Camera

Camera, in photography, is the apparatus required to take the photograph, that is to say, to concentrate the rays of light proceeding from the object to be photographed, to bring these to a focus on a sensitive plate or film, and to keep this plate or film steadily in position while these rays are acting on it. It consists essentially of a box, with one end holding the lens that gathers the rays, and the other end the sensitive plate. The sides are bellows-bodied, i.e. built of corrugated flexible material like a bellows or concertina, so as to admit of variation in the position of the plate with regard to the lens, and to enable the camera to pack up into small compass. The camera is generally carried on a tripod stand, which is very convenient for adjustment on irregular ground, and is also readily packed up. Hand cameras are also used.

Before the dark slide, containing the sensitive plate properly protected from the light by a shutter, is placed in position at the back of the camera, a plate of ground glass occupies exactly the same position as the sensitive plate is to take up subsequently. Thus, when the camera is in place, with the cap removed from the lens, and the glass plate in its proper position, the object is definitely focussed on to it, and the inverted image may be clearly seen when surrounding light is prevented from falling on it by covering the back of the camera with a focussing-cloth. The exact focussing, or adjustment of the plate in the position where the image on it is most clearly defined, is effected "usually by a screw movement. When photographing an object with small details, exact focussing should be helped by the use of a magnifying glass. Then the glass plate is drawn back and the dark slide slipped into its place, with the sensitive plate still covered. The cap is placed on the lens, and the shutter removed from the slide. At this instant the sensitive plate is uncovered, but the camera being light-tight, no actinic effect is produced on it. But so long as the lens is open there is exposure to the light rays from the picture. In instantaneous work the lens is only allowed to remain uncovered for a small fraction of a second, 1/8th to 1/25th. Cap exposures vary from 1/5th of a second to an almost indefinitely prolonged period, depending on the amount of light available. For chemical aspect of photography see the article Photography.