Originally posted by desertscape You say it has this softness at every f-stop in the corners. Of the seven aberrations, only lateral chromatic and distortion are not affected by the diaphragm . Your softness does not seem to be either of those. So edge softness should be reduced when stopping down. What is going on with your 75mm is odd. Curvature of field(an aberration) is not flattened by the diaphragm but is masked by the increased depth of focus when stopping down and has the appearance of being reduced. So your corner softness is not from curvature of field.
The problem does appear worse when the corner features foreground landscape. It might be that the corners themselves, featuring closer objects are simply not in focus. Indeed the problem is still apparent, but much less so when the corners feature objects at infinity. I will play around with close focus with objects at a uniform distance, like a brick wall. Thanks much for the great information.
Oh yea, I have the 35mm f/3.5 SMC Takumar for my spotmatic, it's basically the same lens design and I have never noted this effect.