Add "exclude center" to autofucus. This, along with "crop auto focus frame" should reduce the focus effects on the edge of field. i.e. if focus is maxed in the very center, the edges will be out of focus. If you max focus half way between edge and center, both the edge and center will be similarly focused.
I saw a discussion that almost touched on this, but it wasn't explicitly described.
See these mock ups.
