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Message: <blockquote data-quote="henry link" data-source="post: 1614482" data-attributes="member: 6806">spacepilot,That's exactly what happens, only the cat's eye opening results from a vignetted binocular exit pupil at the edge of the field. I've noticed that the vignette tends to be more pronounced in the horizontal direction compared to the vertical because the pupil of the eye seems to naturally rotate in the direction of the field edge being viewed in the horizontal, but looking down tends to naturally be done more from a pupil position shifted above the optical axis which minimizes the vignette. Pupil positioning is one of the things that makes evaluating edge sharpness so difficult and subjective. Obviously binocular optics don't have different off-axis sharpness in different directions, but the positioning of the eye can make it seem that way.You can actually see the shape of the vignette at the edge of the field that results from different pupil placements by looking at an out of focus star or glitter point, which is an image of the exit pupil (or your eye's pupil overlapping with it). Move the out of focus disc from the center to the edge of the field in different  directions and move your pupil around. You'll see that the disc is round near the field center but turns into a cat's eye shape and finally a narrow slit toward the edge. How narrow the cat's eye shape is depends on several factors, like exit pupil size vs the stopped downed size of the eye and the pupil position of the rotated eye.Henry</blockquote>

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