The remarkable innovations behind the top RF super-telephoto lenses

Canon is synonymous with super-telephoto lens innovation. From FD through to EF and the latest RF primes and zooms, Canon's groundbreaking long lenses have been relied on by professional sports and wildlife photographers for decades.

With the launch of the Canon RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM, another milestone in mirrorless camera lens design has been achieved. No other lenses in their class come close to offering the combination of extreme reach, compact size, fast AF and outstanding optical quality.

The development of the pioneering EOS R System presented Canon's lens engineers with the opportunity to go back to the drawing board with these extreme focal lengths. Freed from the constraints of conventional lens design, it has been possible to reevaluate how a super-telephoto lens is made, resulting in the technical innovation and cutting-edge performance of the RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM lenses.

So, what were the challenges of constructing these super-compact, state-of-the-art super-telephoto lenses, and how have they been overcome?

Here, the Canon lens development team takes us behind the scenes and provides insights into the design, engineering and operation of the Canon RF 800mm F5.6L IS USM and the world's longest focal length AF lens for mirrorless cameras, the RF 1200mm F8L IS USM.

Compared with the Canon EF 800mm f/5.6L IS USM, the RF 800mm F5.6L IS USM is around 3cm shorter and 1.36kg lighter, and the focusing distance has been halved. The size and weight savings that have been made with the Canon RF 1200mm F8L IS USM compared with the legendary EF 1200mm F5.6L USM are even more impressive. Not only is it around 30cm shorter and a staggering 13kg lighter, it focuses down to 4.3 metres – approximately 10 metres closer than the EF version.

So how was this impressive feat achieved? The breakthrough came with the novel idea of positioning the magnifying optics near the back of each lens, explains the development team's Mechanical Design Leader, Katsuhiro Inoue. "The RF 1200mm F8L IS USM shares a similar optical design with the RF 600mm F4L IS USM but introduces a new dedicated magnifying optical system that has made it possible to significantly reduce the size of the lens compared with the EF 1200mm F5.6L USM."

It's a similar story with the Canon RF 800mm F5.6L IS USM. The front half of the lens shares similar functionalities with the Canon RF 400mm F2.8L IS USM and it is compatible with the same lens hoods and accessories. But moving the new magnification optics closer to the camera enables the RF 800mm F5.6L IS USM to be much smaller and lighter than the Canon EF 800mm f/5.6L IS USM.

"In the past, these big magnifying optics have been at the front of the lens, with the focusing group at the back," says Canon Europe's Senior Professional Imaging Product Specialist, Mike Burnhill, "but with the RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM we've essentially flipped it around. Putting the magnification optics at the back, nearer the camera sensor, means that the optics no longer need to be, say, 160mm in diameter, which massively reduces the weight and size."

To correct for chromatic aberration and ensure a high-quality image, a UD lens element has been incorporated into the design of the new magnification optics, explains Optical Designer Tomohiro Ino. "In addition to this, ASC coating that significantly suppresses flare and ghosting is used in those parts of the lenses that have been inherited from the RF 400mm F2.8L IS USM and RF 600mm F4L IS USM. We have also applied a fluorine coating to the front of the lens so that it is easy to remove any dirt and dust that adheres to the surface of the glass."

The innovative optical arrangement of the Canon RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM means that the weight of the lenses is distributed more centrally compared with their front-heavy EF counterparts. Factor in powerful Image Stabilizers, and you've got a brace of super-telephotos that are practical to use handheld, as well as a highly mobile solution for tripod or monopod work.

"We have a long history of pursuing compactness, reduced weight and high image quality in our super-telephoto lenses," says Katsuhiro Inoue. "In the process, for example, in terms of materials, we have adopted a magnesium alloy that has both high rigidity and light weight, which was not used in the era of the Canon EF 1200mm f/5.6L USM lens. As simulation technology has evolved, it has also become possible to reinforce, thicken/thin and optimise the structure of parts with dramatically higher accuracy than before."

Both the RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM are fully weather sealed and have specially strengthened barrels. "In terms of the mechanical design, the biggest challenge has been to ensure the parts that hold the magnification optics securely in place are strong enough," says Takami Hirasawa. "Our simulation technology has been very useful to help develop a lightweight and robust solution. Additionally, we have incorporated measures to hold the heavy lens elements more firmly while maintaining optical performance."

The RF 1200mm F8L IS USM's maximum aperture is one stop smaller than that of the EF 1200mm f/5.6L USM, but this has considerable benefits for handling. "The f/8 maximum aperture contributes to the substantial reduction in size and weight compared to the conventional EF model," Tomohiro Ino confirms. "As the RF 1200mm F8L IS USM supports Dual Pixel CMOS AF, it is capable of fast and accurate autofocus despite the darker maximum f-stop. High image quality, autofocus and IS are also still achievable when the Extender RF 1.4x or RF 2x is attached to the RF 800mm F5.6L IS USM or RF 1200mm F8L IS USM."

Indeed, the only discernible effects with the lens extenders attached are the drop in maximum aperture and a reduction in the AF coverage. "When the RF 800mm F5.6L IS USM is used with the Canon EOS R3, EOS R5 or any other camera that supports Dual Pixel CMOS II AF, you get full focus tracking across 100% of the frame, even with Extender RF 1.4x attached," explains Mike. "With the Extender RF 2x, you still get 80% AF coverage horizontally and vertically.

"The RF 1200mm F8L IS USM still gives you 80% AF coverage horizontally and vertically with the Extender RF 1.4x, and it drops to 60% x 40% coverage with the Extender RF 2x. So, even with a 2400mm f/16 lens, you're getting a focusing area that's roughly the size that DSLR users are accustomed to. There's no impact on AF performance, apart from the size of the area."

As with the Canon RF 400mm F2.8L IS USM and RF 600mm F4L IS USM, the new lenses support electronic full-time manual focus. The ring-type USM motors used in the Canon RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM feature 'power-doubled focus drive', which allows two power inputs, enabling users of the EOS R3, with its high-capacity battery, to experience faster autofocusing.

"The AF performance of these two lenses is in fact fast enough for general shooting without the need for the dual focus power drive," says Electrical Design specialist, Hiromi In. "However, the RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM will benefit from this feature when the focus preset function is used."

As well as being able to rapidly jump between two focus presets when shooting stills, this function can also be used to achieve smooth focus pulls when shooting video with the RF 800mm F5.6L IS USM and RF 1200mm F8L IS USM. "Both of these lenses also support ultra-low speed drive of the USM motor for movie shooting," says Hiromi. "In combination with the EOS R5, EOS R6 and later EOS R System cameras (including the EOS R5 C), the focus drive is much smoother."