Can a Pentax lens be used on a Canon XSI camera, which has Canon's EF-S style bayonet mount? The short answer is no. The long answer is kind of, but at a compromise that probably renders it not worth pursuing, especially since Canon makes some inexpensive lenses that have very good optical quality.
The Short of It
All camera manufacturers have a proprietary lens mount, whether it is Canon, Nikon, Olympus, Pentax or Sony, who bought out Minolta and the rights to use their mount. Olympus partnered with Leica to create a specific digital mount, so Leica makes some lenses that are compatible with Olympus D-SLRs. Nikon and Canon and Sony license their mount to third party lens companies such as Sigma, Tamron and Tokina, and those companies in turn, make lenses for these cameras. Often, third party manufacturers will make the same lens and change the mount and release different versions for Nikon, Canon or other cameras.
The Long of It
While lenses from Pentax won't immediately fit a Canon EF-S mount, with the use of an adapter, they can be made to work. These adapters can run from $30-$175 as of 2009. While using the adapters allows the lens to mount on the camera, they often limit the functionality of the lens. For instance, the lens may not meter properly, and the lens will not autofocus. All functions will be manual. With the cost of the adapters involved, it is probably only worthwhile if you have several Pentax lenses, but want to use an XSI camera.
Check the mount
If you have old, or even new, Pentax lenses around and want to get an adapter to use them with a Canon EF-S mount SLR, check what type of mount the Pentax lens uses. Pentax has made several different mounts over the years; their current mount is the K mount, and has been in production since 1975. Other Pentax mounts include the screw mount (M42) and the T mount.
Tags: Canon EF-S, Canon EF-S mount, EF-S mount, lens mount, lenses that, makes some