Canon EF

Introduced in 1987, 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.

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 (including those for their cameras from other companies), for EOS cameras, on 22 April 2014. Sources including EF-S and Cinema lenses).

The EF mount replaces its predecessor, the FD mount. The standard autofocus lens mounting technology of the time used a motor in the camera body to drive the mechanics of the focus helicoid in the lens by using a transfer lever. The key innovation of the EF series was to use a motor inside the lens itself for focusing. This allowed for autofocusing lenses which did not require mechanical levers in the mount mechanism, only electrical contacts to supply power and instructions to the lens motor. The motors were designed for the particular lens they were installed in.

When the EF mount was introduced in 1987, it had the largest mount diameter (54 mm internal) among all 35 mm SLR cameras.

The EF series includes over eighty lenses. The EF series has encompassed focal lengths from 8 to 1200 mm. The EF-M mount was introduced with two lenses, a 22mm prime and an 18–55mm zoom. Many EF lenses include such features as Canon's ultrasonic motor (USM) drive, an image stabilization system (IS), diffractive optics (DO) and, particularly for L-series lenses, fluorite and aspherical lens elements.

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