Lee Saxon
Well-known member
From what I understand, you can't make a FLR for the M lenses, due to the fact that they aren't retrofocus designs, or something like that. OTOH, they're so small, it almost doesn't matter.
A retrofocus design isn't required for a focal reducer, the issue with the M lenses is simply that there's not physically enough space for the optics; M mount flange is only 7.8mm farther from the sensor than the RF mount flange, and M lenses can often have optics protruding rearward beyond the flange.