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Message: <blockquote data-quote="typo" data-source="post: 3172235" data-attributes="member: 83808">Thanks Kimmo for explaining that. I've not seen published comparisons of MTFs for varying objective diameter but Kimmo may well be right that other aberrations may be more constant. From my own tests a bad resolution at 42mm can be either good or bad at 20mm though a very good result at 42mm will usually still be good at 20mm of course. I'm more than happy if it's good at 20mm.... usually.There was a fairly inexpensive 8x42 binocular I had to review which in casual testing might look quite good for one view and quite poor for another. It turned out that 20mm it gave a fairly typical resolution figure for the price but at 25mm and 30mm the results were significantly worse before recovering a bit at 42mm. I was getting quite obvious variations in sharpness depending on the luminance of the scene. Good when it was bright, poor when it was gloomy and OK at twilight. I wouldn't know if this was spherical aberration or some other problem but at least one other user reported a similar pattern. That was an extreme case, but I suspect smaller degrees of variation are quite common.David</blockquote>