Range Finder, an optical instrument for measuring distance. Range finders are used mainly in photography for focusing cameras and in gunnery for aiming artillery. There are various non-optical, electronic devices that perform the same function as range finders.
The range finders used with cameras are coincidence range finders. Such a range finder uses mirrors or prisms to produce two or more images of an object. The viewer sights the object through a viewfinder and adjusts a knob or other mechanism to bring the images into alignment. The amount of adjustment is shown on a scale from which the viewer can read the distance to the object. The range finder is usually coupled with the camera's focusing mechanism so that the viewer does not need to know the distance; when the images in the range finder are in alignment, the lens is in focus.
In gunnery, both coincidence and stereoscopic range finders have been used. A stereoscopic range finder has a pair of eyepieces through which the viewer sees a single image of the object and a pattern of lines that appear to float in a space. The viewer turns a control mechanism until the pattern appears to be at the same distance as the object; the viewer then reads the distance from a scale. Military range finders were largely replaced by radar after World War II.