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The EF lens mount allows all the Canon EF lenses to be used on any of the Canon EOS line of cameras made by Canon Inc. Unlike the EF's breech-lock predecessor, the FD mount, the EF mount uses a bayonet-style mount. EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens. All communication between camera and lens takes place through electrical contacts; there are no mechanical levers or plungers.
When the EF mount was introduced in 1987, it had the largest mount diameter of 54 mm (internal) among all 35 mm SLR cameras, allowing large aperture lenses to be designed for the EOS system.
Unlike the standard autofocus lens mounting technology of the time, which used a motor in the camera body to drive the mechanics of the focus helicoid in the lens by using a transfer gear, the EF series used a motor inside the lens itself for focusing. This allowed for autofocusing lenses which did not require mechanical contacts in the mount mechanism, only electrical ones to supply power and drive information to the lens motor. Thus, the mounting mechanism was simpler and more durable. Also, the motors were designed for the particular lens they were installed in. One benefit of this arrangement is that if the motor in a lens breaks, other lenses will of course retain full function. If the motor of an in-body driven AF system fails, no autofocus is possible.
The EF series has since grown into a mature system with over 50 lenses for all types of applications, from landscapes to portraits to sports and photojournalism. Canon EF lenses also demonstrate successful commercialisation of mechanical and optical technologies, such as the Ultrasonic Motor (USM) drive, Image Stabilizer (IS), Diffractive Optics (DO) and using fluorite and aspherical elements in the construction of their L-series lenses for superb image quality.
Although Canon does not endorse (and in fact warns against) the use of third-party lenses and adapters, the EF lens mount is ideal for the use of adapters, due to its large diameter and the relatively short flange focal distance of 44.0 mm. It is thus possible to mount lenses using the Nikon F mount, Olympus OM, Leica R and universal M42 lens mounts (among others) by the use of a simple mechanical adapter. Ironically, the earlier FD mount is still not usable for general photography unless adapters with optical elements are used because its flange focal distance was only 42.0 mm; infinity focus would be lost with an adapter which lacks optical elements. The high quality Canon FD-EOS adapter is rare and quite expensive, as it was only sold to professionals with a substantial stock of FD telephoto lenses. It is only usable with certain FD telephoto lenses. Of course, with a purely manual connection, the aperture and focus controls of the lens cannot be controlled or read from the camera; the lens must be focused manually. In addition, since the only possible metering is through-the-lens, the lens must be manually stopped down to accurately meter at anything less than full aperture. (This is called stop-down metering.)
For other lens types, an adapter would act as an extension tube, causing the lens to lose the ability to focus to infinity. Alternatively, the lens adapters would include optical elements and act as weak teleconverters, as well as possibly losing considerable optical quality. Because of this, it is generally preferred to use lenses that can be mounted at their preferred offsets.
Third-party lenses compatible with EOS electronics are manufactured by Sigma, Tamron, and Tokina. The manufacturers of these lenses have reverse engineered the electronics of the EF lens mount. The use of these lenses is not supported by Canon. However, many users find these lenses to be cheaper, and sometimes superior alternatives to Canon lenses. Wikipedia |
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