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Diffraction is primarily a function of absolute aperture, not f-stop. The circumference of the aperture where diffraction happens is linear at 2 Pi R where the area (where light gets in) is Pi r square. That is why astronomical lenses are specified in size to determine resolution limits, their f stop (such as it is) becomes a function of magnification.Modern telephoto camera lenses are similar in employing magnifying elements (like internal extenders), otherwise an 800mm lens would be at least 32 inches long.Magnification, optical design, and placement of aperture all affect this in real terms which is hy you see the measurements you do, but the physics is simple, and all the lenses we discuss are 'small'.This is also why the sharpest lenses are often the really big ones (200/2, 400/2.8), despite lots of glass, because the upper diffraction limit of a 6 inch lens (400/2.8) is so much higher than that of a 1 inch (50/2.0)..... H
You are correct HaroldP.
His latest post that you quoted.