>
> Diffraction limiting from small apertures seems to kick in sooner and with
> greater prominence with the shorter focal lengths than with the longer ones.
>
> -- John
>
Actually, I believe it is strictly a function of the F-ratio.
Assume you have a 50mm Lens at F50. You have a 1mm aperture
hole and the L/A angle is whatever milliradians and the light has to
travel 50mm to the film so you have the spot size as (2*L/A*50)mm =
(100*L/A)mm.
If you now go to a 100mm lens at F50 your aperture is now 2mm.
Call this A'. Well A' = 2A (the aperture size of the 50mm lens at F50).
And the total spot size is (2*L/A'*100)mm = (200*L/A')mm =
(200*L/2A)mm = (100*L/A)mm, which is the same spot size on the
film as the 50mm F50 lens.
So the smaller angle of diffraction by going to a longer focal LENGTH
lens of the same focal RATIO is exactly offset by the longer DISTANCE
the light has to travel to the film plane. Thus, the spot size is a function
of the focal ratio.
Mark Hammons
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