Fast forward another nine years, and the EF-M lens mount was introduced. This is a dedicated lens mount for Canon's new APS-C mirrorless system, the EOS M-series. You can attach EF and EF-S lenses, but via an optional accessory – the EOS-EF M adapter.
Then, in 2018, a fourth new lens mount was introduced, this time for Canon's new full frame mirrorless system – the RF lens mount. Again, you can attach EF and EF-S lenses, but via a different mount adapter – the EOS-EF R. However, whilst you can adapt EF-S lenses to an EOS R-series camera, the camera automatically switches to a cropped mode, to compensate for the smaller image circle produced by EF-S lenses.
The image circle of EF-S lenses is smaller because the APS-C sensor they cover is smaller than a full frame sensor. There’s more information on EF-S lenses via EOSpedia.
RF-S launch
In May 2022 Canon took an unexpected step, introducing yet another lens type – this time called RF-S. These lenses are designed for the new APS-C sensor branch of the EOS R system, launched by the EOS R7 and EOS R10. However, RF-S lenses uses the exact same mount point as RF lenses, not a hybrid one like EF-S. This means that you can use RF-S lenses on full frame R-series cameras, something that isn’t physically possible with full frame DSLRs and EF-S lenses. To compensate, when an RF-S lens is attached to a full frame R-series camera, the camera automatically switches to crop mode, accommodating the smaller image circle of the RF-S lens.
Each lens mount is marked with a symbol, which not only indicates which lens(es) the camera accepts, but also shows the mounting point – there’s a corresponding mark on every lens.
