Coincidence rangefinder

Coincidence rangefinders were important elements of fire control systems for long-range naval guns and land-based coastal artillery circa 1890–1960.

Because the distance between A and B on a coincidence rangefinder is typically fixed, once the angle is set correctly the operator need only read the range from the scale.

The device consists of a long tube with a forward-facing lens at each end and an operator eyepiece in the center.

By rotating the prisms in opposite directions using a differential gear, a degree of horizontal displacement of the image can be achieved.

[2][3] In November and December 1941, the United States National Defense Research Committee conducted extensive tests between the American Bausch and Lomb M1 stereoscopic rangefinder and the British Barr and Stroud FQ 25 and UB 7 coincidence rangefinders, and concluded "that the tests indicate no important difference in the precision obtainable from the two types of instrument — coincidence and stereoscopic.

They do indicate, however, that the difference in performance between large and small instruments is by no means as great as would be anticipated from simple geometrical optics.