The choice of EF lenses for EOS cameras is huge: focal lengths range from 8mm to 800mm. There are special lenses for the EF mount, such as the TSE tilt-and-shift lenses or the MP-E 65 magnifying lens. Canon lens adapters for the EOS R system allow EF lenses to be used with EOS R cameras without restriction.
Why are adapters necessary? The newly developed RF lens mount for the EOS R system has a shorter flange focal distance than the EF mount on mirror-equipped EOS cameras. With DSLRs, the distance between the sensor and the lens mount is 44mm. With the EOS R system cameras it is only 20mm. Although the bayonet diameter remains the same at 54mm, the smaller distance means that EF or EF-S lenses cannot be mounted directly on an EOS R camera.
The Canon solution is an adapter that compensates for the missing 22mm depth. The three EF-EOS R adapters allow SLR lenses to be used on an R camera. Not only does this work with EF lenses, but also with EF-S lenses, which can now be used on a full-frame mirrorless EOS R system camera. The camera recognises EF-S lenses, with the sensor automatically 'cropping' by a factor of 1.6 to account for the smaller image circle of EF-S lenses. Thanks to the adapter, moving from an EOS DSLR with an APS-C sensor to an EOS R system camera with a full-frame sensor becomes even more interesting.
The performance of the lenses is not compromised by the adapter: Focus speed and image quality remain the same. There are also no restrictions when it comes to the dust and splash protection of the L lenses.
What's more, the EF-EOS R bayonet mount with lens control ring lets you change camera settings without taking your eyes off the viewfinder. For example, you can control aperture, shutter speed, exposure compensation or ISO with this additional control. Canon has come up with a third interesting option that makes use of the extra space: A filter can be inserted between the camera and the lens. To do this, you need the EF-EOS R bayonet adapter with filter holder for the variable ND filter or circular polarising filter. The big advantage is that one filter can be used for all EF or EF-S lenses. Even lenses with a very curved front element, such as the EF 8-15mm F4 FishEye USM or the TS-E 17mm F4 L, can now be used with filters.
Conclusion: The adapters are not a compromise, but rather an extension of the possibilities for exploring new photographic avenues with the EOS R system. EOS R-series technologies, such as the Focus Assistant when used with a TS-E lens, make manual control even easier and open up new possibilities for the use of EF lenses.