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A simple and precise method of measuring AFOV (1 Viewer)


Well-known member
Speculation about the AFOV in a recently linked binocular test prompted the following:-

AFOV is often calculated by multiplying true FOV by magnification, e.g. 10 x 6,4° = 64°, or using the ISO standard, which for the same binocular (112 m @ 1000 m) gives 58,5°.

However, angular magnification distortion (pincussion), the tangent error or magnification deviations from a precise integer can falsify the results.

For the measurement the binocular is set up horizontally on a tripod, and with the eyepieces pointed towards a distant building, one views through one of the objectives. The tripod head is then panned so that the edge of the building coincides with the edge of the field and the angle on the panorama scale noted. The procedure is repeated for the other field edge and the difference between the two angular readings is the AFOV.

The binocular should be focussed at or near infinity and the lateral movement of the objectives while panning does introduce a small parallax error. In my case this was about 8 cm for a 60° swing, but over a distance of 50 m the error is about 0,1° and can be ignored. I am confident that the accuracy of this method is well under +/-1° and confirmed Swarovski's figure of 60° for the 10x42 SV, where the ISO or simple magnification rules as applied above could be misleading.

It is also possible to measure the AFOV for the intermediate settings of a zoom ocular on a scope.

The method was adapted from one proposed by Walter E. Schön on a German forum. He shone a green laser through an objective of the binocular, where the eyepieces were pointed perpendicular to a wall. If the distance between the extremities of the laser projection is 'a' and the distance of the objectives from the wall is 'b' then the AFOV is 2x arctan (a/2b).

One possible error source would be that the binoculars or the scope do not have exactly the stated magnification, only an approximate value.
In particular with zoom eyepieces, where you could be quite confident that the painted markings are only indicative, not precise.

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