Field curvature is an aberration that isn't commonly seen by itself where it's bad enough to be easily identified. It is commonly overshadowed by things like astigmatism, coma, and spherical abberration, and if it's present it's in small enough quantities that it's hard to identify. Occasionally I will find a lens where it can be seen on an image. I recently found it easily visible on a microscope objective - Edmund Optics 5x/0.225. The two images below show how it works. Field curvature is pretty simple, the focus plane is curved and not flat like the camera detector. The easiest way I can describe it is to think about looking down at a ball. If you focus on the top, only the top will be in focus. As you focus farther down the ball, the center will come out of focus and a ring-shaped portion of the ball will come into focus. As the focus continues down the ball, the ring of in-focus ball will enlarge. The first image shows the focus on the center of the image. The second image shows the area of sharp focus is now a ring while the center and the far edge are out of focus.
