om2@xxxxxxxxxxxxxxxx wrote:
>Also, in general the longer the focal length, the smaller the minimum
>aperture can be. Is that because of the more telecentric light rays?
>Anyone? This is evidenced by Large Format lenses for 4x5, which routinely
>go to f/45, and 8x10 or 11x14 lenses, which can go to f/128.
>
>
To quote myself from about 4 hours ago:
"Since this is a phenomenon of the physics of light, it is independent
of focal length and depends only on the wavelength of the light and the
size of the hole. The effect is not dependent at all on the focal length
of the lens, but the size of opening for a given f-stop is dependent on
the focal length. So, for example, a physical aperture size that gives
f22 on a 200mm lens gives f5.6 on a 50mm lens. (With complex lens
designs, this isn't exactly true, but the generalization is accurate
enough.) "
Thus, to restate, the longer the focal length, the larger the actual
aperture opening for any given focal ratio and the less the diffraction.
Moose
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