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But I still don't really fully understand the 15mm. I got images with very good sharpness from edge to edge while other images shot at the same f/11 have terrible sharpness at the edges. It has probably something to do with field curvature and focus distance.
Yes, that's what I have noticed myself as well. It must be field curvature. Lloyd Chambers also had this remark. I noticed that the corners nearby are sharp most of the times, but those at infinity not always. Particularly when pointed the camera a little downwards. But it might be more complex than that.
Besides this I use the 15 more than the 21 at this moment, certainly with interior shots in churches and castles.