Some people sent me email to ask about my previous posting about
diffraction limits of lenses, and I thought I would add the following:
First, a precisely calculated value can only be obtained at a particular
focal distance. When light passes through an aperture, diffraction
takes place, and the light starts spreading out. The smaller the
diameter of the aperture the faster the *rate* at which it spreads
out, and so the further it travels after that the worse the effect.
What this means is that at close focus, a lens is farther from the
film than at infinity focus, so at a given aperture, the diffraction
limit of a lens is a lower resolution value at close focus than
at infinity focus. Thus, you would have to calculate a precise
range of diffraction limits for a given aperture not one precise
value.
Second, this effect is why teleconvertors degrade macro shot images
less than landscapes. John Shaw, in his book, "Closeups in Nature"
notes that teleconvertors have a lower impact on quality of a
lens in macro situations but does not give the reason.
The reason is that in macro situations, the lens is further
from the film plane, and the lens is usually well stopped
down for depth of field. These effects result in a lower diffraction
limit for the lens, and the loss of resolution due to the teleconvertor
is bounded by the difference between actual resolution of lens + TC
and the diffraction limit. This is because even a perfect lens could
not do better than the diffraction limit, so a lower diffraction limit
on the aerial resolution of a lens in macro situations translates to
a tighter bound on the loss of quality due to teleconvertor.
Cheers,
Joseph
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