[Brightcolours]

Just curious about what happened when Canon switched from FD to EOS. In particular, I was wondering about the flange distance, where FD was 42mm compared to the 44mm of EOS. Did they say why that change was made?

A lot of thought went into the EOS mount. The flange distance got a bit longer because that made the mirror box a bit bigger, providing enough room for AF sensors. The diameter is bigger than other SLR mounts, to provide for big apertures easily (50mm f1, 50mm f1.2, 85mm f1.2). The contacts provided aperture and AF control with in-lens motors. The mount got a bayonet locking mechanism, which turns out to be more reliable and easier to use than the FD breach lock.

44mm is still relatively short compared to other current full frame compatible DSLR mounts. Sony are at 44.5mm, Pentax at 45.5mm, and Nikon at 46.5mm (values may be rounded).


I have a converted FD 50mm f/1.2 and it is annoying that when used at distant focus, the mirror hits the lens rear! I have heard of some people shaving their mirrors, but the lens isn't worth it for me.

They never considered the FD lenses because an adapter needs to be a certain thickness, which then would have meant an even shorter flange distance for EOS than for FD, and less room for AF sensors. Canon had enough insight in the matter to know what all was involved, after their early AF endeavors (T80 for instance).