|angled view of test. I centered my test image on|
the brightly-colored recessed books in the middle.
Testing up close it's clear to me that field curvature is in play - at 20mm from a foot away, the center of an image looks great but not the sides. I convinced myself of the issue by shooting a bookshelf with books at the edges pulled forward; by doing this I can have both center and edge books in focus. Considering this only appears when shooting really close + wide and shooting a flat-plane surface, it is not a real-life kind of image for me; I'll let the Tamron 90/2.5 take care of those shots!