There is no particular reason to use want one particular lens for a format than another. I have both the 420 mm Repro-Claron and the 450 Nikkor-M. I have tried both on my 5x12 and they work fine. The 450 is my choice for most of my shooting with that format though. I did use my 420 during a workshop last summer when I was too close to get the shot I wanted with the 450. So it is nice to have a range of lenses.