Field curvature is an aberration (distortion) in the image of an optical instrument (such as a telescope), such that it brings the image in focus on a curved surface rather than a plane, so when used with a (flat) focal plane, the best focus is either in the center, or some ring concentric to the center, the rest being fuzzier. A Schmidt camera is a reflector telescope using a spherical mirror which produces spherical aberration, then a lens (the Schmidt corrector plate) which reduces the spherical aberration but leaves it with such field curvature. (The advantages of the Schmidt camera are an easier-to-manufacture mirror, and a good image despite a short focal length.) I believe curved photographic plates were used with them, but the CCDs used now are generally flat. I read that a current strategy is an additional lens which compensates for the field curvature (a field flattener lens).