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Message: <blockquote data-quote="Holger Merlitz" data-source="post: 2756739" data-attributes="member: 84262">It has been mentioned above: The ISO standard is valid for binoculars which are free of rectilinear distortion, while the angle condition (a = m*A, a = apparent angle, A = true angle, m = paraxial magnification) considers the presence of a significant amount of pincushion distortion (in fact, it is correct for binoculars with zero angular distortion).Now, in real life no binocular is perfectly free of distortion. The (relative) distortion is defined asVr = [tan(a/2)/tan(A/2) - m] / mand it amounts to zero in case of the tangent condition tan(a/2) = m tan(A/2). Including this relative distortion, the apparent angle of view is calculated viaa = 2*atan[m*(Vr+1) * tan(A/2)],and, as to be expected, this turns into the ISO formula in the case Vr = 0.Unfortunately, hardly any manufacturer specifies the amount Vr of relative distortion, and therefore it is impossible to calculate the apparent angle of view for a binocular of given true angle of view and magnification. Most of the manufacturers are simply using the angle condition when specifying their apparent angle of view, which is not far from reality when considering their pincushion distortion. Since Nikon is implementing rather less distortion, they decided to use the ISO formula instead, which in turn is rather inaccurate in case of the Nikon EII.Cheers,Holger</blockquote>