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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Minimum focusing distance reduced to half (0.98 m) that of previous lenses

- The minimum shooting distance has been reduced to about half that of previous lenses. What are the background and secrets behind the technology that enabled such a reduction?

Sugita In order to shorten the minimum focusing distance, you either move the focus lens group all the way or increase its power. The former allows close-up focusing by letting you gain room for movement while the latter allows close-up focusing with even just a little movement. Both methods were employed this time. In particular, when the power of the focus lens is increased, the focus changes by a lot even if the lens is moved just a little. Therefore it is very important that the lens is actuated without any rattling. In addition, the movement of the focus lens is also closely related to AF accuracy. Movement has to be extremely quick, but at the same time high precision control is required to stop the lens at precisely the spot that you want. This issue has been overcome by collaborating with mechanical designers to employ a mechanism that does not rattle. If it is just a theoretical optical design, we could, to a certain extent, have come up with a similar design by extending conventional floating technology. To be able to actuate it accuracy, however, finding a balance between the optical design and the mechanical design becomes a major point in trying to achieve a minimum shooting distance of just 0.98 m.

Nagao I will elaborate from a mechanical design point of view. This time, the key issue was how much we could accommodate the optical designers’ request to increase the amount of movement of the high sensitivity focus lens. Ultimately, we were able to fulfil the conditions required to validate the mechanical design with the design specifications this time. Although the focus lens adopts a configuration that is driven by a cam mechanism like that in a zoom lens, this depended on whether a cam mechanism could be implemented in the first place. The next issue was whether the focus lens could be driven precisely along the cam tracks. We resolved this issue by introducing an “Anti-Rattle Mechanism” so that the focus lens could be driven without any rattle while maintaining its accuracy. Next, we had to keep the torque required to drive the lens within a range that could be moved by an actuator. Although the torque could be reduced if the rotation angle of the focus lens was increased, there were a few problems doing so. For example, it was difficult to position the cam within the limited space and keep the mechanical structure including the anti-rattling mechanism. We also thought of making the lens tube thinner but doing so meant that the required rigidity would be lost. I think our key achievement was in coming up with a mechanical design that moved just as the optical designers desired while allowing everything to be located within a narrow space.

- Wouldn’t aberrations increase if the minimum focusing distance is decreased? What kind of technology is employed to suppress this?

Sugita We overcame this by optimizing the overall arrangement of the lens while also allowing a part of the lens to float.

- Even if that is the case, a 50% reduction in the minimum shooting distance is no mean feat……

Sugita Yes, that’s true. The focus lens is designed to move along the combined cam tracks of both the focus and zoom lenses. In other words, an enormous part of this accomplishment is in successfully creating the desired relationship by combining these cams in a complex manner.

- Should this area be known as craftmanship that is unique to the manufacturing of long lenses?

Sugita No. This technique is also used in standard zoom lenses but as the glass in the massive front element moves in this lens, this area is packed with know-how due to the need to balance the changes in the zoom torque resulting from this and the relationship of the movement speed with other lens groups.

New coating technology to suppress flaring and ghosting

In a normal multilayer film coating, an “interface” is created in the boundary with air, resulting in a fixed amount of reflection. However, in the new technology Air Sphere Coating, the formation of an interface is moderated by deploying air spheres on the coating surface to guide the light efficiently. The coating underneath the top layer is the same vapour deposited conventional multilayer film coating.

- The next question is on ASC (Air Sphere Coating). First of all, what kind of technology is ASC and how does it differ from a conventional lens coating?

Sugita ASC is a technology that is going to be incorporated starting from this lens onwards and stands for “air sphere coating”. In conventional coating, multiple film layers are coated using a method called vapour deposition. ASC is also basically the same but it has a characteristic outermost layer that is in contact with the air. The refractive index of this layer is substantially lower compared to normal vapour deposited film. Reflection occurs at the interface of a medium with a different refractive index and the larger the difference in the refractive index of the medium, the larger the reflectance. There is a limit to how much reflection can be suppressed by ordinary vapour-deposited materials at the air interface since a certain level of refractive index exists even in a material with the lowest refractive index. In contrast, vapour deposited film lens coatings have improved greatly with the creation of an “ultra-low refractive index layer”. This coating technology is known as ASC.

- If you look at the cross-sectional view, it has a honeycomb structure that looks almost like styrofoam.

Sugita This is because the air spheres have been arranged in a systematic manner. This technology makes the medium look as if it is a low density material by lining up nano-level air spheres smaller than the wavelength of light on the surface. In other words, by interweaving air on the surface of the coating, the material appears as if it has a low refractive index to light.

- Are there any differences in the surface material from conventional materials?

Sugita No. The material is the same as that of normal vapour-deposited films.

- SWC is construed to be as a technology that takes in light while changing the density at the interface. Is this the same for ASC or does it have a different working principle?

Sugita Even though it is the same in the sense that it changes the apparent refractive index with a structure that is finer than the wavelength of light, the way it is made is completely different. SWC forms a wedge-shaped film that has properties exceedingly close to those of air as there is very little film material and lots of air on the sharp areas of the outermost surface. As you go deeper in, the density of the structure increases. The technology makes use of the theory that reflection occurs in areas where the refractive index changes, so if the refractive index changes gradually, reflection will then occur less easily as well.

- Could you explain the principles of ASC based on that?

Sugita Sure. Unlike SWC in which the density changes gradually, in the case of ASC, just like a normal vapour-deposited film, a method to weaken the reflection at the air interface using interference is employed. However, by using a layer with an ultra-low refractive index in the outermost surface at the interface with air, the reflection, which is the largest at the air interface, is reduced. The concept is based on weakening and cancelling light reflection with multiple layers of coating piled up below.

- If you look at the lens configuration, ASC is carried out on one of the lenses in the 5th lens group. Is there a special reason for doing so?

Sugita No matter what you do, ghosting and flaring will always appear in lenses. Of course, an optical design has been implemented to avoid this but if too much attention is placed on preventing ghosting and flaring in the design, the key optical performance may be affected. Therefore, we have resolved these issues by carrying out ASC on specific surfaces while maintaining the optical balance so as to draw out the lens performance this time.

Improvements in the IS mechanism

- The IS (image stabilizer) has been improved with the introduction of “Mode 3″ etc. How has the IS mechanism changed in the last 16 years?

Nagao The first thing that can be pointed out is the improvement in the structure to reduce frictional resistance where possible when the optical compensation system is in operation. The IS in the previous model employed a structure to slide the guiding axis so as to guide the movements of the optical compensation system. In the latest lenses, including this model, a guiding structure that rolls a ball around is adopted. By improving the mechanical structure and control to reduce loss during actuation, mechanical movements become smoother in response to the compensation signal. This allows us to greatly improve the compensation effect for camera shake.

- What are the major differences with its predecessor in terms of the durability of the lens?

Nagao The biggest difference is in the internal frame structure. The thickness of the parts in the frame structure supporting the lens from the mount to the tip has been increased. Simply raising the thickness alone may increase the strength but as the weight also increases, the thickness at certain parts was reduced as much as possible while maintaining the integrity of the structure. Changes were selectively made, with the thickness cut in unnecessary areas while areas that had to be strengthened were reinforced.

- Compared to the previous model, is the new model better in terms of the anti-dust and anti-spray performance?

Nagao The basic anti-dust and anti-spray structure is based on models from the EF70-300mm f/4-5.6L IS USM generation. Areas that were specifically reinforced include the joints between the parts and the sealing in movement areas.

- I see that areas which are exposed when zooming have been made more air-tight.

Nagao Yes, exactly. Since there is a large amount of area exposed because of the zoom ratio, the large change in volume at both wide-angle and telephoto is a feature of the lens. As a result, an "air drain" section has been installed which lets air but not water and dust pass through the lens, with the air being sucked in and dispelled by the zooming. Of course, this is installed not only in areas visible from the outside but in all internal areas of the lens as well.

Tripod mount structure changed to reflect user feedback

- The tripod mount has been redesigned. What are the new features and how user-friendly is it?

Nagao The mount itself has been redesigned to reflect what users want. Compared to the previous model, the lateral and longitudinal positions can be switched more easily. As the tripod mount can be removed easily, the overall volume of the lens can be reduced when carrying it by hand.

- I see that the main body of the tripod mount is compact. But wouldn’t it be better if it were bigger?

Nagao This is not a problem as the structure is completely different from that of the previous model. Of course, there are some areas which have been made thicker to increase the rigidity of the tripod mount. For this product, we studied the shape carefully to see if there were any areas which we could reduce in size while maintaining sufficient rigidity so as to make it lighter. In addition, there is a sense of continuity in the design of the ring and lens tube, so you can carry it around looking smart. Furthermore, by making the ring non-detachable, the internal structure can be designed more reasonably, allowing the lens to move more smoothly.

- I see that a window has been opened in the lens hood for operating the PL filter etc.

Nagao This directly reflects user feedback. A prescribed length is always required for the hood of a telephoto zoom lens in this class to obtain an adequate shading effect. In the past, when shooting with the hood fitted, this would result in you having to put your hand in to turn the PL filter from the front of the hood after fixing the composition, which may cause the focusing distance to change or the tripod to move. Therefore, in order to improve the operability this time, we have installed a new window for adjusting the PL filter.

- Before we end our interview, is there anything else you would like to tell us?

Yokota If I may quote the words of a company designer, this design places a lot of emphasis on the area of ergonomic function and performance, so be sure to check it out for yourself.

Nagao This lens is an amalgamation of 16 years’ worth of effort and ideas. You should really try it out for yourself.

Sugita I think we have achieved the best in optical performance. This is one product that can fully draw out the performance of a multi-pixel camera. It will be great if customers can get to enjoy the thrill of shooting at a minimum focusing distance of less than a metre with a 400-mm lens.

- Thank you for your time today.