Coincidence rangefinder A coincidence rangefinder ( stereoscopic rangefinder or parallax rangefinder ) is a type of rangefinder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object . The device consists of a long tube , with two lenses facing forwards at either 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 . 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 . 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 . 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 . Again the range to the target is proportional to the degree of rotation of the prisms . Coincidence vs stereoscopic rangefinders . In November and December 1941 , the United States National Defense Research Committee conducted extensive tests between the Bausch and Lomb M1 stereoscopic rangefinder and the Barr and Stroud FQ 25 and UB 7 coincidence rangefinders : " " COINCIDENCE AND STEREOSCOPIC RANGE FINDERS " " The first of these reports is concerned with the comparative test of coincidence and stereoscopic range finders . ( 353 ) In these tests the American stereoscopic Height Finder Ml was operated against the British coincidence type Range Finders FQ 25 and UB 7 , in ranging on fixed ground targets , moving naval targets and moving aerial targets . The coincidence and stereoscopic methods utilize the same basic principles of geometrical optics for the determination of the distance to a target . The two methods differ radically , however , in the nature of the criterion presented for human judgement . These British instruments were of the split field coincident type . American crews were being trained at Fort Monroe to operate the coincidence instruments but this plan was dropped when six British seamen , who were experienced range takers , were made available for the tests . Until recently the British Services had tended strongly to the coincidence type of instrument while the American Services had adopted the stereoscopic principle for long - base instruments at least . The decisions of both the British and American Services apparently grow out of different interpretations of the experience of the Battle of Jutland in World War I and are of no concern in this place . " " Tests were run in November and December 1941 using the British seamen on the British instrument and experienced American observers on the Standard M1 . Bad weather conditions and various experimental difficulties and mishaps made it impossible to obtain a really satisfactory quantity of data before the tests hall to be terminated . Fixed target reading were made on targets from 2 , 700 to 14 , 500 yards . Only five aerial courses could be recorded and these were all level flight courses , at altitudes of 3 , 000 to 4000 yards and slant range between 4 , 000 and 12 , 000 yards Continuous contact was used . Nine courses were obtained on slow moving naval targets at ranges from 4 , 000 to 12 , 000 yards . In these latter courses continuous and broken contact were used at different times . " It was found , throughout the tests , that the performances of the various instruments were more nearly alike when measured in external units ( Reciprocal range ) than when measured in terms of error at the observer ' s eye , in spite of marked differences in physical dimensions of the instruments . The American MI has a base length of 4 . 5 yards and used 12 power ; FQ 25 with a 6 yard base used 28 power and UB 7 , a portable instrument , has 25 power and 3 - yard base . The coincidence instruments did not use internal adjusters but were calibrated on targets of known range . In other words . the net performance of the different instruments were essentially comparable although the instruments exhibited varying degrees of efficiency in performance relative to the size . On aerial courses precision errors of the four instruments were about alike when measured in reciprocal - range units . In UOE , the FQ 25 had comparatively poor precision , while the UB 7 , for three of the five aerial courses , had very small precision errors . The number of aerial courses was too small to yield much information about consistency of observations from one course to the next . " " For the naval target courses , one American instrument was not operating . Precision errors of the other three instruments there similar to those on aerial height courses . In reciprocal range units the three instruments had comparable precision . In UOE the FQ 25 was worse and the UB 7 was better than the American M1 . Consistency error of the UB 7 was smaller than that of the M1 , even when measured in reciprocal range units , while the FQ 25 was similar in consistency to the Ml , again in reciprocals units . On ground targets the same general situation holds . Consistency errors of the four instruments over the 9 - day period were the same when measured in reciprocal - range units . Again the UB 7 was better than the stereoscopic instruments in UOE and the FQ 25 was worse . Consistency over the 9 days was not perceptibly worse than daily consistency for any of the instruments . In other words , the readings over the 9 days did not scatter in total more than did readings for a typical day . " An analysis or these results leads to the following conclusions . ( 1 ) Performance of the coincidence and stereoscopic instruments was about the same when range errors were measured in yards ( 2 ) The UB 7 however , with a virtual base length smaller than that of the American stereoscopic instruments was more efficient than the stereoscopic height finders in terms of performance for its size , while the large coincidence instrument , the FQ 25 . was less efficient in this sense . This situation held for all types of targets — fixed ground , naval , and aerial . ( 3 ) The UB 7 is somewhat better than the American instrument in consistency on naval targets , even when measured in external units . " " This report is attached , as supporting data , to a Report to the Services issued by the Fire Control Division of NDRC ( 20 ) . This points out 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 . " " 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 .