American soldiers using a coincidence rangefinder with its distinctive single eyepiece during army maneuvers in the 1940s.
Eyepiece image of a naval rangefinder, showing the displaced image when not yet adjusted for range
Coincidence range finder
Coincidence rangefinder at a military museum in Overloon, Netherlands
Coincidence rangefinder at a military museum in Brussels

A coincidence rangefinder (parallax, or split-image rangefinder) (stereoscopic is a related type) is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object. Coincidence rangefinders were important elements of fire control systems for long-range naval guns, land-based coastal artillery, and anti-aircraft guns circa 1890–1960.

The device consists of a long tube, with two lenses facing forwards at each end, and an operator eyepiece in the center. Two prism wedges which, when aligned result in no deviation of the light, are inserted into the light path of one of the two lenses. By rotating the prisms in opposite directions using a differential gear, a degree of horizontal displacement of the image can be achieved.


Optical rangefinders using this principle, while applicable to several purposes, were widely used for military purposes—determining the range of a target—and for photographic use, determining the distance of a subject to photograph to allow focussing on it. Photographic rangefinders were initially accessories, from which the distance read off could be transferred to the camera's focussing mechanism; later they were built into rangefinder cameras, so that the image was in focus when the images were made to coincide.

Coincidence rangefinders

The coincidence range finder uses a single eyepiece. Light from the target enters the range finder 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.[1][2] 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.[3][4]

Stereoscopic rangefinders

A stereoscopic range finder uses two eyepieces and relies on the operator's visual cortex to merge the two images into a single picture. A reference mark is separately inserted into each eyepiece. The operator first adjusts the direction of the range finder so that the fixed mark is centered on the target, and then the prisms are rotated until the mark appears to overlap in the operator's combined view.[2][5] Again the range to the target is proportional to the degree of rotation of the prisms.[4]

Coincidence vs stereoscopic rangefinders

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. The report concludes with the belief that stereoscopic and coincidence acuities are about equal. Under favourable conditions existing instruments of the two types perform about equally well, and the choice between them for any given purpose must be based on matters of convenience related to the particular conditions under which they are to be used."[6]

Height finder

The angle of sight of a rangefinder and the range to the target can be combined in a simple computer to produce a measurement of altitude. The resulting instrument becomes a combined height-finder/rangefinder and was standard equipment for land and naval based anti-aircraft units.

See also


  1. ^ Yoder, P.R., "Mounting optics in optical instruments", 2nd Ed., Society of Photo-Optical Instrumentation Engineers, United States (2008), p. 239
  2. ^ a b range finder: Definition and Much More from
  3. ^ "The RN Pocket Gunnery Book, paragraph 359". Retrieved 10 October 2014.
  4. ^ a b FM 4-15, Seacoast Artillery fire control and position finding, pp. 58–71
  5. ^ CHAPTER-16-F
  6. ^ Field Comparison of Instrument Types, in: Rangefinders and Tracking, Summary Technical report of NDRC, Division 7 (Fire Control), volume 2, p17-18. This volume is a US Government report dating from 1947 and is in the Public Domain. pdf, U.S. Defense Technical Information Center

External links

  • The RN Pocket Gunnery Book, section on coincidence rangefinding.
  • Progress in Gunnery Material, 1922–1923, comparison of rangefinder designs.
  • Naval Ordnance And Gunnery Volume 2, Fire Control Chapter 16F, Optics.
  • Detailed data on Barr & Stroud Rangefinders, c1916