 Like the EF-mount, the SA-mount uses electrical communication between body and lens, and in fact uses the same signalling lines and protocol as the EF-mount, despite the physical incompatibility.Sigma has long produced EF-mount lenses for Canon cameras, and thus had the ability to use this protocol.All Sigma SLRs and DSLRs can use manual focus lenses too, which have no electrical communication between body and lens.