20/20 acuity is 120 arcseconds, 20/10 is 60". Magnify that with a binocular 8 times and the maximum detail you can see is 120/8= 15" or 60/8= 7.5". So on paper it doesn't matter how good the binocular is you can't discern more detail than those limits, though worse is certainly possible. Of course that does depend on how accurate your eye test was. I know over here the big chains don't bother to test for better than 20/15 and one place I went to stopped on the 20/20 line (I never went back!). The lab results are normally for individual eyes and two eyes usually gives a slightly better result. Of course we are metric over here but I'll stick with the imperial equivalent.
Binoculars do vary in quality but anything mid range or better will comply with an international standard. For a 42mm objective that will be 5.7" resolution. That sounds better than the 7.5" for 20/10 vision but it's pretty meaningless comparison in practice. All the models over about £250 I've tested reach this standard, and although there are cherry examples, the alphas do not use a higher standard than the others (my two best results are Chinese). 5.7" is significantly worse than the diffraction limit of 2.8 arcseconds so there is resolution loss. What matters is where that occurs in the lens. When the light is bright enough for optimum acuity the pupil of the eye means we only see the image from the middle 20mm of the lens (for an 8x) and it's the optical accuracy of this area that is critical to the user. The diffraction limit is 5.8" for 20mm. I've been suggesting for quite a while that the 20mm resolution is what we should take note of. You could call it effective resolution.
The best pairs I've tested are pretty much "perfect" when stopped down to 20mm giving me a value of around 6", but a significant number are worse with 10.5" being my poorest result for a "high quality" binocular. I strongly suspect that a popular well known model is actually worse than that but I haven't tested it yet to confirm it.
It's this range of 6" to 10.5" that you need to compare to the magnified acuities I mentioned at the start. Someone with 20/10 or 7.5" magnified acuity would readily tell the difference in detail between a 6" and a 10.5" whereas 20/20 with 15" would see the same level of detail, as would 20/15 with 12.5" because they are eyesight limited.
I've only addressed resolution, and sharpness perception is more complex with colour and particularly contrast being involved. Even though the 6 models I've found so far with ~6" centre resolutions show the same level of detail to my eyes they do differ in contrast. Some might argue the higher contrast ones were sharper. I should point out that 12.5" result was for a binocular I owned for a while and I'd rate the contrast as very good. A number of people, including a binocular service engineer, commented on how sharp it was, but to my eyes it was soft. Same binocular, different visual acuities, and different perceptions of sharpness.
I have found some examples of Chinese made binoculars sharper to my eyes than some examples from Swarovski, Zeiss and Leica, but others, particularly with different acuity to me using the same samples may have a different answer.
David
) It's not down to resolution differences and from what I've seen of the Opticron version and the Kowas not strictly contrast either. I'm not really convinced of this but there may be an aspect at play which might best be explained by comparisons with photography. If you are old enough to have used black and white film I'm sure you will know the difference between the high resolution Ilford Pan-F 50 and the high speed films, particularly when cooked for even higher ISO. You could trade detail for contrast and make low definition objects more distinct. This has carried over into the setting on digital cameras though the actual "sharpness" setting play a few computational tricks which are not relevant to binoculars. However it is also possible to manipulate what's called the MTF profile through camera lens design as well. Some are tuned to give higher contrast at lower resolution frequencies and reduce the detail compared to others. They appear "sharper" while delivering less information. I'm not convinced that these image manipulations are deliberately employed in binoculars but I do see differences in this regard in normal use and it's more obvious when you boost the resolution. Some models have much better contrast near the diffraction limit than others. It's not something I've tested on any of the three models in question but I suspect the Kowa is more the Pan-F type and the Swaro a faster ISO. Quite where the Minox sits I'm not sure. If, and it's a big if, the Minox was tuned intentionally or otherwise to low resolution/high 'sharpness' (in camera terms) then it might be easier for you to distinguish detail though the actual resolution story is probably rather different.
