I have a Canon XTi (EF-S lens mount). Can I get an adapter to mount a Pentax M42 lens? If so, do I lose any functionality?
You can use an adapter to mount a lot of lenses to a Canon EOS body, due to the short flange distance.
You will lose auto-focus, and the camera cannot set the aperture of the lens when taking the shot.
That means you will have to focus manually, and meter by stopping down.
just to add to the answers - you'll loose all electrical connections from the body to the lens. so you will loose Automatic focus, but it'll be a manual focus lens anyway and automatic aperture control, but it'll be manual anyway.
Everything else (metering for example) will be fine.
not sure about it hitting the mirror on full frame bodies ? the adaptor will move the lens away from the body moving it away from the mirror ?
But I would expect to get vignetting on full frame, that the smaller APS sized sensor will avoid - I know I have 85mm russian lens on my 7D.