Slightly dark disk in each eye - pupil or glass artifact? (1 Viewer)

[syncrasy]

In my older (1983) 7.5x42 binoculars I have noticed an odd phenomenon: a disk shape that is almost the size of the image field but slightly darker. (See my re-creation in the attached illustration.) The disk is visible in any level of brightness (whether pointing at the sky or an interior wall). The disk appears in each ocular independently, so it's not related to barrel distance or stereoscopic alignment. When I move my eye, the disk moves in the opposite direction. Possible important fact: These binoculars have a larger than average exit pupil: 5.6 mm. (I don't see this artifact in my other binoculars - 8x30, 10x25, 10x40, of varying ages and quality, all with smaller exit pupils.)

What is causing this phenomenon?

My lay hypotheses: (1) normal artifact of viewing a 5.6 mm exit pupil image with smaller (3 mm) pupils; (2) a reflection of my dark pupils in the ocular lens; (3) a reflection of an internal image; (4) an artifact of an older binocular that doesn't have modern glass coatings. Or perhaps some combination of all of these.

I'm hoping the experts can help me understand what I'm seeing. Any ideas?

Thanks!

 

[WJC]

In my older (1983) 7.5x42 binoculars I have noticed an odd phenomenon: a disk shape that is almost the size of the image field but slightly darker. (See my re-creation in the attached illustration.) The disk is most apparent when aiming the binoculars at a solid-colored object (e.g., a white wall) in medium or low light. During normal outdoor use (e.g, birds in trees or with busy backgrounds) it's not apparent. The disk appears in each ocular independently, so it's not related to barrel distance or alignment. When I move my eye, the disk moves in the opposite direction. Possible important fact: These binoculars have a larger than average exit pupil: 5.6 mm.

What is causing this phenomenon?

My lay hypotheses: (1) normal artifact of viewing a 5.6 mm exit pupil image with smaller (3 mm) pupils; (2) a reflection of my dark pupils in the ocular lens; (3) an artifact of an older binocular that doesn't have modern glass coatings. Or perhaps some combination of all of these.

I'm hoping the experts can help me understand what I'm seeing. Any ideas?

Thanks!

Field stop in the wrong position ... OR your eye. :cat:

Bill