Practical Value of Rangefinders

As optical instruments, rangefinders have finite accuracy and precision. Manufacturers and naval services attempted to identify and quantify sources of error and limitation in an effort to make their instruments more helpful in the fire control process.

This article will focus first on Coincidence Rangefinders, as they are easiest to characterise in optical and mechanical terms.

Sources of Error

The Royal Navy estimated that the human eye can detect misalignment of half images in a coincidence rangefinder when the angle at the eye reaches 12 arc seconds, or 0.0000582 in circular measure. For a rangefinder, this meant that the uncertainty of range estimation was:

error = (0.0000582 * range * range) / (magnification * baselength)

In other words, the error in ranging would vary with the square of the range, and would be reduced linearly by the baselength of the instrument and by its magnifying power.