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[Part 2] The challenge of EF lenses – EF200-400mm f/4L IS USM Extender 1.4x

In Part 2 of the story behind development of EF200-400mm f/4L IS USM Extender 1.4x, we dive into the massive test procedure this lens went through before it was unveiled before our eyes. Repetitive trial is the key to the high image quality and durability of this epoch making lens.

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First Chapter

Challenging development

Lens made of human sensibility

The Optics R&D Center in Utsunomiya was put in charge of developing the EF200-400mm f/4L IS USM Extender 1.4x, an ultra telephoto zoom lens with a built-in extender offering super-high image quality rivaling single focal length lenses.

Lens development starts with optical design. Members of the optical design section rolled up their sleeves to face up to "a tough, but worthwhile challenge." Correction of chromatic aberration is a particularly big hurdle in designing optical systems for telephoto lenses. With the new lens, it was necessary not only to clamp down thoroughly on chromatic aberration, but also to minimize fluctuation in various other aberrations caused by zooming.

This was easier said than done. Inserting a 1.4x extender also increases various aberrations by a factor of 1.4, meaning that high image quality for just the 200-400mm range would be insufficient. The targeted image quality also had to be achieved at the maximum focal distance of 560mm.

Fluorite lenses and UD lenses are often used for reducing chromatic aberration. Placement of those special lenses would be the key to minimizing fluctuation in the chromatic aberration level throughout the entire zoom range. Optical system designers worked with an optical CAD (Computer Aided Design) application day after day to determine the ideal lens combination and placement.

The optical CAD system is a computerized optical design assistance application, featuring simulation software and various other technological arsenals developed by Canon. It helps designers realize targeted optical performance precisely and efficiently.

But ultimately, optical CAD is nothing but a tool that enables designers to mold lenses that reflect their intentions. This is apparent from the fact that the best solution simulated by software may not necessarily produce the best results evaluated by human eyes.

Natural fluorite, artificial fluorite crystals and fluorite lenses

Solving such conundrums as how the lenses would best be combined, and what solution would be most ideal for a particular lens, requires more than what computers and software are capable of. It requires years of know-how at Canon as well as optical designers' sensibility and judgment.

Designers pursued the best combination and placement by utilizing optical CAD tirelessly. It was a continuing process of trial and error. But designing the world's first ultra telephoto zoom lens with a built-in extender was no easy task. Their challenge continued.

33 lenses in 24 groups - the highest number in the EF lens series

From the start of development, it was decided that the built-in extender would be operated manually, because no actuator can beat human hands in terms of speed and response. Using a manual extender also made sense in terms of reliability and stability.

One of the major issues at the optical design stage was to determine where the extender should be placed with consideration toward the optimal minimization of aberrations. Normally, extenders would increase the overall length of the lens, boosting the distance of the optical axis from the lens to the focal plane. But a built-in extender had to increase the focal length range by utilizing the lens' refractive power, so strengthening the high power refraction index of its lens groups could have amplified the resulting aberrations.

Optical system

Extender in 1x setting (focal length: 200-400mm)

Extender in 1.4x setting (focal length: 280-560mm)

Red line: indicates IS unit

Optical designers thought the best placement for the extender lens group would be naturally determined if the new lens was to switch between the focal ranges of 200-400mm and 280mm-560mm by swapping lens groups at the location where the impact of aberrations is minimal.

Designers came up with an extender that consisted of eight lens elements in four groups. This optimized optical system was selected after its designers paid attention to the issue of aberrations. But it turned out that the extender lens group was too large and interfered with the optical system for the in-lens image stabilizer. However, the designers persisted with the placement of the extender lens group, imploring mechanical designers to develop a thinner image stabilizer unit.

When the composition of lenses and lens groups was finalized, the EF200-400mm f/4L IS USM Extender 1.4x was given 25 elements in 20 groups for normal use. Once the extender was inserted, however, the number jumped to 33 elements in 24 groups - the greatest number of elements and groups in the history of EF lenses.

Artisan skills made the large lens group mechanically possible

With optical design completed, the development process moved to the mechanical design phase, where the biggest challenge was how to move a large, second lens group with high precision.

If you turn the zoom ring while looking into the EF200-400mm f/4L IS USM Extender 1.4x through the outermost lens, you can see a large glass mass move forward and backward over a considerable distance. This mass is the second lens group - the optical key to the zoom ratio and image quality.

The new lens' second group became extraordinarily heavy, with its total weight in glass exceeding 200g/7 oz. No previous EF lens had been made to move such a heavy glass mass smoothly.

To overcome the challenge, the zoom-cam lens barrel that moves the lenses and the frame that supports the lens barrel had to be stronger. Mechanical designers examined various materials, shapes and surface finishes for the parts. There was no design standard available for dealing with a 200g-plus glass mass. So, the designers worked out rough figures based on their experience and input the numbers in their 3D CAD system. Just concentrating on achieving sufficient strength for the section around the moving lens group would likely result in a large, heavy, excessively strong lens. To avoid such a result, it was important to strike the right balance in every respect, including the placement of the actuator and the diffusion of stress if the lens were to suffer the shock of impact caused by inadvertent dropping.

The number of parts increased to more than double that of a conventional ultra telephoto single focal length lens. Designers drew many parts with new shapes they had never seen before on the 3D CAD screen. They believed they would come closer to realizing a new, highly reliable and easy-to-use lens if they combined those uniquely shaped parts.

The complicated design of the new lens includes roughly 900 parts, which sets a record for the EF lens series. Because the scale of the design was so huge, the designers had to repeat high-speed simulations again and again. "The more we worked, the higher our hurdle became as we realized how difficult the project was," one mechanical designer recalled.

Mechanical designers' experience and knowledge filled gaps that simulations could not answer for. Based on numerous past examples of lens development, mechanical designers determined how thick individual parts should be, distances between the parts and countless other parameters. Here, mechanical design was carried out with artisanal craftsmanship.

Enormous time spent on prototyping and tests

The only way to verify the adequacy of a design is to conduct tests and analyze the results. Canon has developed a number of test criteria and evaluation standards for conditions related to the environment, shock and vibration resistance, durability and other areas.

Team members in charge of quality evaluation approached the new lens with extra caution. Due to the unprecedented number of its lens units and its elaborate mechanical structure, the lens could have presented problems.

This concern prompted quality evaluation personnel to decide that they would repeatedly carry out rigorous reliability tests using the toughest possible working environments for professional photographers.

For example, the development team spent days switching the extender operation lever on and off tens of thousands of times in order to determine the operational limit of the extender insertion-extraction mechanism.

To test the lens' resistance to the shock of falling, prototype lenses were dropped while they were still attached to cameras. In a testing room, an awful sound that photographers dread occurred repeatedly as prototype lenses and cameras were dropped on the floor.

Extender switch lever

Prototypes that passed these tough tests then were subjected to a series of image quality evaluation tests. To ensure high image quality across the entire zoom range, the image resolution, contrast and fluctuation in aberrations were checked as the focal length was changed in small steps. Then, the whole process was repeated again with the extender inserted. Creating a single prototype of the new lens required enormous time and effort equivalent to what would normally be required to develop several conventional zoom lenses. Then, more than a month was spent on testing the prototype.

Mechanical designers gathered the test results for reanalysis and redesign. This led to the creation of another new prototype, which was also subjected to tests. Time flew by throughout this repeating cycle as dozens of prototypes were developed.

Intelligence brought to the new lens by developers

EF lenses are composite products that include optical, mechanical and electronic components. For this reason, the design of electronic control circuits for a new lens starts in the early stages of product development.

Each EF lens stores a high volume of data in its onboard ROM. The ROM was given plenty of extra capacity in anticipation of future system advancements.

But with the EF200-400mm f/4L IS USM Extender 1.4x, lens data generated by the optical design team was too large to be stored entirely in the ROM. Compared to a zoom lens that is incompatible with extenders, the new lens has twice the volume of data simply because it has a built-in extender. Data used when the EXTENDER EF1.4X III is attached boosts the data volume by two-fold. Attaching the EXTENDER EF2X III again doubles the data volume. So, a simple calculation suggests that the new lens has to store roughly eight times more data than a conventional zoom lens that does not support extenders.

ROMs for EF lenses are custom-designed, so it is not rational to simply expand the data storage capacity for several reasons. For example, adding capacity to the ROM slows down the data communication speed between systems. So, people designing the electronic control circuit attempted to compress the data.

At the same time, they embarked on a new challenge - to upgrade the data communication system between camera and lens (EOS system) that supports a lens with a built-in extender.

One of the key product philosophies of the EOS system is that each product in the series should function accurately, even when a body and a lens that are from different eras or that employ different technological specifications are combined. This means that Canon has an obligation to fulfill its promise and make new mechanisms, such as a lens with a built-in extender, accessible to every EOS user.

The promise holds true even for old film-based EOS cameras. For this reason, the new lens was tested thoroughly with all EOS camera models. The patient efforts required in conducting these tests made it possible to incorporate mechanisms in the lens to make it compatible with all EOS cameras. Conducting the tasks may not have been exciting, but the lens could not have seen the light of day without this testing process.

The evolution of the EOS Digital series presented another challenge in designing the electronic control circuit for the new lens. The electronic control program for the EOS system is written in consideration of AF system interaction with EOS cameras. Because AF system of the EOS-1D X was more advanced than that of the EOS-1D Mark IV, the data required from the lens was different. With the data to make the lens compatible with the new EOS-1D X added, designers compressed program codes further. After all the work was done, the massive volume of data could be fully stored in the ROM.

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