I picked up a 13" fl Buhl triplet the other day and was expecting it to help with my brightness (switching from a std S15 triplet). The aperture is about 3.5" vs. the S15 2.5". However, when I swapped this bad boy in, I had rounded corners and the brightness in the corners was dimmer.

This lens is much longer than the S15, so I am thinking that the spaces are just too big. I took a look at the pro lens mod, but I don't totally understand what moving each of the lens around is actually accomplishing. How can I get the buhl lens to give me better brightness and eliminate pincushion at the same time?

thanks for your help
Greg