Range finder

Range finder, instrument used to ascertain the distance of an object from the observer. In coincidence range finders, used in many cameras, light from a distant object passes through two separate apertures, forming a double image that can be viewed through the eyepiece; a mirror in one aperture can be rotated by a knob until both images coincide exactly, and a calibrated scale on the knob indicates the distance of the object. In the stereoscopic range finder, which has mainly military uses, adjustment is made until a stereoscopic image produced by a special optical system coincides with the image of a reference mark; the range may then be read from a calibrated scale.

See also: Radar.