Excerpted this for a discussion on another forum and thought it might be of interest here.

**C.B. 269 – HANDBOOK FOR BARR & STROUD NAVAL RANGE-FINDERS AND MOUNTINGS – 1916**

Page 156

TABLES OF APPROXIMATE UNCERTAINTY OF OBSERVATION WITH RANGE-FINDERS OF VARIOUS BASE-LENGTHS AND MAGNIFICATIONS, UNDER FAVOURABLE CONDITIONS OF OBSERVATION

"The Tables of Approximate Uncertainty of Observation are based upon the fact that, on the system adopted in the range-finders, and under favourable circumstances of observation, it is possible to detect a want of alignment between the images when they subtend at the eye and angle of about 12 seconds, or, say, 0.0000582 in circular measure. The expression –

dR = [0.0000582] x [R(sq)] / [MB]

gives the error in range or the uncertainty of observation due to the visual limitations above mentioned, when R is the Range, M the Magnification, and B the Base Length, R and B being, of course, expressed in the same units of measurement.

From the above expression it will be seen that the errors within which any one range-finder can work at various ranges are proportional to the squares of the ranges and that the comparative accuracies of the range-finders are proportional to the products of their respective base-lengths and magnifications."

APPROXIMATE UNCERTAINTY OF OBSERVATION VALUES -

FQ2 – 9-ft Base Length;

By 1916, being produced with dual 20x/28x magnification.

Range >> 5,000 yds --- 10,000 yds --- 15,000 yds --- 18,000 yds (max listed)

28x ------ 17 yds ------- 69 yds --------- 156 yds ------- 224 yds

20x ------ 24 yds ------- 97 yds --------- 218 yds ------- No value given

FT24 – 15-ft Base Length;

By 1916, being produced with dual 20x/28x magnification.

Range >> 5,000 yds --- 10,000 yds --- 15,000 yds --- 19,000 yds --- 20,000 yds

28x ------ 10 yds ------- 42 yds --------- 94 yds ---------------------------166 yds

20x ------ 15 yds ------- 58 yds --------- 131 yds ------- 210 yds ------ No value given

Scrutiny of the above values will show the convincing technical superiority of the 15-ft range-finder, especially at long ranges.