Super telephoto lenses are a wonderful tool to have for capturing vivid images of that rare, elusive subject from afar, but weight isn't exactly one of their known strengths. Canon's lens development team managed to make the recently-released EF400mm f/2.8L IS III USM and EF600mm f/4L IS III USM significantly lighter than their predecessors—not easy indeed. Here's what we learnt about this aspect from an interview with them.

 

Hallmark improvements on the flagship lenses

The EF400mm f/2.8L IS III USM and EF600mm f/4L IS III USM both boast an eye-catching list of improvements: 

1. Significant weight reduction (The EF400mm f/2.8L IS III USM weighs as much as 26% less than its predecessor; EF600mm f/4L IS III USM weighs just over 22% less)
2. High image quality
3. Heat shield coating
4. Improved operability and convenience (5 shutter speed stops of IS performance, electronic focusing ring, new accessories)
5. Superior reliability (ruggedness, durability, dust-proof & drip-proof structure, and fluorine coating)
6. High quality design

Achieving the first point required humongous effort on the part of the developers, who didn’t simply make the lens lighter but redesigned the entire lens to ensure balance. This is the focus of the interview excerpt below.
(Answers been edited for conciseness and clarity. The link to the original interview is below.)

 

1. The design vision and approach

To create a lens that would be the “pinnacle of next-generation lenses”

Nagao (Development Leader): When we first started developing the lenses, we asked ourselves, “What would the pinnacle of next-generation lenses be like?” We also listened to a lot of feedback, especially from people with critical views. This helped us to imagine an ideal lens that was not bound by the framework of existing lenses. 

Shimada (Product Planning): Our primary concept was to make the lens lightweight. The point was to see just how light we could make them while still achieving high image quality.

 

EF400mm f/2.8L IS III USM, weighing approximately 2,840g.

 

EF600mm f/4L IS III USM, weighing approximately 3,050g.

 

Shot with EF600mm f/4L IS III USM

 

2. The innovative new optical system layout

Nurturing the best idea that emerged from a high-level competition of ideas

Nagao (Development Leader): We are always developing technologies for future projects. Even before we started designing the EF400mm f/2.8L IS III USM and EF600mm f/4L IS III USM, the Optical Design team members had already been generating ideas for new, lightweight optical systems that used new glass materials. The new optical system was the product of nurturing the best idea that emerged from that high-level competition of ideas.

The optical characteristics of the new lenses

Saito (Optical Design): The EF400mm f/2.8L IS III USM is characterised by its optical system. It utilises fluorite and Super UD lenses as well as new glass materials, and features a rear consolidated optical system layout where all lenses from the second lens element onward are located toward the rear. This actually helps to prevent flare caused by light from outside the image frame, and allows more contrast detail to be depicted even when there is backlight. Having the front (first) lens element by itself also helps to reduce internal reflection.

Nakahara (Optical Design): This EF600mm f/4L IS III USM has a similar optical design. As it is difficult to reduce the first lens element in size, we aimed to shrink the diameters of the second and subsequent lens elements and also maximise the distance between the first and second lens elements. We completely revamped the entire lens configuration. The first lens element uses a new glass material that has never been used in any Canon product before.

 

Cross-section of EF400mm f/2.8L IS II USM and EF400mm f/2.8L IS III USM
The EF400mm f/2.8L IS III USM (right) has only one lens element that is different from its predecessor (left). However, the rear consolidated optical system layout enables smaller-sized lens elements to be used after the second lens element, which makes the lens lighter.

 

Cross-section of EF600mm f/4L IS II USM and EF600mm f/4L IS III USM
Just like the EF400mm f/2.8L IS III USM, the EF600mm f/4L IS III USM is also characterised by the distance between the first lens group and the subsequent lens groups concentrated at the back.

 

As much as 26% weight reduction

Hayakawa (Mechanical Design): We managed to reduce the weight of the EF400mm f/2.8L IS III USM by approximately 1,010g (26% lighter than version II). With the EF600mm f/4L IS III USM, we reduced approximately 870g (approximately 22% reduction).

 

Changes in glass materials and lens construction (EF400mm)

 

Shot with EF400mm f/2.8L IS III USM

 

3. New approaches to structural design and materials used were also necessary

Innovation made the impossible possible

Hayakawa (Mechanical Design): Further weight reduction was not a simple task. There are limits to how much we can remove from the lens barrel and exterior, which could affect [structural] strength and optical performance. We had to ensure that reliability and optical performance were not sacrificed. So we: 

1) Changed the base construction by using state-of-the-art injection molding (thixomolding) and new alloys,
2) Renewed components, such as by implementing an electronic focus ring, and
3) Utilised the latest optical adjustment mechanisms.

Switching to an electronic focusing ring allowed us to minimize the number of components required to mechanically link the focusing ring with the focusing unit (USM motor). This not only reduces the lens weight, but also has benefits for lens assembly and layout precision.  

Nagao (Development Leader): We also decided to use a new carbon reinforced magnesium alloy. For the first group lens barrel on the 400mm lens, injection molding helped us to achieve a barrel thickness of 0.8mm, 20% thinner than on previous lens models. It is lighter but still sufficiently strong. The III series lenses use an internally-produced optical adjuster that was designed for super telephoto lenses. This enables us to carry out digital adjustment on many fine details, achieving a precision that is not possible through conventional manual methods. 

 

The first group lens barrel was one of the development team’s weight reduction hotspots. It is made from carbon-reinforced magnesium alloy, which is even lighter than aluminum alloy.
 

 

The significance of a lighter super telephoto lens

On super telephoto lenses 400mm or longer, mobility and portability have always been yearned-for ideals that were technologically hard to achieve. The significant weight reductions on these two new super telephoto L lenses are a breakthrough that provides us with more freedom to shoot as we please, with the same reach and high image quality that we enjoyed on their popular predecessors. 

 

What else went into ensuring high image quality on the new lenses? What changes helped to achieve 5 shutter speed stops of image stabilisation? What was it that motivated the engineers to keep improving on design aspects all the way to the last moment? 
Download the full interview for these behind-the-scenes stories and more. 

 

The developers interviewed

(from left)
Shota Shimada: Product Planning
Yuki Nagao: Development Leader
Shinichiro Saito: Optical Design
Makoto Nakahara: Optical Design
Makoto Hayakawa: Mechanical Design

 

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