Quote Originally Posted by Yashinoff View Post
I'm sure you know what I mean.

(in case you didn't: that diffraction is correlated to the size of the aperture, and the f stop number is a fraction of the focal length of the lens.)

Technical Note: Independence of Focal Length
Since the physical size of an aperture is larger for telephoto lenses (f/4 is has a 50 mm diameter at 200 mm, but only 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size).