Canon's RF mount explained

Canon's innovative RF lens mount debuted in September 2018 in the EOS R, the first of Canon's mirrorless EOS R System cameras. Coming just over 30 years after Canon's acclaimed EF mount was introduced, the RF mount was designed to enable a new generation of optical technology and creative possibilities.

All EOS R System cameras have the same RF mount whether the sensor in the camera is APS-C (as in the EOS R7 and EOS R10) or full-frame (as in other models). This means that all RF and RF-S lenses can be used on all EOS R System cameras, although the field of view of any given lens will be different on an APS-C camera from a full-frame camera, because of the APS-C sensor's "crop factor".

Here we look at some key aspects of the RF mount's design and the technical advances it has made possible.

The Canon RF mount retains the same wide 54mm diameter as the EF mount, but with a big reduction in the back focus distance – the distance between the mount and the sensor – from 44mm in the EF mount to 20mm in the RF mount. This has come about because EF-mount DSLRs had to be designed around the camera's reflex mirror mechanism, but RF-mount cameras are mirrorless, which allows lens designers to prioritise optical performance and makes new optical designs possible.

In particular, lenses used to require additional optical engineering to shift the optical system forward to avoid the mirror of the camera while retaining the same focal length, but with the RF mount this is no longer the case. As a result, RF lenses can often be made more compact and more lightweight than their EF mount equivalents. RF lenses can also be constructed with larger diameter, rear-positioned elements. This type of design helps reduce the bending of light rays as they pass through the lens, which reduces aberrations and improves overall image quality. It is now possible to have larger apertures for a given focal length and achieve corner-to-corner sharpness with minimal light fall-off.

The RF mount has a 12-pin connection between the camera and lens, compared with 8 pins in the EF mount. This enables much faster communication between lens and camera, and much greater bandwidth for data transfer. This unlocks many benefits, and gives developers scope to add even more features in the future.

One benefit is the new controls this makes possible, particularly the Lens Control Ring featured on all RF lenses in addition to the standard focus and zoom rings. This ring can be customised to control a variety of settings such as shutter speed, aperture, exposure compensation and ISO, giving you a quick and tactile way of adjusting key settings without having to take your eye from the viewfinder.

Another benefit is in-camera correction of optical aberrations and real-time Digital Lens Optimisation (DLO). In the past, when you purchased a new lens, you had to download and register its lens correction data, so that your camera could apply the adjustments required to compensate for the optical quirks of that particular lens. Thanks to the speed of the RF mount connection, the amount of data it can handle and the processing power of the latest image processors, DLO data is stored within RF lenses and can be read automatically. What's more, DLO data can be used during continuous shooting without affecting shooting speed or the number of shots you can take, as it did in the past.

Even more notably, the speed and bandwidth of the RF mount delivers faster and more responsive autofocus performance – the EOS R5 and EOS R6, for example, can acquire focus in a class-leading 0.05 seconds and then continue to track even fast-moving subjects across the entire frame.

Thanks to the RF mount, lenses can also take advantage of new focusing technologies. The RF 70-200mm F2.8L IS USM is the first lens to feature Dual Nano USM technology – it has two Nano USM motors, each driving different lens groups, working together to produce faster, more efficient focusing. The RF 400mm F2.8L IS USM and RF 600mm F4L IS USM super-telephoto lenses are the first in the RF lineup to feature a double power drive method, enabling cameras to focus faster than ever before. While the optics and mechanics of these two lenses are largely the same as the EF versions, their capabilities are next-level, as they take full advantage of Canon's revolutionary EOS R System and RF mount.

Technologies such as Nano USM, which combines the speed of traditional USM motors with the silent operation and smoothness of STM motors, are a real benefit particularly for videographers as well as sport or wildlife photographers, where they are shooting fast action and super-quiet, smooth focusing is required. What's more, the RF mount enables smoother aperture adjustments when you're shooting video, with all RF lenses supporting smaller increments of just 1/8 stop as compared to the 1/3 stop used for still photography, meaning less noticeable changes in brightness as you adjust the aperture while filming.

The RF mount's speed of connection even allows electronic focus breathing suppression to prevent the angle of view from changing as you focus. Non-RF cinema lenses tend to be very large because they incorporate a mechanical mechanism to help overcome focus breathing, but the RF mount, coupled with the use of Nano USM technology and floating focus groups, means this can now be done electronically, enabling a very compact lens design for video.

One of the most important technical innovations made possible by the innovative design of the RF mount is superior image stabilisation (IS).

The speed and bandwidth of communication between camera and lens mean that even in EOS R System cameras that do not have sensor-shifting In-body Image Stabilisation (IBIS), a Dual Sensing IS system monitors motion information from the camera's CMOS sensor, which is more sensitive than the lens's optical IS to low-frequency camera shake such as that caused by the photographer's heartbeat or breathing, and this extra information is used to deliver enhanced shake correction – in practice half a stop or more of additional stabilisation.

In addition, in those cameras that do have the IBIS system introduced in the EOS R5 and EOS R6, the camera's IBIS can work co-operatively with the optical IS in IS-equipped lenses to tackle a wider range of camera shake than either system alone. Optical IS is especially effective at telephoto focal lengths, for example, while IBIS is most effective at correcting shake that occurs at wider focal lengths.

The RF mount's wide throat also means that lenses with a large image circle can get the full benefit of the IBIS system – the sensor has more room to move to offset image shake without the risk of cutting off the image. This enables the camera's IBIS system to deliver up to 8-stops of stabilisation when using lenses that do not feature built-in optical IS but have large image circles, such as the RF 28-70mm F2L USM and RF 85mm F1.2L USM.

You'll find the current Canon RF lens range on the RF lens product page. The Canon RF lens lineup is continuing to evolve, with new lenses being released every year. A new category of RF-S lenses has been added, including the RF-S 18-45mm F4.5-6.3 IS STM and RF-S 18-150mm F3.5-6.3 IS STM, specially designed for use with APS-C EOS R System cameras such as the EOS R7 and EOS R10. With RF lens options as wide as 5.2mm and as long as 1200mm, the lineup already offers the widest range of focal lengths available in a mirrorless system, and Canon intends to provide a focal length for every subject, for every photographer.

At the heart of the RF lens range are Canon's "Trinity" of f/2.8 zoom lenses – a wide-angle zoom, a standard zoom and a telephoto zoom lens. Together, these three should give professional photographers enough flexibility to shoot almost any subject in any situation.

Canon's RF Trinity lenses are the RF 15-35MM F2.8L IS USM, RF 24-70MM F2.8L IS USM and RF 70-200mm F2.8L IS USM. All feature a fast f/2.8 aperture, optical image stabilisation and a silent Nano USM autofocus motor. Their versatility means they can shoot everything from landscapes and portraits to sport and wildlife.

Canon also offers 1.4x and 2x lens extenders or teleconverters for certain RF lenses, which offer greater reach without sacrificing image quality. This is achieved through the use of high-refraction, low-dispersion glass, which helps reduce the chromatic aberrations that tend to come with magnification.

The design of both RF extenders also features special lens coatings and a three-layer structure to reduce ghosting, and the white exterior familiar on Canon's telephoto L-series lenses prevents the extenders from getting too hot, ensuring optimum performance.

Because of their physical construction, however, only a few lenses can accept lens extenders. Also, using the Canon Extender RF 1.4x incurs a 1-stop reduction in the maximum aperture and using the Extender RF 2x brings a 2-stop reduction, but the lenses can still focus automatically.

Even though they share the same mount diameter, EF lenses cannot be fitted directly on an RF mount. You can however use EF and EF-S lenses on an EOS R System camera, with no loss of quality or functionality, using a choice of EF-EOS R Mount Adapters.

In addition to the standard Mount Adapter EF-EOS R, the Control Ring Mount Adapter EF-EOS R adds a Lens Control Ring like the one on RF lenses, which you can customise to take control over different settings. Alternatively, the Drop-In Filter Mount Adapter EF-EOS R adds the ability to use drop-in filters, removing the need to fit filters on the front of a lens, which is especially useful for wide-angles with a large front lens element. This adapter is available with either a variable neutral density (V-ND) filter or circular polarising (C-PL) filter.

Users of the EOS C70, Canon's first RF-mount Cinema EOS camera, also have the option of using EF lenses via the Canon Mount Adapter EF-EOS R 0.71x. This mount adapter incorporates a 0.71x wide optical converter, so that the lens's full-frame angle of view is maintained on the EOS C70's Super 35mm sensor, as well as an increased light transmission of approximately 1-stop.

All these mount adapters enable full communication between an EF lens and the EOS R System camera. This means autofocus functionality, chromatic aberration correction and lens metadata are all fully supported just as they would be on an EF-mount camera.

Do Canon RF lenses fit onto an EF mount?

No, Canon RF lenses are not compatible with Canon EF-mount DSLRs, nor is there an adapter for RF mount to EF mount. Find out more about lens compatibility.

What does RF mean in Canon lenses?

When the Canon EOS system was introduced, the name stood for Electro-Optical System. As the EF mount was a fully electronic mount, EF stood for Electronic Focus.

Canon's project to develop the next generation of EOS cameras had the aim of "Reimagining optical excellence" and was hence codenamed Project R. This in turn led to the official name of the EOS R System. For the lenses, a fusion of EF and R resulted in the name RF lenses, which simply means Canon lenses designed for use with EOS R System cameras, which have the new RF mount.

Why are Canon RF lenses better?

Canon RF lenses are better than EF lenses because the RF mount architecture enables much faster communication between the camera and the lens, much greater data transfer, and support for the latest focusing, image stabilisation and optical technologies. The reduced back focus distance also allows for lens designs with no performance compromises.

What are RF-S lenses?

The category of RF-S lenses was introduced to provide affordable, general-purpose lenses designed for use with APS-C EOS R System cameras, so the RF-S 18-45mm F4.5-6.3 IS STM is the ideal kit lens to pair with the Canon EOS R10, for example. The RF mount on an RF-S lens is identical to the mount on all RF lenses, and RF-S lenses are compatible with all EOS R System cameras. However, full-frame EOS R System cameras, when fitted with an RF-S lens, will automatically crop the image area to match the APS-C coverage of the lens.