5 Key Elements to Developing the RF10-20mm f/4L IS STM
The RF10-20mm f/4L IS STM is a groundbreaking lens on many levels: Not only is it Canon’s widest zoom lens to date, it is also the world’s widest constant aperture, ultra-wide-angle zoom lens for full-frame cameras. In addition, at just 570g, it is half the weight of the EF11-24mm f/4L USM, its EF mount counterpart. From an interview with the lens developers, we sum up the lens development story into five keywords.
The RF10-20mm f/4L IS STM lens developers are (from left):
- Akihiko Masuki, Development Leader/Mechanical Design
- Masato Shiono, Mechanical Design
- Fuya Mizuochi, Electrical Design
- Makoto Nakahara, Optical Design
1. EF11-24mm f/4L USM
The role model that set high standards
Well-acclaimed for its unique perspective and stellar image quality, the EF11-24mm f/4L USM was the benchmark the developers set out to surpass. The RF10-20mm f/4L IS STM’s development concept imagined a lens that was:
- At least just as wide
- Compact and lightweight for comfortable handheld shooting
- Capable of equivalent, if not better image quality than the EF11-24mm f/4L USM
The emphasis on compactness and improved portability addressed feedback from many existing EF11-24mm f/4L USM users about the lens’ weight, which is approximately 1180g.
“I thought it was our mission as developers to reduce that burden [from the weight of the lens]”
- Akihiko Masuki, Development Leader, Mechanical Design
The developers’ mission was accomplished, as the RF10-20mm f/4L IS STM’s specifications show.
2. Front aspherical lens
The key to high image quality at 10mm
“The development and manufacturing teams worked together to conduct many simulations and achieve the ideal front element shape that could realise high performance and be mass-produced with high precision.”
- Makoto Nakahara, Optical Design
10mm achieves coverage and dynamic perspectives not possible at 14mm or 15mm. It enables users to capture both distant and nearby subjects in one frame. However, the wider the angle of coverage, the more difficult it is to correct lens aberrations and achieve outstanding image quality.
The key to achieving both the image quality and size reduction goals on the RF10-20mm f/4L IS STM lay in the front lens element, which needed to be of a very specific material and shape:
- Large diameter
- High curvature
- Double-sided aspherical lens
The larger an aspherical lens, the harder it is to process with high precision. A high curvature also makes it difficult to apply special lens coatings such as SWC (Subwavelength Structure Coating), which suppresses ghosting and flaring, evenly. These challenges were overcome with the help of simulation technology.
A: UD (Ultra-low dispersion) lens
B: Glass molded aspherical lens
C: UD Aspherical lens
The glass-molded large diameter high curvature aspherical front element greatly contributes to the 10mm wide end, high image quality, and compact size and weight.
Know this: Optimising image quality over the entire zoom range was challenging too, especially since the EF11-24mm f/4L USM had set such high standards. This was also made possible by advancements in the inhouse-developed optical design simulation technology.
3. Lightweight and robust
Teamwork and simulation technology
“By checking the details in the simulation while proceeding with the design, we have been able to maintain the same robustness as conventional L lenses while reducing the number of metal parts.”
- Masato Shiono, Mechanical Design
The RF10-20mm f/4L IS STM was designed to be the same size as the EF14-35mm f/4L IS USM. This was achieved through the combined efforts of the optical design, mechanical, and manufacturing divisions to optimise materials, parts, and lens placement and configuration. As with optical design, simulation technology helped the mechanical engineers devise innovative ways to maintain robustness.
The grooves in this cam cylinder that supports two lens groups help to disperse impact and ensure the durability and reliability expected of an L lens.
4. Peripheral Coordinated Control
4. Peripheral Coordinated Control
More about how the system works
EOS R5 + RF10-20mm f/4L IS STM @ 10mm, f/11, 0.5 sec, ISO 100
Contrary to popular belief, ultra-wide-angle lenses benefit from image stabilisation. They are particularly prone to peripheral blurring because of the ultra-wide-angle perspective. This is easier to correct by moving an in-lens IS unit, and Coordinated Control with the camera’s In-Body IS helps to correct it further.
“While it is possible to design the lens IS to compensate for camera shake across the entire frame at a specific focal length or focus distance, it is difficult to perform such compensation all the way to the peripheries over a range of focal lengths or subject distances. Peripheral Coordinated Control leverages on the coordination between Optical IS and In-Body IS to perform enhanced correction of peripheral blurring.”
- Fuya Mizuochi, Electrical Design
5. STM
Why does STM work best on this lens?
Left: The RF10-20mm f/4L IS STM’s STM
Right: The focusing lens with the STM
The RF10-20mm f/4L IS STM is the first L lens to utilise an STM (Stepping Motor) for AF drive. As its focusing lens is small and it is designed with a short stroke, the lens developers decided that an STM would perform just as well as a USM (Ultrasonic Motor), which presents a stronger benefit for long stroke lenses. The smaller STM improves the layout freedom of the IS lens group, allowing it to be placed nearer the image sensor where it can reduce peripheral blur more easily.
The STM on the RF10-20mm f/4L IS STM is equipped with a position sensor that helps shorten the motor startup time. This helps to achieve faster focusing and overall performance comparable to a USM.
Conclusion: A new ultra-wide-angle world
Conclusion: A new ultra-wide-angle world
Every RF lens is developed with the intent to provide value that the EF lenses cannot. The 10mm wide end of the RF10-20mm f/4L IS STM captures a unique view that gives a vastly different perspective of the scenery in front of us, and its compact size and portability improve its usability. The lens developers unanimously expressed their hope that users will experience the stunning view of this lens for themselves, and discover its power as a tool for expression.
From left:
- Makoto Nakahara, Optical Design
- Masato Shiono, Mechanical Design
- Akihiko Masuki, Development Leader/Mechanical Design
- Fuya Mizuochi, Electrical Design
Learn the development story behind the EF11-24mm f/4L USM in:
[Part 1] The Unprecedented Ultra-wide Angle World of an 11mm Lens
