The craft behind Canon’s EF 85mm f/1.4L IS USM lens: the developers reveal all

Before starting work on the new EF 85mm f/1.4L IS USM lens, Canon’s aim was simple: use its latest technologies “without reserve” to produce a class-leading portrait lens. In a rare opportunity to hear from the lens designers themselves, we discover the depth of detail and design expertise that has gone into its making. The new lens is not just about headline-grabbing specifications, but providing photographers with an unparalleled combination of image quality and usability.

Yamaguchi, Canon's Product Planning Lead

The EF 85mm f/1.4L IS USM is the fourth 85mm lens in the Canon EF line-up, following on from the EF 85mm f/1.2L USM, EF 85mm f/1.8 USM and EF 85mm f/1.2L II USM. Yamaguchi explains why this new lens is so important, and where it fits into the Canon range.

“The 85mm lenses are a popular focal length for portrait photography,” says Yamaguchi. “With the current line-up, however, the EF 85mm f/1.2L II USM is not affordably priced for many users, while many EF 85mm f/1.8 USM users have said they want greater bokeh.”

“During the planning of the EF 85mm f/1.4L IS USM, we talked with many professional photographers, including wedding photographers, who would have many opportunities to use this lens. They told us that EF 85mm f/1.2L II USM is highly valued for its excellent expression and bokeh thanks to its large aperture. Being a heavy lens, however, it’s easy to get tired with extended use, and the autofocus speed is not necessarily satisfactory. Some said that even though it’s a large aperture single focal length lens, the addition of image stabilisation would allow worry-free shooting.”

Iwamoto, Canon's Optical Design Lead

Iwamoto explains the key optical design aims, including the high resolution, maximum aperture and excellent bokeh quality in the peripherals – but, he explains, this had to be achieved within strict size and weight limits.

“In 2006, Canon released the EF 85mm f/1.2L II USM as the flagship lens in focal range, and the lens has gained an outstanding reputation for its superb bokeh quality. The optical design of the EF 85mm f/1.4L IS USM, on the other hand, leans more towards sharp and crisp delineation, while maintaining the rich expression of its older brother.

“We focused on striking a balance between clarity of resolution for the centre of the screen that’s a feature of an L lens and the beautiful bokeh on the edges, ideal for popular portrait compositions with subjects in the centre of the frame, while taking realistic weight, size and cost into consideration.”

“All the lens units in a large aperture, mid-telephoto lenses require a certain diameter. As the diameter of the image stabilisation unit controlling a large and heavy lens increases, so does the final product’s size and weight. Therefore, an important factor in this lens’s optical design is to reduce the diameter and weight of the stabilising optical system.”

In general, the diameters of the front lenses are greater than the rear lenses, and the lenses near the diaphragm have the smallest diameter. The solution was to place the stabilising optical system on the mount side of the lens near the diaphragm. (Fig. 1)

In spite of this, a certain amount of weight is unavoidable and the stabilising optical system of this lens is similar in size and weight to that used in the EF 400mm f/2.8L IS II USM super-telephoto lens. (Fig. 2)

The new optical design was made possible by the large diameter of the EF lens mount used on Canon DSLRs.

“Due to the nature of optical design, the further the diaphragm (EMD unit) is from the rear of the optical system (mount side), the larger the rear lens diameter must be. Since the image stabilisation unit is also positioned on the mount side, components must be arranged in the following order: lens front, diaphragm, image stabilisation unit, and rear lens group.

Since the diaphragm is located relatively far from the mount in this lens (see Fig. 3 below), the diameter of the rear lens group becomes larger. And because large aperture lenses already have large diameter lens elements, and lens diameter tends to increase when equipped with image stabilisation, the rearward positioning of the diaphragm is important. It was difficult to configure the stabilising optical system and the rear lens group with as few lenses as possible while maintaining optical performance.”

The wide EF mount is one of the factors that has helped make the EF 85mm f/1.4L IS USM’s optical design possible.

The integration of a stabilising unit was not the only challenge for the optical design team. The size and weight requirements of the new lens called for a special optical design for the focusing group too (Fig. 3 above).

“In order to reduce the size of the entire lens, both the focus group and stabilising optical system must be lightweight and only move a small amount. For this lens, we used the following new optical arrangement from the front of the lens: the front group, the focus group, the diaphragm, the stabilisation optical system and the rear group.”

The stabilisation and focusing systems were not the only considerations in the optical design, however, since the new lens also had to deliver high levels of aberration control: “The most important factor for the EF 85mm f/1.4L IS USM is correction of axial chromatic aberration and spherical aberration. Correcting the curvature of field is also necessary for achieving high image quality since the depth of field is very shallow. High refractive index glass and glass with anomalous dispersion characteristics are effective for correcting these aberrations.”

It’s easy to overlook the importance of glass technologies, materials and advances. Indeed, as Iwamoto explains, more than half of the glass elements in the EF 85mm f/1.4L IS USM are made of materials that were not yet put into practical application when the EF 85mm f/1.2L II USM was released in March 2006.

Correction for axial chromatic aberration (colour fringing in defocused areas) is important in any lens designed for use at wide apertures, and Canon’s design team had the answer.

“To correct axial chromatic aberration, we placed glass with anomalous dispersion in front of or near the diaphragm. In addition, we used high refractive index glass for the positive lens, which is essential to improve image plane characteristics. In this lens, axial chromatic aberration and image plane characteristics are favourably corrected through the optimal arrangement of glass, having both high refractive index and anomalous dispersion characteristics.”

By using new glass materials and aspherical lenses that did not exist at the time the EF 85mm f/1.2L II USM was developed, the design team was able to correct each type of aberration and reduce the size of the lens at the same time.

There was one further technical challenge to overcome – ghosting: “For this lens, we used a three-element cemented lens for the rear lens. Precisely aligning the axes of the three lenses in a three-element cemented lens is difficult, but since the lens surfaces on the rear lens are particularly prone to ghosting, we decided to use it to reduce the air contact surface. And while equipping the lens with image stabilisation increased the number of lenses, we still were able to reduce ghosting to a minimum.”

Okuda, Canon's Mechanical Design Lead

Okuda talks about the mechanical challenges of including image stabilisation in the design of this new lens. It might sound like an ordinary request, but Okuda explains that adding image stabilisation while achieving the operational performance photographers would expect was not so straightforward.

“The development team’s first thought was that coming up with a reasonably sized and appealingly priced 85mm f/1.4 lens with image stabilisation, as requested by the product planning department, would be almost impossible.

“However, we had the idea that it could be possible by rearranging optics, using new glass materials and layering 0.1mm scale compact mechanical components.”

Okuda explains how the size and weight requirements affect the operation of the EF 85mm f/1.4L IS USM’s autofocus and image stabilisation systems.

“Ultrasonic motors (USMs), which are the actuators that drive autofocus, come in several choices depending on diameter size. To keep the outer diameter small enough for extended handheld shooting, we decided to use a relatively small type of USM that is in many EF lenses, including the EF 35mm f/2 IS USM and EF 85mm f/1.8 USM. Starting with the optical system, we examined the design with the optical design lead so that each component would fit in the limited USM diameter.”

The current EF 85mm f/1.2L II USM uses the large diameter USM in the EF 400mm f/2.8L IS II USM super telephoto lenses. How much smaller the USM used in the EF 85mm f/1.4L IS USM is can be seen below.

With a large aperture lens, depth of field is very shallow and autofocus requires extremely high accuracy. As you’d expect in a lens of this type, the focus lens group is also heavy and the durability of the entire drive mechanism is important.

“Using highly durable ball bearings in the drive mechanism for this lens reduces load and improves resolution and accuracy. The use of ball bearings also contributes to high-speed autofocus. The heavy focus lens group must be driven with the limited power of the motor, but the focus lens can be driven quickly since the ball bearings reduce the load.”

The image stabilisation mechanism posed a particular mechanical challenge because of the size of the stabilisation group in this lens, which is comparable in size and weight to that of a super-telephoto lens.

“We needed to design a reasonably-sized image stabilisation unit that included the drive system. For that reason, we reduced drive load by using an extremely low friction drive mechanism that relied upon multiple ceramic bearings to support movable components. This reduces the necessary drive force and contributes to a compact and lightweight image stabilisation unit design.”

The designers have achieved an image stabilisation effect of 4 stops (85mm focal length, using an EOS-1D X Mark II, which is CIPA standards compliant).

There’s a further aspect of mechanical performance that’s easily overlooked: reliability. This isn’t just the lens’s long-term durability and consistency, but its ability to withstand knocks, bangs and impacts in everyday use. Okuda explains Canon’s thinking and design solutions in maximising this reliability.

“Although we devised solutions for a lightweight design, impact force, such as when dropped, will be significant as the lens is still close to 1kg. To improve reliability, we put a damper mechanism at the front of the lens. The filter holder is shaped to retract when pushed. The damper distributes the impact when something collides with the end of the lens.”

Many users might be surprised to learn that some lenses have built-in shock-absorption components, but this is characteristic of the depth and detail that goes into the design of Canon’s L lenses.

“We decided to use a damper since this is a fixed length lens with a large aperture. This is not used for all EF lenses, but is used as needed, based on lens configuration and simulations. In addition to this lens, it is also used with the EF 24-70mm f/4L IS USM, EF 11-24mm f/4L USM, EF 35mm f/1.4L II USM and EF 24-105mm f/4L IS II USM.”

Yamaguchi’s conclusion

Yamaguchi sees the EF 85mm f/1.4L IS USM as an outstanding portrait lens, with an excellent balance of resolution, image stabilisation, weight and size.

“When shooting indoors or in dim light, this lens allows you to capture sharp images without the need increase the ISO setting. You can now enjoy handheld portrait shooting without getting tired thanks to the light weight and image stabilisation. We definitely want you to experience the greater expression this lens provides; expression that was previously not possible.”