The Canon EOS mount can take other lenses is due to its shorter lens to sensor flange distance at 44mm, which is shorter than all of the mounts listed above. For example, M42 mount has a flange distance of 45.46mm. This means that if an adapter with a thickness of 1.46mm or thinner, is used to mount the M42 lens to EOS mount, the lens can focus to infinity. Some mounts, like the Exakta and Rollei, have a flange distance of 44.7mm and 44.6mm respectively, which means the adapter must be less than 1mm in thickness. Most of the adapters on the market that claim you can focus your Extaka and Rollei lenses to infinity are lying; none of the ones I bought could do that, except one I just got yesterday, which I will get to later. An adapter that's thinner than 1mm has at least two limitations: 1) it's hard to make it strong enough, 2) very hard to mount it on the lens without it cutting your hand with it's sharp, thin edge.
Adapters are just one of the issues. Even if you manage to get your lens to focus to infinity, for some lenses, there is another problem: the dreaded mirror blockage. What happens is that the rear of some lenses goes deeper into the mirror chamber than others and the mirror would hit the lens, especially at infinity focus. The worse Canon camera bodies with this issue is the 5D classic and 5D mark II. 1-Series of bodies are better, and cropped sensor bodies are the most compatible with the least trouble on manual focus lenses. In order to use their favourite lenses, some people has gone as far as cutting part of the reflex mirrors on their 5D/5D II! If you want to see an example of this, check out this link and scroll down until you see the 5D with its mirror shaved. Or, just image search on Google for 5D Mirror Cut. Personally, I wouldn't go this extreme. The good news is that newer cameras with Live-View partly solve this problem, by flipping up the mirror and shoot like a mirrorless camera.
Next time we will look at some of the lenses that won't work with the full frame Canon 5D/5D II.
Bokeh - Canon 5D Mark II & Carl Zeiss Jena Pancolar 50mm f2 @ f2. Click for larger.