Originally posted by Clavius but suddenly stop worrying about it when diffraction calculations come into play does puzzle me.
Pentax K-3 users aren't the only ones that should be worried about diffraction...
36MP full-frame users should also be worried about it too, but concerns about diffraction don't seem to arise in the Nikon or Sony threads around here or on the web, despite folks sometimes spending many thousands of dollars on FF bodies (D800, A7r) with a pixel density close to 24MP APS-C. That puzzles me too.
Indeed, it is interesting to note that there is never much discussion of diffraction being a major issue for users of 24MP APS-C's like the D3200, D5200/5300, D7100, A77, A65, NEX-7, or indeed users of even smaller sensors like the 16MP micro-4/3's, and even amongst users of compact cameras like the Sony RX100 that sport very high pixel densities.
I guess the reason may be that, as Digitalis says, in all these cases the onset of diffraction is gradual, not immediate, and may only progressively eat away at sharpness past smaller apertures, depending on the lens. So it's not something people may usually see, especially if conditions aren't ideal and other things (motion blur, the use of crappy UV filters on a lens) may cause more problems for image sharpnness than diffraction.