A rangefinder (sometimes range-finder or R.F.) is an optical instrument used to estimate range to an object. A single such observation, or "cut", was subject to errors which generally increased in proportion to the actual range.
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 practice and particularly in action, the accuracy of rangefinders proved far less than the builders and users had hoped.
The largest rangefinders were the most accurate. Generally, such instruments used mirrors or prisms to allow a single observer to compare views from vantage points separated by 6 to 15 feet — the greater this separation distance, the better. The Royal Navy's large rangefinders were generally coincidence RFs, while the Germans employed stereoscopic RFs.
As guns rely on a precise knowledge of the target's range before they may be effectively employed, the largest rangefinders on a ship were largely to support gunnery.
Some applications (such as station-keeping or inland piloting) did not require exceptionally accurate ranging and/or required that rangefinders be near at hand. Additionally, sometimes there just was not much money or room (as in a submarine's periscope) for fitting a large rangefinder. In such cases, compact unmounted rangefinders of 1m or 80cm length or even a stadimeter might be used.
- Moss, Michael; Russell, Iain (1988). Range and Vision: The First Hundred years of Barr & Stroud. Edinburgh: Mainstream Publishing. ISBN 1851581286.
- Admiralty, Gunnery Branch (1909). Information Regarding Fire Control, Range-finding and Plotting, 1909. C.B. 1127. Copy 137 at Admiralty Library, Portsmouth, United Kingdom.
- Admiralty, Gunnery Branch (1916). Handbook for Barr and Stroud Naval Range-Finders and Mountings. C.B. 269. The National Archives: ADM 186/205.