Good Afternoon, Chuck,
I haven't really experienced any vignetting, even with the condensers located around 1/4 inch above the 4 x 5 mark on the scale. As I indicated above, I normally use that slightly higher placement to keep any dust speck on the condenser lens from showing up as a blurry lighter spot on prints. Coverage and evenness of light just hasn't been an issue with my MCR-X as long as the appropriate lens is used to fit the format. I'm sorry that I have no idea of what is causing your problems.