I have tried out compact binoculars with and without spectacles. It seems likely to me that many of these compact binoculars perform badly when used whilst wearing eyeglasses. However without these glasses they perform better.
Does this explain some of the contradictory reviews on this site.
Because compact bins have such small oculars, they do not do a good job blocking strong light from the eyes, so glasses wearers may have more difficulty entering/immersing themselves in the view. I find it very helpful to wear a wide brimmed hat when using pocket bins with glasses. I always wear glasses, incidentally, because most of my correction is for astigmatism.
I'm not sure what you mean by contradictory reviews, because by my read there are very few of them. More often, users simply differ as to what they most care about. For example, some users are content to use crummy optics for "casual" use, so even if they have superb full-sized bins, they might use a cheap pocket roof sometimes. I have very excellent corrected vision, and I seem to be hyper-aware of anything that gets in the way of it (or so my optometrist tells me), so I have no tolerance of optics that don't reach a fairly high threshold for acceptance. I want to be able to pick up any binocular I own and be confident that it will deliver. Since I have very little tolerance for mediocre optical performance, and since pocket bins are inherently optically-challenged, I only tolerate using the very best pocket bins. If I couldn't afford an excellent 8x20, I'd choose to carry a mid-range 8x32 that delivered the view I need, not a lesser 8x20.
You wrote "When I focus them on the top of London's latest skyscraper The Shard, 7 miles away, they show almost as much detail as much larger binoculars." For you, optical performance is apparently not so important because you find that result encouraging, whereas I find it damning because I wouldn't expect to see any difference in detail from such a test. In terrestrial viewing, an object 7 miles away almost always has the finest detail and contrast range wiped away by atmospheric effects, so if you saw any differences I have to assume that either your pocket bin has defective optics, or that you weren't able to line it up to your eyes accurately enough to get its best performance, or that it has issues with backlighting or something.
My issues with the Zeiss Conquest are not so much with its optics (it delivers a reasonably good view in the center of the field) as with its body design, which I think make it a much less capable binocular overall and a much inferior stand-in for full-sized bins as compared to the Leica Ultravid or the Zeiss Victory. All of the top-end pocket bins have pretty good optics, but for me, what sets birding bins apart from other optically fine bins is how quickly/reliably they deliver that view to my eyes under challenging conditions (which for me, occur very frequently when trying to see birds). For me, apart from its other weaknesses, the lack of positive stops and poor hinge tension of the Zeiss 8x20 Conquest eliminates it from consideration for birding, as does its poor hang and inability to take a proper neck strap. Other users, who don't hang their bins on a neck strap won't care about those limitations, and some may be delighted with the Z fold and not care about its cost (i.e. lack of positive stops) to other aspects of handling.
--AP