Because of the high magnification for hand-held binoculars, smaller objective lenses are more detrimental to the sharpness of the view in 10x than 8x binoculars.
The reason smaller lenses reduce sharpness is there's more lens edge for the total lens area, and lens edges scatter light. This cuts down perceived detail, partly through reduced contrast. More magnification exaggerates this effect, because it also shrinks the exit pupil, which also reduces contrast.
Years ago I guided for an outfitter in central Montana for a couple of years. This was when I was doing the optics column for Petersen's Hunting, so I had several binoculars to test, while we did some pre-season scouting, among them a Simmons 10x50 porro prism that as I recall retailed for $50. The outfitter wasn't much of a glasser--like many guys who grew up hunting in eastern Montana, he preferred driving closer.
But one of his previous year's clients had given him, as a tip, a 10x20 Zeiss pocket binocular. He was extremely proud of it, but that didn't last long after I showed up, and the 10x50 Simmons not only provided a much brighter but sharper view.
In my experience, any 10x binocular with an objective size less than about 40mm suffers from this effect, even the very best. I haven't ever looked through a 10x30 or 10x32 that provided as much detail as the same company's 8x30 or 8x32, even when rested very steadily, and perceived detail is why we use binoculars. I do prefer 10x over 8x for open country, and sometimes even in woods, but all my 10x binoculars have at least 40mm objectives, and one has 50mm's.
Dang, you're smart about all this stuff, JB!
