It is surprising how easily vignetting can occur, especially if the camera is set up for a wide angle of view.
(Been there, done that, etc.)
What's amazing is that the Zero Image 45 can cover 4x5 at 25mm without vignetting. I forget the specifics of the angle of view, but it is quite extreme. That is quite a feat of design. I found out the hard way in trying to mimic their design.
Tom, on Point Pelee, Canada
Ansel Adams had the Zone System... I'm working on the points system. First I points it here, and then I points it there...