Originally posted by Smolk I keep thinking, when the Nikon Z 14-30mm (which has terrible curvature) can be corrected by a lens profile, can't the 21mm be too?
This may well be my ignorance in these matters.
The only thing you can really do is just stop down enough that depth of field compensates some. But the DA 15 really needs f8 on APS-C to have focused edges when the center is in focus. I haven't shot the DA 21, but I think it is similar, but not quite as bad.
This is an article that does a pretty good job of explaining the issue:
Field Curvature, A Tricky Problem in Photography | PetaPixel As you can see, field curvature makes focusing really tricky and makes it difficult to have it maintained on both center and edges at the same time without stopping down a bunch.