Lately there have been some questions and remarks about curvature of field. Curvature of field means that the the field of optimum focus of a lens is not flat, but curved. This can be both ways, optimum focus at the edges is farther out than at the center or closer. I've encountered the latter more, often edges that are closer to the camera than the center are surprisingly sharp. Last week I received a Minolta MD W.Rokkor 20mm 1:2.8, a lens that is beautifully sharp in the center even wide-open, but that delivers quite blurry corners due to pronounced curvature of field. Here are 100 % crops from center and corner. Focussing was done with flexible spot in center or corner. The total frame: You can see that this lens can be quite sharp in the corners if focussed there. And no, the unsharpness is not caused by the corner subject being closer, depth-of field should have covered this: the other edge farther away shows the same pattern.