Canon EF mount

The EF mount was introduced by Canon in 1987 and has been the standard lens mount on the Canon EOS family of film and digital reflex cameras. EF stands for "Electro-Focus": automatic focusing is handled by a dedicated electric motor built into the lens. Mechanically, it is a bayonet-style mount and all communication between camera and lens takes place through electrical contacts; there are no mechanical levers or plungers, infact the EF mount replaces its predecessor, the FD mount by, introducing, as the major innovation, a motor inside the lens itself for focusing, thus allowing building lenses without mechanical levers for receiveing physical commands by the camera.

In 2003 Canon introduced a new version of EF: EF-S, specific for APS cameras. EF-S lenses cannot be mount on EF-mount cameras, while EF lenses can be mounted on EF-S cameras.

In 2012 another version of EF was introduced for APS-C mirrorless cameras: the EF-M, compatible with EF/EFS through an ad hoc adapter.