This phenomenon has come up a number of times at Birdforum. Here's the most recent thread: https://www.birdforu...ut-axis.393234/
Increased sharpness on the horizontal axis comes up in the last paragraph of post #6.
I have read the thread a few weeks before. Before I post, I first use search engines with conclusion to this thread:
We need a scientific, anatomical-medical explanation (document) for 2 human eye rotation points or a displacement oft the rotation point during horizontal versus vertical rotation, as I suspect. This would easily explain the effect I described and the effect found in the BF (different kidney beans horizontal vs vertical).
A simple technical paper from zeiss in german language but with pictures: https://www.zeiss.de...ndrehpunkt.html
This website in english I see without pictures: https://www.zeiss.co...-rotation-.html
My theory: The center of rotation of a non-spherical eye (nearsighted or farsighted, eye is an ellipsoid) could shift due to counter pressure from the eye sockets - different for horizontal and vertical rotation. But I'm not near- or farsighted. Only a little farsighted with reading glasses 1 dpt because age, eye lens with too little accomodation. Are my eyes nevertheless ellipsoids, non-sperical?
According the link to birdforum:
" ... BTW, this phenomenon first came up in a different discussion. People occasionally report better edge sharpness near the field edge of the the horizontal axis compared to the vertical axis. Most of the time that's not a binocular defect. It's caused by the very same difference in vertical and horizontal vignetting described above. The extra sharpness at 9:00 and 3:00 occurs when the vignetted exit pupil becomes so much smaller than the eye's pupil that it acts like a camera lens diaphragm went stopped down to a small aperture to create wide depth of field. In a binocular the effect is to bring objects near the field edge, normally unfocused by field curvature, within the enhanced DOF just before they are completely blocked from view by the vignetting.
I'm not agree with this theory, because the tubes of binoculars are rotationally symmetric and therefore the vignetting should be the same on the entire edge of the image field. Minor errors of the alignment of the individual optical elements (lenses, baffles) with respect to the optical axis e.g. during collimation are probably negligible. (Mechanical parts for moving lenses must not cause vignetting, prisms only a little in light, small bins.)
Rotationally symmetric vignetting in a single tube, this is according to my observations. Almost the same "cat eyes" (equal shaped) of the exit pupils on the edges of oculars during oblique view (tilting of the binoculars in different directions against window or so).
CS, thanks for reading and best regards
(If my texts seem hard or bossy: Sorry, english language isn't my mother tongue, not my native language.)
Edited by cnuser, 24 November 2020 - 03:27 PM.