Originally posted by RGlasel Field curvature in a very compact UWA lens has something to do with this. Plus, as you may have heard, sharpness is a bourgeois concept. If I need flat field sharpness, I have the Tamron 90mm in my bag.
I couldn't agree more. This might be a bit off topic but It seems like Original Poster is interesting in get a DA15, so this could be helpful for the OP when use the DA15 (or any Wide-angle lens). Note that Field curvature is common (Read more
here) and it is more pronounce on a higher pixel density sensor like K-3.
Mostly on landscape photography; If you want the whole frame sharped using DA15, try focus 1/3 from the far corner of the frame.
here are 2 examples.
Sample 1. I focus in the center. As see, corner sharpness is somewhat terrible on K-3. I used to use DA15 with K-5 and it was not a problem.
Sample2. I focus around above letter "a" top right corner. It sharp on average over the whole frame.