E., the 28mm 3.5 was recalculated when it went to Ai-s.
Something seems wrong, both my 28mm f/2 don't look like this wide open. It does have front CRC like the 28mm f/2.8 Ai-s (which is an excellent refinement of the f/2 design BTW) and wide open you will see a bit of field curvature so make very sure what you have in focus is what you want, its not Flat Field and the later 3.5 is much better in this regards (and I think you're seeing some of this). Try the test again with more focus bracketing with f/2 you really have to nail the focus even if its a broad/landscape type shot. IMO the f/2 lens strength is mid to close distances (1-2.5 meters) when its wide open or close to it in low light.
Right, that's what I thought. Mine is a converted pre-ai, the Op's is a 'K', so both are pre-ai-s and should behave similarly.