I thought that my 10x42 SE, despite its slightly larger exit pupil, requires more precise IPD adjustment/eyepiece positioning than my 8x32 SE. I much more often notice significant vignetting viewing through the 10x42 SE than the 8x32, which I usually try to ameliorate by fiddling with the IPD adjustment or eye alignment, and pushing the bins closer against my face. These measures help, but most times don't completely get rid of the vignetting. Yesterday morning, when I was looking through the 10x42 and again trying adjustments to get the best view, it occurred to me that the vignetting I was fighting could be caused by the fact my entrance pupil is larger than the exit pupil of the bins when it's dark. I've been mainly using the 10x42 during twilight hours, and reserve the 8x32 for better light condition, and that could be the reason why I've been noticing the vignetting a lot more in the 10x42 than the 8x32. I stayed out a little longer yesterday with the 10x42 than usual. As the light started to get brighter, the vignetting also went away in the view, and I was able to see the whole fov with about the same brightness. So that seemed to confirms my hypothesis that the combination of the 4mm exit pupil and dark condition caused vignetting, at least in my 10x42 SE.
I wonder if what I thought was correct. As I don't have other 4mm binoculars to compare to my SE's, I don't know if the vignetting in dark condition is a "feature" of the SE eyepiece design, or a universal phenomenon for small-exit-pupil binoculars. I'm trying to find a rainy-day backup for my SE's, and have been considering the 8x42 and 10x42 formats. If the 10x42 replacement, regardless of design, is going to have the same vignetting problem during darker hours or heavy overcast just by virtue of smaller exit pupil, I'll probably opt for an 8x42, or even a 7x42. Your insight will be most helpful for my decision. Thanks.
Ning
Ning,
I think what you are seeing seems at least partly reasonable, and here is my attempt at understanding the situation.
The "exit pupil" of a binocular is defined for a single viewing direction. There are really a multitude of eps for all the viewing directions within the field. The center of the field always has a nice round ep, corresponding to the full objective. But, eps corresponding to viewing directions near the edge of the field become closed down and football-shaped, as you can see if you hold the bino out and watch the ep as you tilt the binocular around. When the light is dim, your eyes open up large enough to receive the entire ep from any direction, and it is then apparent that the eps near the edge are smaller, which causes a darkening in that region. But as the light gets brighter, and your eye pupil closes down, it becomes so small that it cannot receive all the light even from the reduced-size eps near the edge of the field. As a result, the eye , now the limitation, gets equal illumination from all parts of the field, and the field appears uniformly bright.
The hard part to understand is the marked difference between the 8x32 and the 10x42. I don't believe a 0.2mm difference is the culprit. Something else that is different about the optical trains must make the edge more heavily vignetted in the 10x. You can check this yourself, by examining the exit pupils in the two binoculars near the field stop. Does the ep in the 10x42 appear to be the skinnier "football"?
All binoculars of a given central exit pupil size will by no means suffer the same degree of vignetting, so I don't think you can infer from the SEs how any other binocular of a given specification would perform in this regard. I think you'd have to try it out, or at least inspect the exit pupils as described above.
Maybe somebody else who is bored in the post-Thanksgiving leftover days will comment and straighten me out if I am wrong.
Ron
PS: The 0.2mm difference, while it seems very small, would in fact work in the observed direction, and might be sufficient to explain this if all else fails, in which it would appear that your understanding is 100% correct, most admirable. Taken at face value, that is a difference in surface brightness of 10%, which would be noticeable to many of the forum participants. But do compare the shapes of those eps.
I bought a pair of 7x50s (Porro) specifically for use under dim conditions, and apart from their extra weight, these are by far the easiest of my binoculars to use wrt setting their interpupillary distance. My other bins are 7x 35 Porro, and a pair of compact 8x25 roof prism - and these are easily the most difficult to get to their optimal spacing.