This page assesses what happens to image quality from f/8 to f/16, the contrast-robbing effects of diffraction being the concern.
For in-depth coverage of diffraction and depth of field and more, see Making Sharp Images.
Contrast and diffraction
Similar contrast loss should be expected from the Pentax 645D, but the degradation from diffraction depends on how good the optics are to begin with, and might also depend to a small extent on sensor technology.
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