An example of a plastic EF-S lens mount.
The Canon EF-S lens mount is a derivative of the created for a subset of with sized image sensors. It was released in 2003. Cameras with the EF-S mount are with the EF lenses and, as such, have a of 44.0 mm. Such cameras, however, have more clearance, allowing lens elements to be closer to the sensor than in the EF mount. Only Canon cameras released after 2003 with APS-C sized sensors support the EF-S mount.
The "S" in EF-S has variously been described by Canon as coming from either "Small image circle" (the lens projects a smaller than normal EF lenses to match the sensor), or "Short back focus" (the smaller mirror used in APS-C cameras also allows optical elements to protrude further into the camera body, reducing the minimum distance between the sensor and the back element of the lens). The combination of a smaller sensor and shorter back-focus distance enhances the possibilities for wide angle and very wide angle lenses, and enables all lenses designed for the EF-S mount to be made smaller, lighter (containing less glass), faster (larger ) and less expensive.
Although not all Canon EF-S lenses use this short back focus, they cannot be mounted on with sensors larger than APS-C. However, some lenses produced by third-party manufacturers may feature the standard EF mount if they do not require the shorter back focus but only have a small image circle. Such lenses will give noticeable if used on a or cameras. To a lesser degree, vignetting also occurs with sensor sizes, such as several (now discontinued) cameras of the 1D series.
The cameras that can use the EF-S mount are:An EF-S compatible body, the , with open lens mount An EF-S compatible mount close-up, from the . Note the circular indent inside the mount, which corresponds to the raised ring on the lens.
By design, it is physically impossible to mount EF-S lenses on EF-only cameras. This is because the increased proximity of the lens to the sensor means that on full-frame sensor or 35mm film EF cameras the lens itself would obstruct the mirror's movement and cause damage to the lens and/or camera. While it is possible to modify the lens such that the physical obstruction is removed, allowing for mounting to EF mount cameras, the rear of the lens would still obstruct the mirror. An additional reason is that the lenses produce a smaller of even illumination (circle of no ). An EF-S lens alignment mark is indicated by a small white rectangle, whereas the EF employs a small red dot. The lens will insert into the body when the alignment marks on each are matched, and the lens can then be rotated and locked into the operating position. EF-S camera bodies have both EF-S and EF alignment marks, while EF bodies have only EF marks. Some have reported success attaching EF-S lenses to full-frame bodies with the use of an ; however, this does not eliminate the vignetting problem, and also removes the lens's ability to achieve . Also, attachment of EF-S lenses on EF bodies can often be accomplished by removing the small plastic ring seen in the photo above. Although vignetting is still an issue, photos can be taken, and infinity focus achieved. This modification comes with caveats, one being that on some lenses, like the EF-S 10-22mm, at the 10mm setting, the element protrudes too far back toward EF mount camera bodies.
The , , and earlier cameras share the -only mount with the full frame EOS camera bodies, and also with the size EOS camera bodies (1D series prior to the 1D X), despite having a smaller sensor and therefore a smaller mirror.
The EF-S lens mount is a relatively new offering from Canon, so the selection of available lenses is limited compared to the full EF range, but it is backward compatible with the EF mount, and can therefore still accept all EF lenses. The variety of EF-S prime lenses is very limited in comparison to EF-S zoom lenses, with three primes to nine zooms. EF-S lenses are very popular due to their lower cost and zoom lenses are preferred by amateur photographers. As of April 2017, no EF-S lens has been produced with the designation or with , and only three EF-S have been produced.EF lens (left) and EF-S lens (right). Note raised ring on EF-S lens. Equivalent focal length