REFLEX CAMERA (Fr., Chambre miroir ; Ger., Spiegel-reflex Kamera) Synonym, reflector camera. A camera in which the image is focused on a horizontal ground glass screen by the aid of a surface silvered mirror inclined at an angle of 45°, the latter being automatically swung out of the light are retarded in their passage, exactly in the same way as the wind is retarded in passing through a clump of thick trees. Take first a sheet of glass, shown in section by A, B, C, D in diagram A, on which falls a beam of parallel light, the successive positions of the wave front being represented by the transverse curves. On meeting the glass at right angles to the surface, A B, there is a slight retardation, but as practic ally the whole of the wave front meets the glass simultaneously the relative positions remain the same and the direction of the ray is unaltered, although there is retardation in the glass, as shown by the curves being closer. If the beam strikes the glass at an angle, it is obvious that way when exposing the plate. The reflector principle was suggested very many years ago (see " Camera" and " Camera Obscura "), but it is only of recent years that the mechanical perfection of the reflex hand camera has been achieved. A and B show respectively the ex terior and interior of a typical reflex camera, the lettering being the same in both ; as is the hinged mirror, E the lens, s the horizontal focus ing screen, n the hood shielding the last-named from extraneous light, at the focusing pinion, R the release that operates both shutter and mirror, and P the position of the plate. The
dotted lines show the manner in which the image is reflected to the screen. The hood folds down flat when not in use. In designing these cameras great care has to be taken to secure identity of focus on the screen and at the position of the plate. The mirror is usually arranged to swing upward by means of springs or levers directly the shutter release is pressed, and to resume its former position after the exposure. In the majority of reflex cameras a focal-plane shutter is fitted. The advantages of the reflector principle are that a full-size image is seen, and may be accurately followed or focused up to the very moment of exposure, using if desired, and seeing the effect of, the rising or swing front, or any other adjust ment. Large aperture lenses and lenses of long focus may also be used on near objects without stopping-down, as may frequently be necessary in press work, whereas with an ordinary hand camera it would be next to impossible to secure sharp focus by scale under such con ditions.
Another form of reflex camera, but with a fixed mirror, is the twin-lens camera.