I have a Zero Image 4x5 camera and at the shortest "focal length" of approximately 25mm (from pinhole to film) the light drop off at the corners is significant.
Significant, but somewhat correctable?

I'm making a foamcore cam right now and I went with 50mm, reasoning that that's shorter than the shortest lens I've ever seen for 4x5. Maybe I should make interchangeable 'pinhole cones'.