Developer Interviews

EF85mm f/1.4L IS USM

A lens infused with the full range of latest technologies,
in the words of development staff

This fourth model* of 85mm lenses is equipped with image stabilization worthy of an L lens.

Yamaguchi, Product Planning Lead

85mm is a popular focal length for portrait photography. In our current lineup, however, the EF85mm f/1.2L II USM is not affordably priced for many users, while many EF85mm f/1.8 USM users have said they want a greater bokeh effect.

During planning, we sought the opinions of wedding photographers and other professionals who would have many opportunities to use this lens. The EF85mm f/1.2L II USM enjoys a favorable impression and is highly valued for its excellent expression and bokeh, thanks to its large aperture. As a heavy lens, however, extended use can lead to fatigue, and there is some dissatisfaction with the autofocus speed. Some users told us that the addition of image stabilization would allow worry-free shooting even with a large aperture single focal length lens.

Based on these opinions, we decided to add the EF85mm f/1.4L IS USM to the lineup. We felt that it would meet the wishes of handheld photographers thanks to its balanced specifications, offering both a large f/1.4 aperture and image stabilization useful for handheld photography, and thanks to its satisfying high image quality for portraiture, as befits an L lens.

To ease handling and photography even during extended handheld shooting, we set our sights on making a product that would meet these goals:
- Lens weight of less than 1 kg
- Compact overall lens size

Wedding photography and other portrait shooting capture precious moments in life, making memories last a lifetime. Our goal was to create a lens for worry-free, beautiful preservation of important moments and memories, while letting wedding photographers and other users personally experience its ease of use.

*The fourth following the EF85mm f/1.2L USM, EF85mm f/1.8 USM, and EF85mm f/1.2L II USM

Achieving both high resolution from an aperture befitting an L lens and excellent bokeh quality right up to the edges

Iwamoto, Optical Design Lead

In 2006, Canon released the EF85mm f/1.2L II USM as the flagship lens in its 85mm focal range. The lens has gained an outstanding reputation for its superb bokeh quality. The optical design of the EF85mm f/1.4L IS USM, on the other hand, leans more toward sharp and crisp delineation while maintaining the rich expression of its older brother. We focused on and struck a balance between clarity of resolution for the center of the screen as befits an L lens, and beautiful bokeh at the edges, an ideal effect for popular portrait compositions with subjects in the center of the frame.

We proceeded with development to achieve such resolution while taking realistic weight, size, and cost into consideration.

Canon's first attempt to develop an 85mm f/1.4 lens equipped with image stabilization

Okuda, Mechanical Design Lead

When the product planning department requested development of this lens, our development department was puzzled by the high degree of difficulty in the design. In large aperture lenses with image stabilization, the arrangement of the focus and image stabilization groups becomes an important factor in determining the size and weight of the lens. Lenses with a small F-number (large aperture) characteristically have large diameter glass. To equip a lens with image stabilization, the image stabilization group, part of the optical system, must be able to move to compensate for camera shake. This makes the image stabilization group heavy. As the force needed to drive that group results in a large and heavy lens, the development team’s first impression was that developing a reasonably sized and appealingly priced 85mm f/1.4 lens with image stabilization, as requested by the product planning department, would be almost impossible.

However, while pursuing the dilemma with the optical design lead, we sensed that the goal could be met by combining advanced technologies, including rearranging the optics, using new glass materials, and layering 0.1 millimeter-scale compact mechanical components. In the end, we decided to take up the challenge and tackle this project.

Making full use of a wealth of glass materials to correct aberrations within size and weight restrictions

Focusing on optics (Iwamoto)

As mentioned earlier, all of the lens units arranged in a large aperture, mid-telephoto lens require a certain diameter or larger. As the diameter of the image stabilization unit controlling a large and heavy lens increases, so do the final product’s size and weight. Therefore, a key point in this lens’s optical design was to reduce the diameter and weight of the stabilizing optical system. In general, the front lenses are greater in diameter than the rear lenses, and the lenses near the diaphragm have the smallest diameter.

To reduce the size and weight of the stabilizing optical system in this lens, we placed the image stabilization unit, including the stabilizing 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 stabilizing optical system of this lens is similar in size and weight to that used in the EF400mm f/2.8L IS II USM super-telephoto lens. (Fig. 2)

Fig. 1
Optical arrangement

The most important features in the EF85mm f/1.4L IS USM are the correction of axial chromatic aberration and the correction of spherical aberration. Correcting the curvature of field is also necessary for high image quality, as the depth of field is very shallow. High refractive index glass and glass with anomalous dispersion characteristics are effective for correcting these aberrations.

More than half of the glass elements in this lens are made of glass materials that were not yet put into practical application during development of the EF85mm f/1.2L II USM, released in March 2006. Thanks to the characteristics of these new glass materials, we were able to both reduce the lens’s size and ensure high image quality, while also equipping it with image stabilization. An effective way to correct axial chromatic aberration is to place glass with anomalous dispersion, like a UD lens, in front of or near the diaphragm. Using high refractive index glass for the positive lens is also essential as a way to improve image plane characteristics. In this lens, axial chromatic aberration and image plane characteristics are substantially corrected through an optimal arrangement of glass having both high refractive index and anomalous dispersion characteristics, which were not yet put into practical application when the EF85mm f/1.2L II USM was developed.

By effectively arranging special glass like fluorite and UD lenses as well as new glass materials and aspherical lenses that did not exist at the time, we were able to correct each type of aberration while reducing the size of the lens.

Generally speaking, optical systems in newer lenses tend to have a more effective optical arrangement due to a wider choice of glass materials, allowing the final lenses to achieve higher image quality in a more compact body. The EF85mm f/1.4L IS USM is a good example of this.

Fig. 2
Image stabilization unit and stabilizing optical system comparison

From left: EF35mm f/2 IS USM, EF100mm f/2.8L Macro IS USM, EF85mm f/1.4L IS USM, EF400mm f/2.8L IS II USM

Optical arrangement that makes the best use of the large diameter EF mount

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. As mentioned earlier, however, since the image stabilization unit is also positioned on the mount side, components must be arranged in the following order:

(Lens front) Diaphragm - Image stabilization unit - Rear lens group (Camera side)

Since the diaphragm is located relatively far from the mount, the diameter of the rear lens group becomes larger. As large aperture lenses already have large diameter lens elements, and as lens diameter tends to increase when the lens is equipped with image stabilization, the rearward positioning of the diaphragm is important. It was difficult to configure the stabilizing optical system and the rear lens group with as few lenses as possible while maintaining optical performance.

In the end, the lens has a diameter that just fits the EF mount. If you actually hold the lens and look at the mount side, we think you will grasp the size of the lens diameter. The large diameter EF mount made this configuration possible, letting us equip the lens with image stabilization.

Fig. 3
EF85mm f/1.4L IS USM optical arrangement

Once the image stabilization unit configuration has been decided, the optimum focus group configuration and then the configuration of the entire lens can be decided. In order to reduce the size of the entire lens, both the focus group and stabilizing optical system must be lightweight and move only a small amount. For this lens, we used the following new optical arrangement, starting from the front of the lens: the front group, the focus group, the diaphragm, the stabilization optical system, and the rear group. With regard to the focus group and stabilization optical system in particular, the characteristics of the optical system change greatly depending on whether these are convex or concave lens groups. By configuring the focus group as a convex lens group and the stabilizing optical system as a concave lens group, and by placing them in adjacent positions so as to cancel each other out, the respective refractive powers can be set appropriately to achieve compactness for both the entire lens and the stabilizing optical system.

In addition, by configuring the focus group in an arrangement of convex, then concave, then convex lenses, we achieved both weight reduction and high image quality. By configuring the rear group with an aspherical lens, we reduced the number of lenses in the rear group while correcting spherical aberration, allowing us to position the diaphragm as far back as possible. By devising the basic configuration of the entire lens and the configuration of each lens group in this way, we managed to achieve our design goals.

Since the number of lenses increases with the addition of the image stabilization group, another technical problem was the suppression of 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 as the lens surfaces on the rear lens are particularly prone to ghosting, we decided to use the technique to reduce the air contact surface. While equipping the lens with image stabilization increased the number of lenses, we were able to suppress ghosting to a minimum by using ASC for the 8th lens.

Fig. 4
EF85mm f/1.2L II USM optical arrangement

Aiming for significant size reduction

Focusing on mechanics (Okuda)

Ultrasonic motors (USMs), which are the actuators that drive autofocus, offer 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 used in many EF lenses, including the EF35mm f/2 IS USM and EF85mm f/1.8 USM. We examined the design together with the optical design lead so that each component, including the optical system, would fit inside the limited USM diameter.

The EF85mm f/1.2L II USM uses the large diameter USM that is used in the EF400mm f/2.8L IS II USM super telephoto lens and other lenses.

Fig. 5
Ring USM

Right: USM used by the EF85mm f/1.4L IS USM

As mentioned earlier, although the image stabilization group of this lens is similar in size and weight to that of a super-telephoto lens, we needed to design a reasonably sized image stabilization unit, including the drive system. We reduced drive load by using an extremely low friction drive mechanism that uses multiple ceramic bearings to support movable components. This reduces the necessary drive force and contributes to a compact and lightweight image stabilization unit design.

Fig. 6
Image stabilization unit comparison
Left: Actual unit Right: Comparison units

Comparison units, from left: EF35mm f/2 IS USM, EF100mm f/2.8L Macro IS USM, EF85mm f/1.4L IS USM, EF400mm f/2.8L IS II USM

We also devised a focus drive mechanism (Fig. 7) on the front side of the image stabilization unit. As the lens has a large aperture, its depth of field is very shallow and its autofocus requires extremely high accuracy. The focus lens group was heavy, as expected, and the durability of the entire drive mechanism, including the motor that drives the group and the brake control that stops it, needed further improvement. This was true for other L lenses as well. Using highly durable ball bearings in the drive mechanism for this lens reduces load and improves resolution, while also improving accuracy. The use of ball bearings also contributes to high-speed autofocus. The heavy focus lens group must be driven by the limited power of the motor, but the focus lens can be driven quickly as the ball bearings reduce the load.

By making the shape of the mechanical components of the ring USM unit follow the outer diameter of the glass, we maximized space efficiency, which in turn contributed to size and weight reduction.

Considering the design in general, we invested a great deal of time examining the combination of mechanics and optics that would overcome the great difficulty of adding an image stabilization mechanism to an 85mm f/1.4 lens while maintaining an appealing size and weight. We brought together layered 0.1 millimeter-scale compact mechanical components into a usable form.

This lens achieves an image stabilization effect of 4 stops (85mm focal length, using an EOS-1D X Mark II; CIPA standards compliant). This is the first Canon lens with a large aperture of f/1.4 to be equipped with image stabilization. We definitely hope you will try out the high operability of this reasonably sized, image stabilization-equipped lens.

Fig. 7
Focusing unit

(ball bearings in a square)

Dedicated to reliability

Although we devised solutions to achieve a lightweight design, the lens is still close to 1 kg in weight, and the impact force from falls or other shocks will be significant. To improve reliability, we implemented a damper mechanism at the front of the lens. As is obvious by touching 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. It is characteristically difficult to distribute shock with a fixed length lens, and as this is a fixed length lens with a large aperture, we decided to use a damper. This solution is not used with all EF lenses; it is used as needed, based on lens configuration and on simulations. In addition to this lens, it is also used with the EF24-70mm f/4L IS USM, EF11-24mm f/4L USM, EF35mm f/1.4L II USM, and EF24-105mm f/4L IS II USM. Although it may be somewhat hard to understand with a zoom lens, if you push the filter holder on a single focal length lens, we believe you will understand.

In closing

In closing (Yamaguchi)

While the demands for greater pixel counts will increase in the coming years, this is a highly usable lens that will allow handheld photography with a sense of security. We can confidently recommend this outstanding lens that came to fruition with an excellent balance of resolution, image stabilization, weight, and size for a portrait lens.

When shooting indoors or in dim light, this lens allows you to capture sharp images without the need to increase the ISO setting. You can now enjoy handheld portrait shooting without fatigue, thanks to the light weight and image stabilization. We definitely hope that you will experience the unprecedented greater expression that this lens enables.