Quote Originally Posted by Diapositivo View Post
The Cambridge In Colour site quoted above specifies that the f/number is relevant, not the absolute diametre of the diaphragm aperture. It says:

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).
Another way to put it is that "Long focus lenses magnify the circles of confusion and Wide lenses make the circles smaller with the same linear aperture size."
All, in all it works out the same "F number" (not linear aperture size) across lenses gives the same CoC size on the film.

When the physics of optics gives such an elegant solution, why do some not want to believe it