The EF100-400mm f/4.5-5.6L IS USM, a lens that has gained widespread support among nature photographers, has undergone a renewal after about 16 years. This article is a summary of an interview with the developers of the new EF100-400mm f/4.5-5.6L IS II USM. (Report and Photos of interviewees by: Ryosuke Takahashi)

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Long-awaited Lens Packed with Canon's Know-how and Technologies

- First of all, can you elaborate on the positioning of the EF100-400mm f/4.5-5.6L IS II USM, and the target photographic subjects and scenes?

Yokota The EF100-400mm f/4.5-5.6L IS USM, the predecessor model that was released in November 1998, has won strong support from photographers ranging from advanced amateurs to professionals. It has been about 16 years since its launch, and we have received many requests asking for its renewal. The positioning of the new model remains unchanged from its predecessor, and thus boasts excellent performance in nature photography. Not only so, it can also be employed for a wide range of scenes from airplanes to motor sports and landscapes. Since we decided to revamp the model, we wanted to deliver a lens with performance that meets the expectations of our users.

- What was the reason for not renewing the model in the past 16 years?

Yokota We needed time to assess the lens performance in order to satisfy our users' needs. Enhancement of the optical performance is one of the considerations that must be included in the renewal, and we spent much time examining how we could do so without increasing the size and weight of the lens, which would make the lens less user-friendly. The EF100-400mm f/4.5-5.6L IS II USM is packed with know-how and technologies that we have developed for our super telephoto lens series as well as lenses such as the EF70-200mm f/2.8L IS II USM.

- Can you give more specific details about what improvements users are expecting to see from the previous model?

Yokota Users' requests have been incorporated as a part of the lens features, which can be largely classified into five aspects. The first is enhancement in the resolving power. We aimed at delivering a high image quality that befits the "L-lens" designation to meet the needs of the digital environment today. Secondly, we have stepped up measures against backlight with the adoption of a new "Air Sphere Coating". There have been voices from users asking us to address the backlight issue, but at the same time it was also a part of our proposal. The third point is a reduction in the closest focusing distance from 1.8m to 0.98m. The fourth is maintaining the same lens size. Users who travel overseas wanted us to "maintain the lens in a size that can be easily stored into the camera bag and carried onto the plane", so we kept in mind to make only minimal changes to the lens size while maximising the optical performance as much as possible. Lastly, we tried to boost the operability by changing the zoom mechanism from the conventional linear extension zoom into a rotating zoom design.

- Can you explain in greater detail the reason for switching from the linear extension zoom to the rotating zoom mechanism?

Yokota Some users prefer the linear extension zoom design, but we decided to adopt the rotating zoom this time after weighing the advantages and disadvantages and taking the voices of advanced amateurs and professionals around the world into consideration. The primary reason is that the rotating zoom mechanism is the mainstream among zoom lenses today, so we wanted the operation system of this lens to be consistent with that trend. Also, another advantage of rotating zoom is that it is easier to adjust to the intended focal length.

- The focal length can be adjusted speedily with the linear extension zoom. What efforts were made to prevent the ease-of-use of the lens from being compromised with the switch to the rotating zoom design?

Nagao For telephoto zoom lenses of this class, the physical extension of the lens elements in the first lens group becomes larger. Lens extension is performed by the cam mechanism. In order to extend further while keeping the rotating angle constant, it is generally necessary to increase the angle of inclination of the cam. However, doing so significantly adds to the load of the zoom operation. So, we adopted a mechanism that enables more efficient extension while maintaining a sufficient level of operability.

- In other words, if we liken cam to a spiral staircase, a larger angle of inclination would mean a steeper slope, am I right?

Nagao Yes, exactly. To give a brief explanation of the mechanism of this lens, it is built in with a deceleration mechanism that makes use of cam and roller. In general, the inner side of a zoom ring comes with a cam, but for this lens, cams are found in both the cam cylinder on the inner side of the lens as well as the cam cylinder that is further within. In other words, multiple layers of cams are combined to achieve a longer lens movement distance without increasing the zoom operation force.

- What has changed with the introduction of a multi-layer structure?

Nagao We are able to reduce the angle of inclination for each cam. For the linear extension zoom design, the first group lens tube is pulled out manually with our hand. In the case of a rotating zoom mechanism, the first group lens tube is moved along the groove of the cam on the inside of the zoom ring. For this lens, the cam cylinder within is designed to rotate out following the movement of the first group lens tube, which in turn moves all members of the optical system required for the focal length displacement.

- Is this the first time for such a mechanism to be adopted?

Nagao We do not have something that is completely identical, but a similar technology is employed on the EF70-300mm f/4-5.6L IS USM. This technology is suited for telephoto zooms, as the torque during operation can be kept at almost a constant level.

- I see. So the zoom operation does not slow down suddenly during the process.

Nagao Yes.

Control Ring with Enhanced Operability

- The EF100-400mm f/4.5-5.6L IS II USM comes with a control ring for adjusting the torque during zooming. How do we make good use of this ring?

Nagao We believe that operability of the zoom when taking a photo is an important consideration to the photographer. You can turn this control ring to increase or reduce the zoom torque. More specifically, you can prevent changes in the angle of view due to impact by setting to a lighter torque when the lens is positioned horizontally, and to a heavier torque when the lens is directed upward. Also, while you are carrying the lens, setting to a heavier torque helps to prevent the lens from extending out under its own weight.

- The previous model is also equipped with a similar system. Are they identical in structure?

Nagao No. If you take a close look at the ring between the zoom ring and the control ring, you will realise that it moves forward or backward when the control ring is turned. The elastic material inside pushes against the zoom ring when this ring moves.

- So the movement is something like a car clutch?

Nagao Yes. It is similar to the idea of changing the pushing strength of the friction clutch. For the predecessor model, the torque tightens in the "direction of the diameter" when the ring is turned, so the structure is fundamentally different from this model. We are unable to disclose the type of elastic material used, but you can perceive it as controlling the torque using multiple components such as springs.

Improvements Made to the Image Quality

- How has the resolving power improved compared to the previous model, and in what scenes can this change be felt?

Sugita In a nutshell, "all". We have raised the MTF throughout the entire zoom range. As a telephoto zoom lens, we are particularly confident with the resolving power around the centre of the image at the telephoto end, as well as at the area around the main subject. The resolving power at the peripheral areas has also improved considerably compared to its predecessor.

MTF Curve

Spatial Frequency	Max. Aperture		f/8
	S	M	S	M
10 lines/mm
30 lines/mm

S: Sagittal (Radial) Direction
M: Meridional (Concentric Circle) Direction

Compared to its predecessor, performance has improved both at the maximum aperture as well as at f/8. In particular, the area around the centre of the image marks a value of at least 0.8 when the aperture is fully open, indicating that it is capable of expressing both thick and thin lines of the subject clearly.

- For telephoto zoom lenses, the key to enhancing image quality lies in the control of chromatic aberrations. Specifically, which aspects of the optical design did you focus on?

Sugita I believe the most important factor is the use of a fluorite and a UD lens element to minimise chromatic aberrations of magnification and axial chromatic aberrations. Not only so, instead of a total of 17 elements in the lens construction of the previous model, 21 elements have been employed and arranged optimally to enhance the performance.

- The use of fluorite and UD lens elements has been a standard technology on Canon's telephoto zoom lenses. In what areas have improvements been made to the optical performance?

Sugita I can't provide you with the details, but I can say that it is largely attributable to the design and manufacturing technologies that Canon has developed and accumulated over the past 16 years. With the increase from 17 to 21 elements, aberration correction is made easier, and this in turn has helped to enhance the performance. However, simply increasing the number of lens elements would lead to larger fluctuations in the precision error due to the use of more components. To prevent this from happening, we have made modifications to the design and manufacturing technology so that the 21 elements can be put to full use.

- I see. So the high image quality is a result of minimising the precision error to achieve an advanced lens design in total, and not because of some silver bullet.

Sugita Yes, indeed.

- The image quality is equivalent to that of the EF70-200mm f/2.8L IS II USM. Despite the large zoom ratio, why is it possible to raise the image quality to this level?

Sugita This has to do with the principles. Performance of a telephoto lens hinges on the length of the lens with respect to the focal length. As the EF70-200mm f/2.8L IS II USM adopts an inner zooming system, the lens length remains constant throughout the entire zoom range. Meanwhile, the lens of the EF100-400mm f/4.5-5.6L IS II USM extends out on the telephoto side, which helps to boost the fundamental ability of the optical system. However, it is also a fact that the lens tube tends to rattle for telephoto lenses with a front surface that extends out. We worked together with the mechanical design team to minimise rattling on this lens, and this contributed to the enhanced image quality.