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).
Let me (not a physicist) ask this: I can see the above being the case for a simple long focus lens, but what about a true telephoto where the focal length is greater than the physical length of the lens? (I know this has something to do with one of the nodal points but I barely remember my optics class.) The light's "distance traveled" could include some distance before it actually crosses the aperture edge. Or is this simply magnification and not "distance traveled" and a non-issue?
I photograph things to see what things look like photographed.
- Garry Winogrand