The EF lens mount is the standard lens mount on the Canon EOS family of SLR film and digital cameras. EF stands for "Electro-focus": automatic focusing on EF lenses 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. The mount was first introduced in 1987.
In 2003, Canon introduced the EF-S lens mount, a derivative of the EF mount that is strictly for digital EOS cameras with APS-C sensors released after 2003. EF lenses can be mounted on EF-S bodies but EF-S lenses cannot be mounted on EF bodies. In October 2012, Canon introduced the EF-M lens mount, a derivative designed exclusively for mirrorless interchangeable-lens cameras (MILCs) with APS-C sensors. EF and EF-S lenses can be mounted on EF-M bodies via the optional Mount Adapter EF-EOS M.
Canon claims to have produced its 100-millionth EF-series interchangeable lens on 22 April 2014.