Photography Museum Lenses: Canon RF Mount

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Canon’s RF lens mount (2018) used on EOS R cameras and associated RF lenses is an interesting evolution and amalgamation of the previous EF (1987) and EF-M (2012) technologies. The mirrorless specific RF mount has the same 54mm diameter as the EF mount, but has a flange to sensor distance of only 20mm versus 44mm for the SLR EF mount (and 18mm for the EF-M mount). This shorter flange distance allows for more compact wide angle lens designs. It also means that EF mount lenses can be used on EOS R cameras with an adaptor.

The RF mount features 12 twelve electronic contacts compared to the 8 contacts on EF/EF-S and 9 contacts on EF-M. The increased contact count allows for more bandwidth and faster communication between the lens and the camera body.

Like EF-M lenses, Canon RF lenses contain factory-calibrated correction data which is passed to the camera when mounted.

The RF mount was initially only available on full-frame R-series cameras. However, in late 2022, Canon introduced the R1O and R7 bodies with an APS-C sensor. Concurrent with these new bodies, Canon released the first RF-S lenses designed to work with the smaller sensor. RF-S lenses can also be mounted on full-frame R-series bodies, but with an automatic 1.6x sensor crop.

Along-side the zoom and focus rings, many RF mount lenses feature an additional control ring. In some lower end primes and zooms, a switch toggles a single ring between focus and control functions. A control ring is also available on the Control Ring Mount Adapter EF-EOS R. The control ring can be configured in camera to adjust many camera functions, such as aperture, ISO, focus area, exposure compensation, flash exposure compensation, etc.

While Canon never released a professional L series lens for the EF-M mount, the RF mount and R-series bodies were, from the onset, targeted more towards professionals and enthusiasts than amateurs and beginners. In the first two years, Canon released 8 L series lenses and only 2 non-L lenses (a 35mm prime and 10x super-zoom).

As of writing, all RF lenses feature the slower, but virtually silent, STM (stepper-motor) or the faster, and still mostly silent, linear Nano-USM (ultra-sonic motor)1. Because of these two technologies, all RF lenses to date are focus-by-wire and work equally well for either stills or video.

Canon has chosen to prioritize compactness for RF L-series zoom lenses which feature extending lens barrels, compared to most EF L-series lenses which were of a bulkier internal-zoom design.

To date, all L-series zoom lenses also feature IS (image stabilization), except for the fast and already very large RF 28-70mm F2 L USM.

Adapted EF lenses (Canon or third-party) work seamlessly via the EF-EOS R adaptors.

Canon have chosen to keep the RF mount closed to third-party autofocus lenses.

The new Lens Dust Cap RF is compatible with EF lenses, but not the other way around.

In fact, my biggest complaint about the RF mount is the dust cap design. Because the bayonet tabs on the EF mount were arranged symmetrically, the older EF dust caps could be mounted at any of three angles (0°, 120°, 240°). Without looking you could quickly get the dust cap on just by feel. The bayonet tabs on the RF mount are not symmetrical. The new dust cap will attach in only one orientation and the alignment marker is quite hard to see. I plan to highlight all of my RF dust cap alignment markers with silver marker or vinyl strips so they are easier to see and still look classy.

RF Prime Lenses

  • Canon RF 16mm ƒ/2.8 STM
  • Canon RF 24mm ƒ/1.8 Macro IS STM

RF Zoom Lenses

RF Miscellaneous

  • Canon Mount Adapter EF-EOS R With Control Ring

  1. The only exception is the RF 5.2mm F2.8 L Dual Fisheye Lens which has fixed focus.