I'm in the market for a new high MP camera. Mostly for archival work. I have used a 5DSR in the past, with a 24-70 f2.8L II, which was not the right lens (field curvature caused problems, but I didn't have enough space to use a 100mm macro, or 85mm). Seems like the time to go mirrorless, but with which lens? The RF 35mm macro? Any info on the field curvature for that lens, or field curvature on the new RF lenses in general? (Has mirrorless helped field curvature?) And when might we guess the new high MP body will arrive? I could definitely use more than 50mp.
As near as I can tell the only "natural" help for field curvature is to be far away from the subject. It's a matter of your geometry, not the lens. If you're close to the center of your object, you're significantly further away from its edges, proportionally speaking. (You can visualize it as a right triangle. The horizontal leg is one half of your subject, the vertical leg is your line of sight, camera to center of object, and the hyptoenuse is your distance to the edge of the object you're photographing. The closer your camera is, the shorter the vertical leg, and if you compare the vertical leg to the hypotenuse, the hypotenuse is longer. Whereas if you're relatively far away, long vertical leg, your hypotenuse is almost the same length. Since the edges of your object are significantly further away than the center, on your pic you'll see the center bulged out.
It's possible there's some sort of lens that distorts in such a way to cancel this out, but to be honest, that strikes me as a bad idea (because the amount it would have to distort would depend on distance to the target); and if I am right about this surely the designers see it too. (If I am wrong, the designers are right.)
It may be possible to photoshop your curvature out, too. If you keep your camera in a fixed location, you can photograph a grid, play around in photoshop until the grid doesn't look like barrel distortion, remember that setting, and apply it every time you photograph something (there's probably an automated way to do that in photoshop; but I don't use it, personally).