LENSES

Canon's RF mount explained

The RF lens mount is at the heart of Canon's EOS R System, enabling hugely increased communication speed and bandwidth between camera and lens. Find out about the numerous technical innovations and design improvements the RF mount has made possible.

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.

Canon RF mount dimensions

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.

An illustration showing the short back focus distance between lens and sensor in a Canon camera with RF mount.

The large diameter and short back focus distance of the RF mount makes it possible to position lens elements nearer the focusing plane and achieve greater freedom in optical design. Because space does not need to be left for a mirror, the camera and lens system overall can be made shorter and more compact.

Cutaway views of a Canon EF 70-200mm f/2.8L IS III USM lens and an RF 70-200mm F2.8L IS USM lens, showing how much smaller the latter is than the former.

A direct comparison of Canon's EF 70-200mm f/2.8L IS III USM lens (top) and its RF counterpart, the RF 70-200mm F2.8L IS USM (below). Thanks to the technical advances delivered by the RF mount, the developers of the RF lens were able to incorporate innovative technologies and cutting-edge design to ensure it is not just an RF-mount equivalent of the EF lens but a shorter, lighter, super-compact high-speed telephoto lens for a new generation of photography.

12-pin connection

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.

A cutaway illustration of an EOS R System camera with an RF lens attached, highlighting the Control Ring and the data flow between lens and camera.

The innovative, customisable Control Ring on RF lenses gives you a versatile additional way to adjust various settings, such as shutter speed and aperture. It is made possible by the RF mount's huge increase in the speed and bandwidth of communication between camera and lens.

A man holds up a Canon EOS R5 C with no lens attached to the camera, showing the RF mount and sensor.

Canon has brought the advantages of the RF mount and the growing range of RF lenses to pro video cameras, including the EOS C70 and the EOS R5 C shown here.

Faster focusing

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.

A cutaway illustration showing communications between the lens microprocessor and image processor.

The Digital Lens Optimizer (DLO) technology built into the EOS R System takes full advantage of the RF mount's super-fast communication between lens and camera body, and the power of the image processor, to get the best from your lenses.

A cutaway illustration showing the image processor, imaging sensor, lens microprocessor and Nano USM lens focusing motor.

The Canon RF 24-105mm F4L IS USM lens has a Nano USM motor (labelled Nano USM here) controlled by a microprocessor (labelled Lens Microprocessor), which communicates at high speed with the Dual Pixel CMOS AF system in the sensor of EOS R System cameras and the camera's processor (labelled Image Processor), delivering super-fast autofocus performance. This is one of many technical innovations made possible by the technology in the RF mount.

IS technology

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.

A diagram showing how the motion of the image sensor in the EOS R5's In-body Image Stabilisation system remains within the image circle.

Thanks to the large diameter of the RF lens mount, the sensor in a camera with In-body Image Stabilisation (IBIS) such as the EOS R5 can move around more to provide unprecedented levels of image stabilisation. The mount opening (1) is large enough that when the sensor moves from its default position (2), even the maximum amplitude of sensor motion (3) is well within the image circle (as highlighted in red), meaning that light reaches the entire image sensor no matter how much the sensor moves to maintain image stabilisation.

A long-exposure image of a horizontal red light trail emerging from a cave in a cliff face on a sandy beach.

Commercial photographer Rob Payne used a four-second exposure to capture this striking image of a drone in flight – or more accurately, the red light fitted to it; the moving drone itself is invisible. Even though he shot handheld in low light with such a long exposure, the background is still sharp, down to the tufts of vegetation against the sky on the hillside, demonstrating the effectiveness of the camera's image stabilisation. Taken on a Canon EOS R6 at 24mm, 4 secs, f/5.6 and ISO1600. © Rob Payne

Canon RF lens roadmap

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.

Canon's RF Trinity lenses

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 RF lens extenders

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.

Three Canon RF zoom lenses, sitting on a paint-spattered wooden surface. A rowing scull with a white-and-red hull is out of focus in the background.

Canon's "Trinity" of f/2.8 RF lenses encompasses ultra-wide, standard and telephoto zoom ranges, covering focal lengths from 15mm up to 200mm with a constant f/2.8 aperture, for shooting in practically any professional situation from landscapes, portraits and sports, to interiors, fashion and cityscapes.

Can Canon EF lenses be used on an RF mount camera?

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.

A close-up view of a photographer's hands fitting a Drop-In Filter Mount Adapter EF-EOS R with V-ND filter to a Canon EOS R5 C camera.

The Drop-In Filter Mount Adapter EF-EOS R not only enables you to use EF lenses on an EOS R System camera but also adds the ability to use drop-in filters. A variable ND filter such as the one in the adapter being fitted here to an EOS R5 C will help control the amount of light entering the camera, which is useful for filmmakers wanting to shoot with wide aperture/slow shutter speed combinations in bright light.

Canon RF Mount FAQs

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.

Kirjutanud Jeff Meyer & Alex Summersby


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