One issue that I'm choking on is that the calculations for diffraction
limitations are not accounting for asperical element lens designs. A modern
lens is almost always utilizing aspherical elements which do not expand the
diameter of the airy disk in a linear manner. You can see this very clearly
when comparing bokeh of one lens to another lens even though they are the
same focal length and the subject-sensor distance remains the same.
I have said repeatably that modern lenses seem to have a "zone of focus" vs
a "plane of focus". For example, a classic, old 50/1.8 may have a plane of
focus only two centimeters thick at 1 meter subject-film distance, but a
modern zoom may have that plane of focus that is effectively four or five
centimeters thick. If the optical engineers have been able to accomodate
this, it is entirely possible that they've "gamed the system" to exceed the
theoretical limits of diffraction.
A classic lens design is pretty straight-forward. Especially so if the lens
is symmetrical. However, newer lenses are actually extremely different. For
example, that symmetrical 50mm lens is truely a 50mm lens front to back.
But a modern lens (zoom or fixed) could actually be a 100mm lens with a 1/2
multiplyer mechanism to give an effective 50mm focal length.
Now, imagine with me a lens design that is, say 50mm, but has
the diffraction characteristics of a 100mm lens. I believe this is exactly
what the optical engineers have been able to do.
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
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