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Message: <blockquote data-quote="typo" data-source="post: 3171175" data-attributes="member: 83808">Another go at explaining it.Those in the UK will know the easiest way to distinguish a Red Kite from a Buzzard at long range is the shape of the tail. It's something with 20/20 (6/6) vision you might be able to distinguish up to about 500m and those with  20/10 up to 1000m on a good day. That's acuity or the resolution of the eye. With a 10x binocular on a tripod those with 20/20 should be able to do it at 5000m with any half decent binocular. It would take an exceptional binocular for those 20/10 to get to 10000m as frequently the effective resolution of the binocular is limiting. For acuity and resolution we are talking about the limit of detection. Sharpness is different. With the naked eye the distance where the tail distinction is clearest, or sharpest would be much closer, around 70m or 700m with a 10x due to the contrast sensitivity function of the eye. (If I understand it correctly. I don't have any information on how that changes with acuity.) As MTF has had a mention, the resolution limit is normally quoted in cycles/degree for 5% contrast whereas the figure for 50% contrast is considered a good indicator of perceived sharpness. Binocular aberrations and internal stray light may affect the 50% 'sharpness' readout more significantly than the 5% 'resolution' readout. In camera systems it's common to tweak the sharpness at the expense of resolution but I don't know if this could be applied to binoculars but suspect it might be counterproductive if it was.So it's possible for a binocular to have a high resolution and poor sharpness and alternatively poor resolution and high sharpness in theory. I can't think of any examples of the latter though. On the whole it appears to me that if you get good effective resolution, good sharpness follows, but undoubtedly colour bias, stray light and other factors play a role as well.David</blockquote>

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