I'd like to comment on the "diffraction can start to occur earlier on full frame camera bodies.", too, Dubaiphil:
If you're not into pixel-peeping where (almost) everything depends on the size of the individual pixels it generally holds true that diffraction starts 1 stop later
on FF/FX-bodies than on DX/APS-C bodies.
But as Canon APS-C and Nikon DX bodies have almost the same sensor size you can bet any amount of money that the N-f/22 = C-f/11 is utter B.S.
From my current testing of the two DX macro-lenses from Nikon I can attest to f/8 being the sharpest and diffraction visibly setting in at f/11 (but only if you look very close). At f/16 it is more easily visible and at f/22 (and beyond) it really gets bad.
With macro shots you always have to balance the gain in dof and the loss of overall sharpness when stopping down. So in this case a shot at f/16 might even produce a more satisfying photo than f/8.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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