able to shoot the scene successfully at all unless he has an extremely fast lens, for he would have to use a lens o})ening of f/1.1 to match our exposure. Another point to consider is that by using the smaller lens stop, the scene will have much greater depth of field than it would if shot with a camera having a smaller shut¬ ter opening; and this difference would be increasingly noticeable as the lens was opened wider or focused on near¬ er objects, as for closeups. On the other hand, in the matter of getting clear, blurrless pictures of fast-moving objects, the camera with the smaller shutter opening offers a distinct advantage. Fast moving objects will be registered on the film in a shorter exposure interval. Because they will have less time to move during the exposure, less blurr will result. Obviously, the solution to the shutter problem for the ad¬ vanced amateur is the adjustable shutter, same as found on standard 35mm motion picture cameras, which will per¬ mit adjusting the shutter opening to suit the shot. All pro¬ fessional 35mm cameras used in the studios have variable shutters and most of them permit adjusting the shutter opening while the camera is running, if necessary. This has proven a valuable adjunct in shooting scenes where the camera moves in and out of dark areas, or for trick effects where speed of a person or an object is to be altered with¬ out stopping the camera. The ability to make fades and lap-dissolves in itself justifies the variable shutter as standard equipment for all cine cameras. Next, and much more important, is the con¬ trol of exposure afforded by the variable shutter. Reduc¬ ing the lens opening increases depth of focus, and increas¬ ing the aperture size reduces depth. Such changes between closely related scenes are not pleasing; and when light fluc¬ tuations, such as a cloud passing over the sun, for example, make such changes necessary within a scene, they are doubly objectionable. But by controlling the light with the shutter instead of the lens diaphragm, such depth of focus contrasts can be avoided. If, for instance, we are shooting on one of those days, say, when small clouds are obscuring the sun, we can set our lens at, say, f/8. 3 and the shutter at 90° and get the same exposure as though we shot at f/11.3 with the shutter opened at 170°. With cameras having a variable shutter,, when a cloud suddenly appears to reduce the light falling on the scene, the exposure can be kept uniform (in relation Continued on Page 54 JANUARY • 1959 41