The EF 300mm lens refers to a family of telephoto prime lenses made by Canon, five of which have been sold to the general public and one of which was only made on special order. The lenses have an EF type mount which fits the Canon EOS line of cameras.
Image: Canon 300mm f 28 MG 2036
Image: Canon EF 300mm L IS I USM
Image: Canon EF 300 f 2.8
Image: Canon EF 300 F4 L
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.
The electronic contacts (gold-plated) of an EF mount lens.
The EF mount of a Canon EOS 50
Electronics of an EF-S lens
An EF lens showing its different controls and features