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 ishandled by a dedicated electric motor built into the lens. Allcommunication between camera and lens takes place through electricalcontacts; 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 cameraswith a 1.6x crop released after 2003. All other EOS cameras that onlyhave an EF mount will not mount EF-S lenses.

History

The EF mount of a Canon EOS 50

When the EF mount was introduced in 1987, it had the largest mountdiameter (54 mm internal) among all 35 mm SLR cameras, allowing large aperture lenses to be designed for the EOS system.^[1]

Unlike the standard autofocus lens mounting technology of the time,which used a motor in the camera body to drive the mechanics of thefocus helicoid in the lens by using a transfer gear, the EF series useda motor inside the lens itself for focusing. This allowed for autofocusinglenses which did not require mechanical contacts in the mountmechanism, only electrical ones to supply power and instructions to thelens motor. The motors were designed for the particular lens they wereinstalled in.

The EF series includes over sixty lenses. The EF series hasencompassed focal lengths from 14 to 1200 mm. Many EF lenses includesuch 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.

[edit] Versatility

Electronics of an EF-S lens

Although Canon does not endorse (and in fact warns against) the useof third-party lenses and adapters, the EF lens mount works withadapters due to its large diameter and the relatively short flange focal distance of 44.0 mm^[2]. 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. The earlier FD mountis not usable for general photography unless adapters with opticalelements are used because its flange focal distance was only 42.0 mm;infinity focus would be lost with an adapter which lacks opticalelements. The Canon FD-EOS adapter is rare and is only usable withcertain FD telephoto lenses. With a manual connection, the aperture andfocus controls of the lens cannot be controlled or read from thecamera; the lens must be focused manually. Since the only possiblemetering is through-the-lens, the lens must be manually stopped down toaccurately meter at anything less than full aperture. (This is calledstop-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 actas weak teleconverters, as well as possibly losing optical quality.

Third-party lenses

Third-party lenses compatible with EOS electronics are manufactured by Sigma, Tamron, Tokina, and Carl Zeiss. The manufacturers of these lenses have reverse engineeredthe electronics of the EF lens mount. The use of these lenses is notsupported by Canon. Sometimes compatibility problems arise, as no thirdparty has access to Canon's specifications for camera-bodycommunication.^[3]These compatibility issues mostly occur when using a newer body whithan older third-party lens. Over time, most of these issues have beenresolved by the major third-party brands. Nevertheless, it is notaccurate to call these lenses EF mount, as that term is reserved byCanon for its own lenses exclusively.

Controls and features

An EF lens showing its different controls and features

Canon EF lenses typically have a number of controls, switches andphysical features, used by the photographer so they can control thelens. 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 complexhaving over a dozen different controls and switches.

This is a list of the different controls and switches found on mostCanon EF lenses, along with a detailed description on what they areused for.

Lens mount index: This marking is found on all EF lenses. Itis used for matching the EF lens mount to the mount on an EOS body, soone can connect the lens to the body quickly. On EF lenses, this can beidentified as a round red mark, while on EF-S lenses, this will be asquare white mark.

Focusing ring: This control is found on most EF lenses. It is used for focusingthe lens, so the subject that one wishes to photograph is in focus.This control is usually a ring on the lens body, that can be turned. Onsome lenses, such as the Canon EF-S 18-55mm lens, this is simply the inner lens barrel.

Zoom ring: This control is found on EF zoom lenses.It is used for changing the focal length of the lens. The zoom ringusually has certain, common, focal lengths marked on it. To set thezoom ring to any given focal length, one must turn the ring so that themarked focal length matches the zoom index. The zoom index is typicallya white, or black, line found next to the zoom ring.

Distance scale of an EF lens

Distance scale window: This feature is found on most EFlenses. This feature, while not a control or switch, is useful to thephotographer for determining, or setting, the lens's focus distance. Itis used in conjunction with the Focusing ring, when rotated,the distance scale will also rotate to show the changing focusdistance. On some lenses the distance scale also has an infrared index.These are shown as red markings below the distance scale. This is usedfor making focus adjustments when the photographer is doing infraredphotography. To make an adjustment, first focus the subject, then turnthe Focusing ring so it matches the corresponding infrared index mark.

Focus mode, and focusing range switches

Focus mode switch: This switch is found on most EF lensesthat have an autofocus feature. It is used for setting the lens toeither autofocus mode, or manual focus. When set to autofocusmode (AF), the lens will autofocus when commanded by the camera. Whenset to manual focus (MF) the photographer must focus the lens by usingthe Focusing ring.

Focusing distance range limiter switch: This switch is foundon most longer focal length lenses, and macro lenses. It is used forlimiting the focusing distance range of the lens, when using it inautofocus mode. Most lenses have two settings, these are usually fullfocus range (from minimum focus distance to infinity), and distantfocus range (from half way point of focus range to infinity).Other lenses have three settings, with the additional setting usuallybeing near focus range (from minimum focus distance to half way pointof focus range). The reason for this feature is to shorten theautofocus time for the lens. Longer focal length lenses, and macrolenses, have a longer travel distance for the focusing mechanism insidethe lens. So when the photographer knows they will not need a certainpart of the focus distance range, limiting it will help shorten theautofocus time, and possibly prevent "focus hunting".

Soft Focus Ring: This ring is found only on the 135 mm 'SoftFocus' prime lens, and enables a variable soft focus effect fromcompletely sharp (0) to very soft (2), although it has little effectwhen used with apertures over f/5.6. Although variable two 'stops' areimplemented at positions 1 and 2.

Both types of image stabilizer switches

Image stabilizer switch: This switch is found on all EFlenses that feature an image stabilizer. It is used for turning theimage stabilizer "on"( | ), or "off"( o ).

Image stabilizer mode switch: This switch is found on most EFlenses that feature an image stabilizer. The switch has two settingsMode 1, and Mode 2. Mode 1 is normal mode, used for typicalphotography, where the subject does not move. Mode 2 is used forpanning, this is useful for sports or wildlife photography, where thesubject moves constantly and one will need to pan. One should not useMode 1 for panning as this will typically cause blurred photographs.Most older lenses that have an image stabilizer, but do not featurethis switch, are permanently in Mode 1. However newer lenses, such asthe Canon EF-S 18-200mm lens,are able to detect if they are being panned in either axis and willauto disable the stabilization for the axis parallel to movement.Therefore they do not need this switch.

Autofocus stop buttons: These buttons are found on supertelephoto EF lenses, evenly spaced around the front collar of the lens.They are used for temporarily stopping the autofocus feature of thelens. Only one button needs to be pressed to activate the feature. Touse this button, one must first have the autofocus active, then whenone wishes to halt autofocus, they will press and hold the button. Whenthey wish to resume autofocus, they will release the button.

Focus preset: The focus preset feature is found on most supertelephoto EF lenses. The focus preset feature uses 1 switch, 1 button,and 1 ring. It is used for presetting a given focus distance intomemory, so that the photographer can quickly recall the focus distance,without the need for autofocus. The switch has three settings "off"( o), "on"( | ), or "on with sound"( ((- ), and is used forturning on the feature, and deciding if sound is desired. The "set"button is used for saving the focus distance into memory. The focuspreset ring is used for recalling the memory save point, it is a thinknurled ring, usually located in front of the Focusing ring. Touse this feature, one must set the switch to either "on", or "on withsound", then the photographer will focus the lens to the point theywish to save at, then press the "set" button. After this, when thefeature is turned on, the photographer can turn the focus preset ring,and the lens will recall and focus quickly to the save point that wasset. This feature is useful for sports and birding photography

Rear gel filter holder on an EF lens

Filter mounting: This mount is used for attaching filters toEF lenses. There are three types, front threaded mount, inner drop-inmount, and rear gelatin holders. Front threaded filters are used onmost lenses, and are attached by threading, and tightening the filter.Inner, drop-in filter mounts are used on super telephoto EF lenses.They are attached by first pressing the two "opposing" buttons on thefilter mount, and pulling it out. Then either a 52 mm round threadedfilter is attached, or one can use a gelatin filter. Rear gelatinfilter holders are used by cutting out a sheet of gelatin, to the sizeshown on the back of the lens, then it is slid in the holder. Filtermounts are useful for all types of photography, every EF lens haseither one, or on some lenses two, of the three types used.

Lens hood mount: This feature is found on most EF lenses.This mount is used for attaching the lens hood. The hood mount is of abayonet style on most EF lenses, though a clip-on style hood mount isused for a small selection of current lenses (85 mm f/1.2L USM, 85 mmf/1.8 USM and 100 mm f/2 USM)

Tripod collar: This feature is found on most longer focallength lenses, and macro lenses. The tripod collar is used forattaching the tripod ring. There are two main styles of tripod rings.One type is opened up, placed on the lenses tripod collar, then closedand tightened. The other type, does not open, but instead is slid upthe lens from the mount end, and tightened. To set the tripod ring sothat it is level with the lens, rotate the ring until the index mark onthe tripod ring matches the index mark on the distance scale. Thetripod ring is used for attaching to a tripod/monopod, instead of thecamera body.