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Message: <blockquote data-quote="henry link" data-source="post: 3220892" data-attributes="member: 6806">6130m,I hesitate to enter these waters again as there are a few too many red herrings swimming around <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" /> , but while I can't seem to find any of my old posts on the subject of a perception of oval shaped sweet spots it has come up here several times. I have a pretty good idea of why you and your friend see this in various binoculars, even without eyesight astigmatism. It's not really an instrument characteristic (that's why you found that it doesn't rotate with the instrument). It results from the off-axis vignetting in every binocular combining with unconsciously asymmetrical eye movements. You can teach yourself to be aware of those movements and choose to induce or eliminate an asymmetrical sweet spot in virtually any binocular by consciously changing the position of your pupils in just the right way as you move your gaze toward the edge of the field. In my experience the darkening of blacks and apparent increase in sharpness along the horizontal axis happens because the eyeballs naturally rotate horizontally in a way that tends to produce more off-axis vignetting than vertical eyeball movements do. There is a way to directly observe the effect of eyeball rotation and pupil position on vignetting by using a defocused artificial star. Set up an artificial star at 3-4 m from the binocular (glitter point in sunlight or pinhole in aluminum foil stretched over a flashlight beam). Set the binocular focus on infinity and observe the defocused disk of the star point. What you see is a focused image of the binocular objective lens or your own eye's pupil depending on which is smaller. Move the disk to the edge of the field and notice how it's size and shape change. What you see near the field edge is the vignetted pupil (eye or exit) that you actually view through when you look at the periphery.I'll leave it to you to play around with the effects of pupil movement in this test, but I think you will quickly notice that lateral horizontal movements of your pupil have a large effect on the amount of vignetting along the horizontal axis and I believe you will notice that natural eyeball rotation tends to create more horizontal vignetting than vertical vignetting. Increased vignetting (of the exit pupil or the eye pupil) has the benefit of decreasing aberrations like field curvature and astigmatism in the binoculars as well as decreasing axial aberrations of the eye, so the periphery of the field in the direction with the most vignetting looks the sharpest and highest contrast.Henry</blockquote>

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