G'day Tony, The field curvature that you have illustrated could be fixed with about half an hours worth of post processing in Photoshop (most any version) if you are interested. It is not as though your images are beyond redemption. I am soon to submit a review of a tutorial DVD to Mike S. But let me just say at this time, that I have in my possesion a DVD of tutorials that are worth significantly more that the selling price,
and one of the tuts addresses exactly what you are dealing with.
best,
Doug