Good Evening,
Each brand of camera has a dimension associated with its design referred to as the registration distance. This is the distance between the front, where the lens mounts, back to the sensor (or film). Canon has one of the shortest distances, while Nikon has one of the longest. Pentax is somewhat in the middle. What this means to you, is that Canon can use other brands of lenses - primary because there is sufficient distance for an adapter - since each manufacturer has a different mechanical mount for their respective lenses.
So in a way, you can mount Pentax lenses on Canon, by way of an adapter - however, not the other way around. Also, when you mount a Pentax lens on a Canon body, the Canon AF does not match or work with the Pentax lens' AF - either the mechanical screwdrive linkage or the electronic AF SMD interface. So, in essence you wind up with a fully manual lens.
Mounting a Canon lens on the Pentax body -
- Physically does not work - as the mechanical mount interface as indicated above is different.
- Because the registration distance is different - longer for the Pentax K mount, the shorter Canon EOS mount distance will not work, since the lens will need to essential sit inside the mirror box in order to be able to focus as Canon intended.
- The Pentax K mount screwdriver and SMD AF will not work because of the differences in both the mechanical and electrical interfaces.
- Yes, you can adapt some other lenses (but certainly not every lens vendor) to Canon's AF interface like the Contax N lenses (the only one that comes to mind), but its very expensive.
- Yes, you can adapt - via a lens mount change, use other lenses on the Pentax K mount - like the older Contax C/Y mount. When mounted they act like a fully manual lens. Pentax's AF confirmation still works, but you the photographer still need to turn the lens' manual focus ring in order to focus. This does not work for all longer registration distant lenses - like Nikon lenses mounting on Pentax with an adapter really does not work that well and there needs to be additional optics, so its on a case by case basis.
- Yes, you can use third party lenses like Sigma, Tamron, Zeiss, etc. on Pentax bodies since these lens manufacturers design and build their lenses to specifically use the Pentax K mount, and the Pentax AF interface.
So, in this case - if you really want to use Canon lenses, the best route would be with a Canon body.
hope that helps....