Coincidence Rangefinders were instruments that offered the operator a monocular view of the object being ranged upon and required him to align two half images to ascertain the estimated range. This type of rangefinder was used in many navies. Most notable amongst them were the variety of Barr and Stroud models the Royal Navy's ships employed for Fire Control.
The quality of the data obtained by visual rangefinding depends on the skill of the operator, the range to the object (short ranges are much more accurately measured), and often the size, nature and level of maintenance offered to the instrument employed. The atmospheric conditions, vibration of the platform and amount of available light also play a role in how accurately ranges would be measured.
In his review of the official history of the Barr and Stroud company, Professor Sumida notes that the authors "argue persuasively that Barr and Stroud instruments were at least equal if not superior in performance of those of Zeiss," their stereoscopic counterparts used by the Imperial German Navy. However, in combat or under stringent testing in exercise, the accuracy of the Royal Navy's rangefinders proved far less than the builders and users had hoped.
- Sumida. Review of 'Range and Vision'. Journal of Military History. p. 112.
- Moss, Michael; Russell, Iain (1988). Range and Vision: The First Hundred years of Barr & Stroud. Edinburgh: Mainstream Publishing. ISBN 1851581286.
- Admiralty, Gunnery Branch (1916). Handbook for Barr and Stroud Naval Range-Finders and Mountings. C.B. 269. The National Archives: ADM 186/205.