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Message: <blockquote data-quote="Surveyor" data-source="post: 1532803" data-attributes="member: 50720">Hello Spyglass2, In my opinion, the rated FOV is a calculated, nominal value. All binoculars have a tolerance limit. ISO 14133-1:2006 for general purpose optics: Magnification a, Γ ± 5 %Field of view in object space a, b ± 5 %Entrance pupil diameter c ± 5 %Exit pupil diameter ± 10 %Eye relief (mm) 5 +− 0,5Zero-setting error of dioptre scale (m–1) ± 1Image rotation (degrees) ± 1,5Disparity of image rotations d (minutes of arc) 40Relative difference in magnification d 2 %Focusing difference of telescopes of binoculars when focused by means ofthe centre focusing mechanism within the focusing range (m–1) 1 As can be seen, since the TFOV is acceptance angle (AFOV) of the eyepiece divided by the power, and power can be off 5%, the exit pupil 10% etc. As Henry Link often points out, even unknown and unspecified aberrations alter both the object space and image space angular values. But the ISO limits the deviation of the TFOV to a 5% measured value. The only way to know your TFOV for sure is to measure it. For my Promaster, the stated nominal is 7.5* TFOV, so any value from 7.125 to 7.875* is acceptable. The actual measured values are 7.43* left and 7.6* right side and an average measured AFOV of 59*. The specified 60* AFOV is probably more correct, the tolerance of my AFOV measurements are unknown at this point. There is a further restriction on the relative power difference between tubes of 2 % and I am not sure if ISO intends for this to be a limit for only that one specification or extend to the determining factors. The differences imply a power difference of 2.3%. This is an academic question since I cannot measure any parameters to any better than a percent or two. Hope this helps. Best.Ron</blockquote>

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