John P. writes:
=======================================
I cranked up Excel for a few minutes and derived the following table. It
assumes a few things, like a resolution for Velvia of 120 lpm, and a lens
resolution of 600 lpm @ f/2.8.
Lens aperture System Resolution Assumed Lens Resloution
f/2.8 80 lpm
600 lpm
f/4 90 lpm
400 lpm
f/5.6 95 lpm
300 lpm
f/8 100 lpm
200 lpm
f/11 95 lpm
150 lpm
f/16 80 lpm
100 lpm
f/22 70 lpm
75 lpm
As you can see, around 2-3 stops from wide open is the "sweet spot" for
overall resolution.
======================================
this doesn't make sense. If you have a lens that is sufficiently
well corrected to achieve its theoretical resolution at f/2.8, then
it is only going to achieve its theoretical resolution at smaller
apertures (which is lower as more diffraction) and so the system resolution
numbers will decrease as the lens is stopped down. such a lens will
be sharpest at f/2.8.
Real lenses improve as the lens is stopped down because stopping down
the aperture removes some aberrations thus getting sharper until
the resolution is diffraction limited, and then the resolution
starts decreasing as the lens is stopped down further.
J. Albert
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