So I took an 80mm Rodenstock Heligon off of a Graflex XL and mounted it onto a recessed lens board for my 4x5 monorail. For anyone not familiar with the XL, this lens was built for 6x9 (2.25 x 3.25). Wider than f.11 or so I start noticeably losing full coverage. I'm having a total brain fart tonight. Can anyone please tell me how to figure out (or just tell me) what the effective focal length of this lens is on my 4x5? Thanks.