In 2014, Canon has put in much effort to enhance the lineup of its interchangeable lenses, which includes the successive launch of two wide-angle lenses, the EF16-35mm f/4L IS USM and the EF-S10-18mm f/4.5-5.6 IS STM. In this article, I will go to the core of the technologies found on the latest lenses, which have a different focal length and maximum aperture value from other existing lenses.(Reported by: Junichi Date)

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Photos (left to right)
Shota Shimada: ICP Group 1, Image Communication Products Operations, CANON INC./ Kunihiko Sasaki: ICP Development Center 1, Image Communication Products Operations, CANON INC./ Shunji Iwamoto: ICP Development Center 1, Image Communication Products Operations, CANON INC./ Makoto Nakahara: ICP Development Center 1,Image Communication Products Operations, CANON INC./ Masayasu Mizushima: Camera Development Center/ Masami Sugimori, ICP Development Center 1, Image Communication Products Operations, CANON INC.

Peripheral Image Quality Improved With 2 Aspherical Lens Elements

― First of all, can you describe the background that led to the release of the EF16-35mm f/4L IS USM?

Shimada It has been more than a decade since the EF17-40mm f/4L USM was released. During this period, the pixel count of cameras has increased significantly, and there are growing calls for lenses with a higher level of reproduction power. In particular, severe feedback has been received from Canon's customers regarding astigmatism that is found at the peripheral areas of the image. In response to these voices, we have strived to achieve a high level of optical performance that would meet the demand of users ranging from advanced amateurs to professionals when we developed the EF16-35mm f/4L IS USM, with emphasis placed on enhancing the peripheral image quality. Also, it is the first ultra wide-angle EF lens to be equipped with the IS (Image Stabilizer) feature. The aim of this feature is to help broaden the scope of expression, such as by enabling handheld shooting without the use of a tripod for dimly-lit scenes indoors, in the morning, or at dusk.

― One of the characteristics of the EF16-35mm f/4L IS USM is the "clear image quality up to the four corners of the image". What are the differences compared to other existing ultra wide-angle lenses? When I compare it with the lens construction of the EF16-35mm f/2.8L II USM, they do not seem very different except for the addition of the IS feature to the optical system.

Iwamoto Both the EF16-35mm f/2.8L II USM and the EF16-35mm f/4L IS USM employ two aspherical lens elements in the first group, but those used for the EF16-35mm f/4L IS USM are aspherical on both sides of the lens. To further improve the peripheral image quality, we have optimised the shape and material of these two aspherical lens elements.

― What are the key points for enhancing the peripheral image quality?

Iwamoto In the design of wide-angle lenses, it has remained a longstanding challenge trying to correct distortions while maintaining the focal plane flatness at the same time, which are two contradictory properties. For example, reducing distortions alone would lead to the loss of focal plane flatness. On the contrary, seeking focal plane flatness by minimising field curvature tends to result in larger distortions. To address this challenge, the material and shape of the two double-sided aspherical lens elements are optimised on the EF16-35mm f/4L IS USM to enable both focal plane flatness and the correction of distortions at a high level. Additionally, chromatic aberrations of magnification are improved with the introduction of two UD lens elements and the latest optical design.

― Are there any existing lenses that also make use of double-sided aspherical lens elements?

Iwamoto The EF16-35mm f/4L IS USM employs two double-sided aspherical lens elements in the first group, one of which has a large diameter. It is the first time for us to adopt a double-sided aspherical lens with such a large diameter. Our success in establishing the technology necessary for the precise production of such a lens has helped to enhance the design performance of the EF16-35mm f/4L IS USM by a large extent.

Shimada Besides the EF16-35mm f/4L IS USM, the EF-S10-18mm f/4.5-5.6 IS STM has also contributed significantly to the advancement of our production technology. If we had come up with this design a few years earlier, we wouldn't have been able to create a lens with the same level of performance. Breakthroughs in both the optical design and production technology have led to the birth of the EF16-35mm f/4L IS USM and the EF-S 10-18mm f/4.5-5.6 IS STM.

― Between the EF16-35mm f/2.8L USM and the EF16-35mm f/4L IS USM, which is higher in image quality? Of course, the result may vary when the aperture is opened up fully and when it is stopped down.

Iwamoto The maximum aperture of the two lenses is not the same (f/2.8 and f/4), so it would be difficult to make a simple comparison. For example, if we take a shot with the aperture stopped down to f/8, the image quality of the EF16-35mm f/4L IS USM is higher because it is equipped with the latest optical technology. That being said, the f/2.8 lens is capable of expression with a shallow depth of field that is only achievable at the maximum aperture of f/2.8.

― The reproduction power of the EF16-35mm f/2.8L II USM is also excellent up to the area covered by the APS-H format sensor, but the image quality deteriorates markedly at the peripheral area. If we look at the MTF chart, the lines for the EF16-35mm f/2.8L USM are "wiggly" from the centre to the periphery. In contrast, the characteristics of the EF16-35mm f/4L IS USM are extremely straightforward, which decrease monotonically as it gets closer to the peripheral area. What is the reason for such a difference in the optical characteristics?

Iwamoto Generally speaking, the rippling MTF curve can, in many cases, be attributed to the curvature in the position where focus is achieved (focal plane). With the flatness of the focal plane improved on the EF16-35mm f/4L IS USM, the lines from the centre to the periphery become straighter and more gradual.

― The sagittal and meridional lines on the MTF chart are seen to diverge as it approaches the peripheral area. What type of aberration causes this to occur?

Iwamoto Generally, this is largely attributable to astigmatism. Astigmatism is a type of aberration due to differences in the position where image is formed in the sagittal and meridional directions. Divergence between the two lines increases when this difference is larger. The design of the EF16-35mm f/4L IS USM aims to minimise not just field curvature, but also astigmatism, so the divergence between the sagittal and meridional lines is relatively smaller.

― It would be difficult simply to maintain the peripheral image quality on an ultra wide-angle lens. Unless the image circle is sufficiently large, wouldn't correction by the IS feature cause the image quality in the peripheral area to deteriorate?

Iwamoto To prevent the image quality from deteriorating when correction is performed by the IS feature, we have optimised the lens construction and achieved a design that delivers a sufficiently high level of optical performance.

Lens Construction: EF16-35mm f/4L IS USM

Comparison of MTF Curve

EF16-35mm f/4L IS USM

EF16-35mm f/2.8L II USM

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

The lines of the MTF curve for the EF16-35mm f/2.8L II USM are "wiggly". The resolution performance of 30 lines/mm is also comparatively low, and deteriorates markedly at the peripheral area. Meanwhile, the EF16-35mm f/4L IS USM maintains a high level of contrast and resolution up to the edges.

Optimising the Lens Layout For a More Compact IS Lens Body

― If the IS were not included in the design, would the lens be lighter and more compact?

Iwamoto Of course, without the IS unit, it is possible to come up with a smaller and lighter design. For this lens, we have adopted a lens layout that helps to prevent the body from becoming overly large due to the IS unit.

Sasaki The IS unit on the EF16-35mm f/4L IS USM has a compact size that is optimally designed for this lens. Although the overall length of the lens may be shorter if we had omitted the IS, the weight does not become considerably lighter, so I believe there are more advantages if we introduce the IS unit. We have also modified the shape of the hood for a more compact feel when it is attached to the lens.

― When I first held the EF16-35mm f/4L IS USM in my hands, I thought it was really light, but it is slightly heavier compared to the EF17-40mm f/4L USM. I suppose it feels lighter than its appearance, which is larger with a longer body. By the way, what is fluorine coating and what effects does it have?

Iwamoto The coating is water- and oil-repellent, which prevents dust from settling on it. Even when dust is attached to the surface, it can be wiped off easily. Lately, fluorine coating has become a standard feature on L lenses.

Size and Weight of 3 Wide-angle EF Lenses

The EF16-35mm f/4L IS USM is longer than the EF17-40mm f/4L USM in total length, and close to the size and weight of the EF16-35mm f/2.8L II USM.

― If the lens protect filter is not attached, will there be a higher risk of dirt and dust entering the lens during day-to-day use?

Sasaki For day-to-day use, there is no worry that dust or water droplets would enter the lens even if you do not attach the lens protect filter. However, due to its structure, dust or water droplets may be drawn into the lens unintentionally at the far end of the filter screw when the front lens moves forward or backward during zooming. To effectively protect the lens against dust and moisture in this case, it is recommended that you attach a lens protect filter.

― If a small amount of raindrops accumulate on the inner side of the lens tube when the camera is positioned almost horizontally, can I zoom the lens as usual?

Sasaki Of course, a small amount will not be an issue.

― Wouldn't it be possible to maintain a dust- and water-resistant structure even when the front lens moves, or adopt an optical system with the front lens remaining still?

Iwamoto In optical design terminology, many wide-angle zoom lenses select an optical system with the first group playing the role of a concave lens, and the first lens group is movable in most of the cases. If we adopt the inner zoom type, where the position of the front lens is fixed, the overall length of the lens would be longer and the diameter larger.

Front lens movable inside the lens tube

The front lens moves inside the lens tube, and the length of the lens remains unchanged during zooming. To ensure sufficient dust- and water-resistance in this case, the use of a protection filter is recommended.

― Instead of a structure with the lens moving inside the lens tube, wouldn't it help to eliminate the worry of water accumulating if the lens were able to extend out in the same way as conventional lenses?

Sasaki There are some users who prefer the front lens to remain still without the lens extending out or retracting. For the EF16-35mm f/4L IS USM, the lens tube is shortest not at the wide-angle end, but at the mid-telephoto zoom range. After taking into consideration the focal length at which the lens tube is shortest when retracted, and also the hassle during zooming, we decided that this design was the most well-balanced, since it would be easier to use and more robust if the lens tube does not extend out during zooming while the front lens moves inside the lens tube.

― I see. For an ultra wide-angle lens, the front lens is large and heavy, so instead of having the lens tube and the front lens extend out, the lens is able to better withstand external shock and hardly rattles due to wear and tear, am I right?

Sasaki Yes, exactly. In particular, the first group of an ultra wide-angle lens plays a very crucial role, so maintaining its precision is also the key to maximising the image quality according to the design value. Also, we paid meticulous attention to the slightest wobble in the focusing lenses, and made improvements to the mechanism for maintaining the optical system so as to achieve a stable level of optical performance. In addition, we were also mindful to enhance the position detection accuracy of the focusing lenses. While all these might sound very dull, it is the gradual enhancement in the precision through such accumulated efforts and revisions that lead to the overall improvement in the image quality.

Iwamoto We were also committed to maintaining the high level of optical performance we have achieved without any variation during mass production. Regardless of how excellent the design performance is, it would be meaningless if the lens is unable to deliver the inherent level of performance due to manufacturing errors. By working together with the production team to address the issue of how we can introduce the highest possible level of design performance into our products, we were able to maximise the performance of the EF16-35mm f/4L IS USM. 

 

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