Originally posted by Zig ..... however, I'm not certain if there is a point of too much f-stop that creates a loss of IQ.....
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Yes, there is IQ loss at high f-stop, and it is exacerbated when shooting macros.
A more accurate equation for the Airy disk diameter (diffraction spot diameter) than usually given is:
D= 1.22*f-stop*(1+m)*wavelength (for greenish light, D_microns~(2/3)f-stop(1+m) )
So in your photos, the effective f-stop with respect to diffraction was actually greater than f22.
Guessing the mag was about 1/2, and using the wavelength of greenish light, the diffraction spot diameter is about 22 micrometers. This is 4 times larger than the distance between K-x pixels, 5.5 micrometers, so it is likely that resolution was lost.
It might be worth your time to take a series of photos at different f-stops to find out when resolution begins to degrade for your camera and lens.
Dave
Last edited by newarts; 10-14-2010 at 05:32 PM.