I have read that a 50mm f2 lens does not have to be a retrofocus design on a SLR as there is plenty of room between the film plane and rear element of the lens for the mirror box. With a 50mm f1.4 it does need to be a retro focus design (just, depends on camera make there is some variation in this distance) and with f1.2 definitely because the rear element is closer to the film plane than the mirror box will allow. With a rangefinder - no mirror box so a 50mm f1.4, f1.2 etc does not have to be of a retrofocus design, therefore can be made simpler and smaller (and more easy to higher optical corrections). However, with the light meter in modern 35mm RFs, there needs to be room to take the light measurement, for example, the Zeiss ZM needs 15mm between the film plane and rear element. My question can you work out when a lens on a 35mm rf needs to be a retrofocus design because of its focal length and/or its aperature e.g. 25mm f1.4, 28mm f1.4, 21mm f2, 12mm f3.5...? Also, the size of the rear element of the lens, for say a leica m mount camera, increases in size with aperature. When would this cause problems in the m mount? 25 f1.4? Any equations out there to calculate this stuff?