The "flange back difference" is the distance from the lens mount to the film or sensor. It's measured in sub-millimeters. If it's off even a bit, the light rays leaving the lens won't focus where they should. I'm hardly an expert on this stuff but can tell you that when I sent my Canon 50/0.95 TV lens to Ken Ruth to be modified to be used on my Leica M3, he asked that I send him the camera too. He specifically calibrated the lens to be 27.80mm from the film plane on my M3. When you're talking about using a high speed lens wide-open, a precise flange distance is essential.
Originally Posted by David Lyga