The EF lens mount is the standard lens mount on Canon’s EOS family of 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 electronic contacts, with no mechanical levers or plungers.
The EF mount was first introduced in 1987, superseding the FD mount, and bridged Canon’s transition from film to digital camera bodies. Canon announced the production of its 100-millionth EF-series interchangeable lens on April 22, 2014. The last new EF lens design was released in 2018, with Canon’s focus shifting to their new RF lens system.
EF lenses can be used via an adapter on RF camera bodies, but RF lenses cannot be used with EF cameras. EF has no interoperability with the FD mount system.
History
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.
The EF mount reversed the mechanical logic of the FD mount. The FD mount provided the three-eared bayonet fitting on the camera body, and each FD lens provided a breech-lock receptacle to register and fasten the lens to the bayonet. The EF mount reverses this logic, providing the bayonet on each lens, and a receptacle on the camera body.
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, encompassing focal lengths from 8 to 1200 mm. 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.
Versatility
Its large diameter and relatively short flange focal distance of 44.0 mm allows mechanical adaptation of EF camera bodies to many types of non-EF lenses.W.J. Markerink maintains an article on Camera Mounts & Registers which gives much more detail about flange focal distances and lens compatibility. It is 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 mechanical adapter without electronic control of the aperture or autofocus. In contrast, parfocal adaptation of EF lenses to non-EF camera bodies is not possible with only a mechanical adapter that does not contain optical elements.
EF mount lenses are somewhat compatible with newer Canon bodies, though the reverse is not true:
EF-S lens mount cameras can mount EF lenses without an adapter EF-M lens mount cameras can mount EF lenses with an EF-EOS M adapter RF mount cameras can mount EF lenses with a variety of adapters
Lenses for the earlier Canon FD lens mount are not usable for general photography on an EF mount cameras, unless adapters with optical elements are used because they are made for a flange focal distance of only 42.0 mm. Infinity focus would be lost with an adapter which lacks optical elements. The Canon FD-EOS adapter is rare and is only usable with certain FD telephoto lenses. With a manual connection, the aperture and focus controls of the lens cannot be controlled or read from the camera; the lens must be focused manually. 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.)
Third-party lenses Compatible third-party lenses with the EF lens mount are manufactured by Yongnuo, Samyang, Schneider, Sigma, Tamron, Tokina, Cosina and Carl Zeiss. The manufacturers of these lenses have reverse engineered the EOS electronics—except Zeiss, which does not have the rights to use the autofocus or the electronic aperture control of EOS cameras . The use of these third-party lenses is not supported by Canon. Sometimes compatibility problems arise, as no third party has access to Canon's specifications for camera-to-body communication. These compatibility issues mostly occur when using a newer body with an older third-party lens. Over time, most of these issues have been resolved by the major third-party brands.
Third-party cameras Due to the high market penetration of EF-mount lenses, other camera manufacturers began to offer EF-mount cameras. Since the EF-mount was created for SLR cameras with their long focal flange distance, mirrorless interchangeable-lens cameras can use EF lenses with a mechanical adaptor that bridges the distance.
Red Digital Cinema Company offers various camera models that can be equipped with an electronic EF-mount. Many Blackmagic Design cameras are sold in EF-mount variants. For Sony E-mount various adaptors enable using EF-mount lenses with full electronic control.
Controls and features
Canon EF lenses typically have a number of controls, switches and physical features, used by the photographer to control the lens. The types and number of the controls can vary from lens to lens. With the most basic lenses having only a few, to the most complex having over a dozen different controls and switches.
This is a list of the different controls and switches found on most Canon EF lenses, along with a detailed description on what they are used for.
Lens mount index: This raised, round red mark is found on all EF lenses. It is used for matching the EF lens mount to the mount on an EOS body, so one can connect the lens to the body quickly.
Focusing ring: This control, found on most EF lenses, is used for focusing the lens. It is usually a ring on the lens body, that can be turned.
Zoom ring: This control is found on most EF zoom lenses. It is used for changing the focal length of the lens. The zoom ring usually has certain, common, focal lengths marked on it. To set the zoom ring to any given focal length, one must turn the ring so that the marked focal length matches the zoom index. The zoom index is typically a white, or black, line found next to the zoom ring.
Distance scale window: This feature is found on many EF lenses. This feature, while not a control or switch, is useful to the photographer for determining, or setting, the lens's focus distance. It is used in conjunction with the Focusing ring. When rotated, the distance scale will also rotate to show the changing focus distance. On some lenses the distance scale also has an infrared index. These are shown as red markings below the distance scale. This is used for making focus adjustments when the photographer is doing infrared photography, as lenses typically focus infrared light at a different point than visible light, and therefore achieving correct focus using visible light will result in an out-of-focus infrared image. To make an adjustment, first focus the subject, then turn the Focusing ring so it matches the corresponding infrared index mark.
Focus mode switch: This switch is found on most EF lenses that have an autofocus feature. It is used for setting the lens to either autofocus mode, or manual focus. When set to autofocus mode (AF), the lens will autofocus when directed to by the camera. When set to manual focus (MF), the lens is focused using the Focusing ring. Some lenses support full-time manual focusing (FT-M), which allows the photographer to focus the lens manually even with the mode switch set to AF, without damaging the lens (as could happen if a lens without FT-M is manually focused while in AF mode).
Focusing distance range limiter switch: This switch is found on most longer focal length lenses, and macro lenses. It is used for limiting the focusing distance range of the lens when using it in autofocus mode. Most lenses have two settings; these are usually full focus range (from minimum focus distance to infinity), and distant focus range (from halfway point of focus range to infinity). Other lenses have three settings, with the additional setting usually being near focus range (from minimum focus distance to halfway point of focus range). Longer focal length lenses and macro lenses have a relatively long travel distance for the focusing mechanism inside the lens; this feature shortens the autofocus time. When the photographer knows they will not need a certain part of the focus distance range, limiting it will help shorten the autofocus time, and possibly prevent "focus hunting".
Soft focus ring: This ring is found only on the 135 mm 'Soft Focus' prime lens, and enables a variable soft focus effect from completely sharp (0) to very soft (2), although it has little effect when used with apertures over f/5.6. Although the ring can be set to any position, two 'stops' are implemented at positions 1 and 2.
Image stabilizer switch: This switch is found on all EF lenses that feature an image stabilizer. It is used for turning the image stabilizer "on"( | ), or "off"( o ).
Image stabilizer mode switch: This switch is found on many EF lenses that feature an image stabilizer, particularly those of longer focal lengths. The switch has two settings on most lenses: Mode 1 and Mode 2. The newest IS Mark II versions of certain EF super telephoto lenses (the 300mm f/2.8L, 400mm f/2.8L, 500mm f/4L, and 600mm f/4L), plus the 200–400mm f/4L IS and 100–400mm f/4–5.6L IS II, have a third setting, Mode 3. Mode 1 is normal mode, used for typical photography, where the subject does not move. Mode 2 is used for panning; this is useful for sports or wildlife photography, where the subject moves constantly and one will need to pan. Mode 3, intended to track action, is similar to Mode 2 in that it ignores panning; however, it only applies stabilization when the shutter is released—the viewfinder image is not stabilized. One should not use Mode 1 for panning as this will typically cause blurred photographs; the image stabilizer will attempt to correct for all motion, including the panning motion, but cannot do so due to the limited range of motion of the IS mechanism. Older lenses that have an image stabilizer, but do not feature this switch, are permanently in Mode 1. Some newer lenses, such as the Canon EF-S 18-200mm lens, are able to detect if they are being panned in either axis and will automatically disable the stabilization for the axis parallel to movement and therefore do not require this switch.
Autofocus stop buttons: These buttons are found on some super telephoto EF lenses, evenly spaced around the front collar of the lens. They are used for temporarily stopping the autofocus feature of the lens. Only one button needs to be pressed to activate the feature. To use this button, one must first have the autofocus active, then when one wishes to halt autofocus, one presses and holds the button. To resume autofocus, one releases the button. Some newer bodies allow these buttons to be assigned to perform other functions; for instance, the Canon EOS 7D allows the photographer to set these buttons to perform any of six functions.