Originally posted by Iberia Thanks to all for your replies.
I shot the following JPEG/sRGB samples with a k-5IIs mounted on a tripod in manual mode at 100 ISO, self-timer and SR off.
I left "Using the Aperture Ring" off because I have no intention to set f/ stop on the lens.
i'm a bit late to the party, but for future reference, the problem with shooting brick walls is that the camera sensor needs to be perfectly parallel with the wall... i use bubble levels and even a tape measure sometimes.
if your camera was setup close to that, i'd say that lens was weak on the left side, particularly in the upper left corner... it was probably a defective copy.
i tested a bunch of 28mm legacy primes on the a7r; canon, minolta, konica, olympus, etc., pentax was the winner for me, don't give up on the brand.
the best way that i've found to test for decentering in a lens is to shoot big wide landscapes, with equidistant objects that are located in the flat plane of focus.