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Message: <blockquote data-quote="jcbouget" data-source="post: 1538105" data-attributes="member: 17837">The two methods described here are interesting.In the first method ( described by John Russell), I don’t see why the laser beam must be pointed at the center of the objective : if there is severe off-axis vignetting, the rays passing through the center of the objective don’t pass through the eyepiece.  I have two binoculars like that : the Zeiss 10x40 BGAT and the Meade 10x50. The problem in the Meade is eyepiece vignetting. In such binoculars, the laser beam should be decentered to give valid results.Until now, I used a procedure similar to this one : <a href="http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/720055/page/0/view/collapsed/sb/5/o/all/vc/1" target="_blank">http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/720055/page/0/view/collapsed/sb/5/o/all/vc/1</a>My estimation of the accuracy is about 0.2°.I have tried the method described by Surveyor with several binoculars. I have a geared mount to do the test. I can measure the rotation angle of the knob in order to have an accurate evaluation of the rotation of the mount. I have found that to increase greatly the precision, it is possible to use a second binocular, which widens the small image seen through the objective of the tested binocular.With all these precautions, the accuracy is again about 0.2°Here are the results for 9 binoculars, first with the method described in the link above, then with Surveyor’s method :Meade 10x50 :   66.02°   -   66.04°Zeiss 10x40 BGAT :   61.38°   -   61.20°Zeiss 10x42 FL :    61.22°   -   61.07°Canon 10x42 IS :   63.22°   -   62.93°Canon 12x36 IS II :   58.78°   -   58.49°Nikon 12x50 SE :   58.47°   -   58.58°Zeiss 15x60 BGAT :   64.01°   -   64.09°Fujinon 16x70 FMT SX2 :   63.01°   -   63.07°Takahashi 22x60 :   48.11°   -   48.18°The difference between the two methods never exceeds 0.3°.On 3 binoculars, I have also measured precisely the real field of view, and the magnification at the center of the image at infinity : Nikon 12x50 SE :   5.02°   -   12xZeiss 15x60 BGAT :   4.33°   -   14.8xFujinon 16x70 FMT SX2 :   4.11°   -   16.05xWith these results, one can calculate a theoretical AFOV according two formulae :The first one is the classic approximation : AFOV = M x FOVThe second one is the ISO method : AFOV = 2 x arctan( M x tan( FOV/2 )) in which M and FOV are measured.It is interesting to compare the two calculated AFOV with the measured AFOV :Nikon 12x50 SE : approx : 60.2°  -  ISO formula : 55.5°  -  measured : 58.6°Zeiss 15x60 BGAT : approx : 64.1°  -  ISO formula : 58.5°  -  measured : 64.1°Fujinon 16x70 FMT SX2 : approx : 66°  -  ISO formula : 59.9°  -  measured : 63.1°My results and those obtained by Henry Link show that AFOV must be measured. I remember someone who claimed that the Nikon Astroluxe 18x70 was much more impressive than the Zeiss 15x60 because the AFOV was supposed to be 72°, instead of 65° for the Zeiss. He had owned both binoculars. Obviously he was more impressed by the official specs than by his own observations. But I have learned the hard way how reviews on the web could be totally misleading.My second conclusion is that the method described by Surveyor is excellent. Just think to examine the image with another binocular to get accurate results.Jean-Charles</blockquote>

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