Be aware that there is a compatibility issue for K-mount lenses on FF Canon SLRs. The aperture actuator on the lens interferes with mirror travel. Non-FF Canon cameras do not have this problem. There are two solutions:
- Surgery on the lens to remove the actuator lever (highly frowned on by people on this forum )
- A minor grinding on the body's mirror that will insure clearance
Things are better with M42 screwmount lenses, but some still have deep enough rear elements to cause a problem. FWIW, this issue is long-standing for adapted lenses on Canon cameras going back 40 years or more.
Steve