page_title: Coincidence rangefinder
section_title: Coincidence rangefinders
hierarchical_section_title: Coincidence rangefinder / Coincidence rangefinders
caption_attribution_description: Range-finding Instruments Eyepiece view of a twelve-foot Barr &amp; Stroud rangefinder Photo: By Courtesy of Messrs. Barr &amp; Stroud
context_page_description: A coincidence rangefinder or coincidence telemeter is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object. There are subtypes split-image telemeter, inverted image, or double image telemeter with different principles how two images in a single ocular are compared. Coincidence rangefinders were important elements of fire control systems for long-range naval guns and land-based coastal artillery circa 1890–1960. They were also used in rangefinder cameras.
context_section_description: The coincidence rangefinder uses a single eyepiece. Light from the target enters the rangefinder through two windows located at either end of the instrument. At either side the incident beam is reflected to the center of the optical bar by a pentaprism. The optical bar is ideally made from a material with a low coefficient of thermal expansion so that optical path lengths do not change significantly with temperature. This reflected beam first passes through an objective lens and is then merged with the beam of the opposing side with an ocular prism sub-assembly to form two images of the target which are viewed by the observer through the eyepiece. Since either beam enters the instrument at a slightly different angle the resulting image, if unaltered, will appear blurry. Therefore, in one arm of the instrument a compensator is adjusted by the operator to tilt the beam until the two images match. At this point the images are said to be in coincidence. The degree of rotation of the compensator determines the range to the target by simple triangulation. Coincidence rangefinders made by Barr and Stroud used two eyepieces, and may be confused with stereoscopic units. The second eyepiece showed the operator a range scale so the user could range and read the range scale simultaneously.
caption: Eyepiece image of a naval rangefinder, showing the displaced image when not yet adjusted for range
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