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Message: <blockquote data-quote="henry link" data-source="post: 1705339" data-attributes="member: 6806">Ok, here's my 2 cents worth.Your eyesight may be the limitation at 67x. To see the true resolution of a scope requires enough magnification so that the scope's resolution in arc seconds multiplied by the magnification is comfortably worse than your eyesight acuity. 1.57 x 67 = 105.19 arcsec. 20/20 vision is equal to 120 arcsec. Lots of people are better than that (on a good day my acuity is around 90 arcsec), but magnifying scope resolution to around 150-160 arcsec makes reading the resolution on the chart much easier for me. I would be inclined to use around 150x to test a 100mm scope using the USAF chart.My experience testing many scopes with this chart is that the best ones show resolution  of about 115/D, where D is the aperture of the scope in millimeters, so a "perfect" 100mm scope would measure about 1.15 arcsec. Birding scopes have design limitations that usually cause then to be considerably worse than that. The only birding scope I've measured that came close was the best of 3 Kowa 883's at 119/D. I would consider any birding scope better than 130/D to be a cherry sample. Average performance might fall between 130-140D. Anything worse than 135/D would normally not make me very happy, but there are other considerations in this case. Your scope may do better than 157/D once you measure it at higher magnification, but even if it doesn't improve the raw figure of 1.57 arcsec is as good as most specimens of expensive 80mm class birding scopes and at 67x it may present as much or more detail than your eye is capable of seeing anyway.It might be interesting to experiment with some stop down masks. It's probable that the image through this scope will actually look cleaner and sharper when it is stopped down to 80-85mm, with no loss of detail and only a slight loss of brightness.Henry</blockquote>

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