Originally posted by JeffB Canon is the only DSLR platform where you can (sort of) easily adapt other SLR lenses without intervening elements because Canon's sensor to mount registration distance is the shortest of any maker.
And because their mount's throat is so wide, at 54mm in diameter. If flange focal distance was the only consideration, you'd think Sigma SA-mount having just as short a flange focal distance as Canon EF would have lots of adapters (and let's face it, they need it a lot more than Canon). But they don't, because their bayonet is mechanically the same as Pentax K, so the diameter is only 44mm -- not really wide enough to adapt anything except M42. (Source:
here.)
Consider also Sony/Minolta Alpha-mount (FFD = 44.5mm, diameter = 49.7mm). The FFD difference between A-mount and, say, Olympus OM or Contax/Yashica is 1.5mm, roughly the same as the difference between Canon EF and Pentax K, for which glassless adapters do exist. Yet the only OM-or-C/Y-to-A adapters out there have glass. Heck, even with Leica R, with a full 2.5mm longer FFD than A-mount, you need to buy a mount replacement kit rather than a simple adapter.