coincidence rangefinder

Definition: Optical instrument designed to measure the distance from the observer to the target being observed. * The principle used is triangulation: the further away the target, the smaller the angle between the target and a fixed base-line at the observer. * E.g. Leica rangefinder: the target is viewed by two optical systems - one sees the target directly, the other via a rotating prism - which create two superimposed images, one fixed, the other moves. As the prism is rotated, the image projected by it changes position: when it is correctly superimposed or perfectly coincident on the fixed image, the prism's setting measures the distance.
Humble origins
This principle, much refined during the world wars for range-finding artillery, is also used in today's compact auto-focus cameras and even some digital cameras. An infra-red beam scans the scene ahead: the angle of beam which gives a maximal reading is a measure of the focusing distance.

Related Terms: active AF

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