Canon EF-S lens mount. The Canon 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 with the EF-S mount are backward compatible with the EF lenses and, as such, have a flange focal distance 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 image circle 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).