Pushing lenses to their limits: How Canon engineers are redefining ultra-wide lens design

When Tatsuro Watanabe joined Canon, he announced his intention to create "the perfect lens". Looking back, he finds the remark he made during his first assignment a little embarrassing – but admits, "I secretly believe the RF 14mm F1.4L VCM just might be the one."

This pioneering ultra-wide prime lens is a feat of optical and mechanical engineering. Such a lens would have been impossible to design for a DSLR: it’s ultra-wide, ultra-fast and, given its high specification, remarkably compact.

Canon's shift to mirrorless camera designs has made it possible to develop lenses that are smaller and lighter than ever, says Tatsuro, who oversaw the optical design for the RF 14mm F1.4L VCM. "We previously offered the EF 14mm f/2.8L II USM for EOS DSLRs. Compared to this model, the RF 14mm F1.4L VCM is actually lighter in weight, yet its maximum aperture is two stops brighter.

"We developed the lens with a strong focus on its potential use in starscape photography – a genre that requires a bright, ultra-wide-angle lens with a focal length around 14mm, in order to effectively capture both the starry sky and terrestrial subjects in a single landscape photo. This allows for the shortest possible exposure time, ensuring that the stars appear as sharp, distinct points of light."

It is perhaps unsurprising that Tatsuro set out to create the best astrophotography lens, given his lifelong dedication to astronomy. Not only did he study the subject at university – where he volunteered to build a 50cm-aperture telescope during his summer holidays – he also developed equipment for the 8.2-metre Subaru Telescope, operated by the National Astronomical Observatory of Japan (NAOJ) in Hawaii.

With the RF 14mm F1.4L VCM, however, Tatsuro got to focus on more compact optics. The secret to making this stellar lens so small was Canon's RF mount, he explains. "The optical designs of RF lenses for the EOS R System have a large-diameter mount and a short back focus, which allows larger-aperture lens elements to be located closer to the imaging plane than conventional EF lenses."

Having larger rear elements positioned closer to the sensor means that light rays don't have to bend as much, making it possible to maintain image quality right across the frame – even at wide apertures. This is an especially important consideration when it comes to the unique demands of astrophotography.

"High optical performance is essential when taking any sort of astronomical photos," Tatsuro notes. "Stars are bright points of light scattered across the entire sky. Wide-angle lenses with short focal lengths will often experience types of aberration that cause image distortion at the edges of the picture. To capture stars as crisp, sharp points, the lens design must improve image quality in the peripheral areas."

It is an area in which the RF 14mm F1.4L VCM excels, delivering corner-to-corner performance that's out of this world. "The RF 14mm F1.4L VCM features a rich lens configuration including GMo aspherical lens elements, fluorite material for chromatic aberration correction, UD lenses, and BR optical elements, along with advanced coating techniques," Tatsuro says. "This is a particularly special lens which is replete with Canon’s powerful optical technologies."

The RF 14mm F1.4L VCM captures stars with remarkable sharpness, even at the edges of the frame, with its three GMo aspherical lens elements playing a key role. These specialised elements reduce sagittal coma flare – the effect that can make stars and other point light sources appear stretched or comet-shaped towards the corners. This is particularly pronounced when using wide lenses wide open – the very combination astrophotographers often seek. Canon's design team, however, has ensured that the lens delivers pinpoint stars across the entire image, even at f/1.4.

"Two of these lens elements sit at the front of the lens, with the third located in the rear focusing group," explains Tatsuro. "The frontmost GMo aspherical element has a large diameter, which makes it difficult to manufacture. By collaborating closely with our production facilities, we systematically addressed each of the manufacturing challenges, one by one."

The team also incorporated a UD lens and BR optical elements to further suppress chromatic aberration. The BR (Blue Spectrum Refractive) optical element is moulded from resin, Tatsuro explains, then sandwiched between convex and concave elements and positioned near the centre of the lens, just behind the aperture blades. "This element significantly refracts shorter wavelengths in the blue range," he says. "By doing so, it minimises chromatic aberration from pinpoint light sources near the centre of the image."

Subwavelength Structure Coating (SWC) and Air Sphere Coating (ASC) were also applied to selected surfaces to suppress flare and ghosting.

"Ultra-wide-angle lenses often feature a large, protruding front element that allows light to enter from various angles," Tatsuro says. "In some cases, integrated lens hoods and standard lens coatings may not fully prevent ghosting and flare.

"SWC is a particularly advanced technology, designed to significantly reduce reflections from light entering the lens at oblique angles. This special coating forms nanometre-scale structures on the lens surface, creating a powerful anti-reflective layer."

ASC, he adds, is especially effective against reflections from light entering at close to a vertical angle. "ASC forms a film containing silicon dioxide and air on the lens surface. By incorporating air – which has a lower refractive index than optical glass – at a specific ratio, it creates a coating with an ultra-low refractive index."

It isn't only in its optical design that the RF 14mm F1.4L VCM breaks new ground. Keeping such a high-performance prime as portable as possible was one of the team's key objectives.

"The use of a VCM (Voice Coil Motor) focus actuator – common to all the RF hybrid prime F1.4L lenses – significantly reduces the size and weight of these lenses," explains Naoki Saito, who was responsible for the electrical design of the RF 14mm F1.4L VCM.

"Previously, the only option for driving a heavy group of lens elements like the ones used in this lens was a ring-type USM. Because the diameter of a ring-type motor determines the minimum diameter of the lens itself, it is extremely difficult to design a more compact lens around this motor technology."

From a mechanical standpoint, the ring-type USM also requires a larger supporting structure, adds Nobuyuki Nagaoka, who led the mechanical design of the RF 14mm F1.4L VCM. "While this isn’t an issue for large telephoto-type lenses, it becomes a major obstacle when trying to reduce lens size at the shorter end. By contrast, a VCM provides greater design freedom, reducing the size and weight of the unit while still offering the high thrust needed to move the focal lens groups."

Despite the use of a compact VCM, the limited space available for internal structural components – known as the 'main base' – presented another challenge, Nobuyuki explains. "The most challenging part of the mechanical design was ensuring that the framework had sufficient strength, while still accommodating the actuators and flexible circuit boards," he says.

It was for this reason that Canon designed the RF 14mm F1.4L VCM purely for higher image quality, with distortion corrected electronically by the camera.

In-camera lens corrections with the latest EOS R System cameras is fast and highly effective, freeing Canon's lens designers to concentrate on optimising lenses for performance and portability.

"As a result, we achieved optical performance that is equivalent to lenses that eliminate distortion using optical methods, yet also managed to reduce size and weight," adds Tatsuro. "This lens design is only possible because Canon manufactures its own lenses and can develop them to work in tandem with the camera."

In addition to delivering outstanding performance for professional photography, the RF 14mm F1.4L VCM is an innovative hybrid lens that is designed with video capture in mind. It is equipped with a dedicated Iris Ring for smooth and silent adjustment of the aperture during filming, for example, and a circular 11-blade aperture for cinematic bokeh and highlights.

The lens designers also focused on minimising focus breathing in order to suppress distracting shifts in the field of view as the lens focuses back and forth. To achieve this, they used a different optical arrangement to the other RF hybrid prime lenses – adding one more lens element to the focal group and incorporating a GMo aspherical concave lens.

"Due to the characteristics of lens construction design, there is often a trade-off between improving optical performance and suppressing focus breathing," explains Tatsuro. "However, by carefully configuring the lens design of this model, we successfully achieved both high image quality and a reduction in focus breathing."

Adding more lenses to the focal group inevitably increases weight but, as Naoki highlights, this is where the skills of Canon's electronics and mechanical engineers come into play.

"VCMs are actuators that deliver high thrust yet operate with little noise or vibration, but it takes a high degree of ingenuity to control their operation," he says. "For instance, careful and precise tuning is needed to move heavy lens groups at high speeds and stop them abruptly, while still suppressing vibration. When recording movies, precise control is essential to limit drive noise to the absolute minimum. Canon was able to overcome these challenges by designing control algorithms and building the mechanical structures that make them possible."

The RF 14mm F1.4L VCM is a testament to the dedication, skill and experience of Canon's lens engineers. It delivers an exceptional image from a surprisingly lightweight design – quality without quantity – and reshapes expectations of what can be achieved with large-aperture, ultra-wide lenses.

"This lens was meticulously crafted with a design that balances various factors, including optical performance, but places a particular emphasis on capturing starscapes," Tatsuro says. "We sought ways to minimise various types of aberration that affect image quality, and the result is a lens that delivers sharp, precise images right to the edges of the picture.

"We aim to develop lenses that allow photographers to enter new realms of expression not previously accessible," he adds. "We seek to expand the range of shooting opportunities by making use of the latest technologies to create lenses that are smaller and lighter – lenses that deliver outstanding performance without the weight and bulk that might discourage users from carrying them. We are certain that the RF 14mm F1.4L VCM will be one such lens."