An optical instrument for measuring distance, usually from its position to a target point. Light from the target enters the optical system through two windows spaced apart, the distance between the windows being termed the base length of the rangefinder. The rangefinder operates as an angle-measuring device for solving the triangle comprising the rangefinder base length and the line from each window to the target point. Rangefinders can be classified in general as being of the coincidence or the stereoscopic type.