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Message: <blockquote data-quote="henry link" data-source="post: 3652880" data-attributes="member: 6806">I don't think this is related to astigmatism in your eye. That would have the same effect at the 6:00 and 12:00 positions of the binocular's field edge. I've observed this same effect in most binoculars and after some experimenting determined that it is caused by variations in exit pupil vignetting due to unconscious differences in way the eyeball rotates when looking in different directions. The way the eyeball rotates looking down seems to naturally cause less exit pupil vignetting than looking in other directions. That might sound like a good thing, but less exit pupil vignetting actually degrades edge of field aberrations. A highly vignetted exit pupil at the edge (enough to make the exit pupil smaller than the eye's pupil) improves the sharpness at the edge by by stopping down the eye and increasing its depth of field You can induce an improvement at the 6:00 position in any binocular by consciously moving your whole eye down a little toward the bottom of the field before you rotate your pupil in that direction. Or try moving your eyeball up and down after you rotate to search for the sharpest image at the edge. The lowest aberrations will occur just before you experience a blackout coming from the top of the field as the vignetting approaches 100%.</blockquote>