If you've ever read a review for a lens, you'll probably have noticed how the review will show crops of an image of a test chart, comparing the resolution at the center of the image and the corners. And in most cases the corners are worse than the middle, sometimes considerably so.
For practical purposes, we can say that the corners of such a lens are soft. However, technically speaking the lens may actually have very sharp corners. The softness may be caused by field curvature.
Field curvature refers to when the plane of focus projected by a lens is not flat. This is the case for almost all lenses, though they differ in the amount and style of field curvature. In this article we'll look at it more in depth, and why in some cases it is not necessarily a bad thing.Read the rest of this entry »