Originally posted by nakey is this in regards to the EOS mount having a 3rd party adapter for pretty much everything?
how do you define the difference between compatibility and prior inversions?
Compatibility:
Canon, olympus and/or I think that Sigma (I am not sure): can mount (at least) Nikon, OM, adaptall, Pentax K, C/Y, M42 with adapters.
Pentax: can mount old K-mount without adapters. M42 and adaptall with adapters and not too much more (maybe some NAI nikkors fit also pentax)
Advantages of using pre-KA mount lenses on Pentax over (i.e) a cropped Canon with adapter=none
Prior inversions (I mean, prior inversions from people that want to stay in the same system):
Case 1) say you have a canon with FD mount and lenses. YOu cant use it with your new canon EOS without an almost impossible to find adapter. You cant upgrade the canon body without having to sell the lens
Case 2) you have nikon AI (or in this particular case, non motorized) lenses and you dont have money for a D300/D3 but for a D40-60 then you start thinking on starting from scratch.
Case 3) you had a K1000, ME, SF, PZ.... and you have lenses for those cameras and want to go digital. Pentax made a dslr that was able to use them all without loosing a bit of their original functionality.