 Physically, the SA mount is identical to the PK mount with a reduced back-focus of 44.0mm (equals Canon EOS) as opposed to the 45.5mm of Pentax PK and M42.The Sigma SA lens protocol is a clone of the Canon EF protocol.This provides a not so obvious but very interesting option that most Canon EF-mount lenses will work seamlessly after exchanging the mount plate with the Sigma DSLRs, but without image stabilization.