II.

The Cine-Camera

Although the fundamental purpose of a cine-camera, the taking of still photographs, is basically similar to that of other cameras (see Photographic Techniques), the need to take so many photographs per second means that the necessity for rapid transport of film through the camera dominates the design of the mechanism. Since, at 24 frames per second, one minute of filming uses over 27 m (90 ft) of 35-mm film, cameras are fitted with magazines that can hold 122 m (400 ft) or 305 m (1,000 ft) of film. In order to draw the film smoothly off such large rolls, and in order to wind it up again after exposure, the film has to run continuously in the camera. However, to take the photographs, the film must move in steps where the individual frames are exposed.

Continuous drive of the film is provided by one or more toothed drums or sprockets round which the film passes, held in contact with the teeth by guide rollers. The sprocket teeth engage in perforations along one or both edges of the film and drive the film smoothly.

The gate is a metal plate, with a rectangular gate aperture, against which the film is held flat, from behind, by the spring-loaded gate pressure pad. In front of the gate is the lens, which focuses inverted images of the subject being photographed onto the surface of the film, through the gate aperture, to form the frames on the film. In cine-cameras, the film usually runs vertically downwards and the height of each frame, plus the space between it and the next frame, is usually the length of four perforations, whereas, in a still camera, where the film lies horizontally, each frame takes up the length of eight perforations.

Between the lens and the film is a continuously rotating shutter, which alternately opens to expose the film in the gate and then closes while a fresh section of film is pulled down into the aperture. A typical shutter consists of a blade that is semicircular, so that the angle of the open section is 180°. The shutter is, therefore, open for half of the time and closed for half of the time. At 24 frames per second, this produces an exposure of of a second. On some cameras the exposure can be varied by altering the shutter angle.

The intermittent movement of the film through the gate is usually achieved by a claw mechanism that moves in a continuous cycle, engaging with the perforations, pulling the film downwards in a step, and then retracting to recover during the exposure. It then repeats the action. In order to guarantee that the film is accurately located and absolutely still during exposure, many cameras are fitted with register pins, which slide into the perforations and secure the film during the exposure and then retract for pull-down. In order to reconcile the continuous and intermittent drive of the film, the camera is loaded with small, free-standing loops of film above and below the gate.

To achieve accurate framing, most cameras have reflex viewfinders. The front surface of the shutter is set at 45° to the axis of the camera and is mirrored so that, during periods when the shutter is closed, the image is reflected at right angles onto a ground-glass screen, or fibre-optic screen, exactly the same dimensions as the frame of film in the gate. The camera operator has a magnifying viewer, through which he or she can view the image on the ground-glass screen, from the back of the screen. Many film cameras also have a miniature video camera that transmits the image on the ground-glass screen to a closed-circuit television system for monitoring and playback. This is referred to as a video assist.

The quality of the projected image depends on the area of each frame. For professional film-making, film 35 mm wide is the norm, although 16-mm film is widely used for television and documentary film-making. Frequently, a version of 16 mm, called Super 16, is used, where only one set of perforations is employed for film transport, freeing up extra space on the film. The aspect ratio of the resulting image can be used for television presentation, with part of the top and bottom of the frame masked in black. It is also well suited for showing on wide-screen televisions and high definition television systems. In addition the image can be enlarged to 35-mm wide-screen for theatrical distribution. Amateur use of 8-mm film has largely been replaced by the use of video camcorders. In some selected cinemas, enlargements of the image onto 70-mm film can be used, to produce the best quality of presentation.