flat field / curved field
what does the following mean in plain English?
"One big thing that sometimes is an issue is that many lenses aren't designed to focus on a flat field (e.g. test chart). Many lenses have a curved field of focus that varies in some complex ways. Thus, if you test one of those lenses on a flat chart with only central focusused, you get very bad results for the edges, where the focus has curved away from the plane of the chart." (from Thom Hogan's lens FAQs)