Canon EF-S Lens Mount
The EF-S lens mount is a derivative of the EF lens mount created for a subset of Canon digital single-lens reflex cameras with APS-C sized image sensors. It was released in 2003. Cameras supporting the EF-S mount are backward-compatible with the EF lens mount and, as such, have a flange focal distance of 44.0 mm. Cameras supporting EF-S, however, have more clearance, allowing lens elements to be closer to the sensor than in the EF mount. Only Canon cameras with the APS-C sized sensor released after 2003 support the EF-S mount.
The "S" in EF-S comes from "Small image circle", meaning the lens is designed to provide a small (smaller than a normal EF lens) image circle. The proximity of the rear element to the image sensor greatly enhances the possibilities for wide angle and very wide angle lenses, enabling them to be made smaller, lighter (containing less glass), faster (larger aperture) and less expensive. Most current Canon EF-S lenses are wide angle.
Lenses designed specifically for APS-C sized sensors are often optically designed to provide a narrower light cone to match the sensor. However, not all such lenses require the shorter back focus, and may feature the standard EF mount. Such lenses will give noticeable vignetting if used on a 35mm film or sensor camera.
Read more about Canon EF-S Lens Mount: Compatibility, List of EF-S Lenses
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