Another test to try out! Thanks! I think by the time I'm done with all these findings, I'll have a pretty strong case on my side to take with me when I talk to the camera company!
Not on a Pentax either, but I have the Sigma 2.8 18mm to 50mm macro as well and the general consensus from my colleagues who are photographers in looking at my photos is that it is very sharp even at f. 2.8
Yeah, I know that my 100-300 f4 is stellar at f4 100mm through 250mm and same again at 300mm when I stop down to f5.6 edge to edge (and the slight drop at f4@300mm is really minor )