As a result of tests made at the National Bureau of Standards on certain range finders of unsymmetrical construction, it was found that the observed range depended upon the position in the field of view at which the coincidence was obtained. Examination of the components of the instruments for the source of this effect was unsuccessful. It was concluded that the effect was a consequence of the difference in distance, by way of the two windows of the range finder, from the target to the viewing telescope. The magnitude of the effect is accounted for by analysis on this assumption.
ROBERT E. STEPHENS, "Cross-Field Error in Unsymmetrical Range Finders," J. Opt. Soc. Am. 45, 146-146 (1955)