[Part 2] Optical Design that Fully Mobilises the EF Lens Technologies
How are distortion and field curvature, commonly found in super wide-angle lenses, suppressed on the EF11-24mm f/4L USM? We find out here in the second part of this series of interviews with the developers of these lenses. (Reported by: Ryosuke Takahashi)
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(From left to right)
ICP Group 1: Shota Shimada
ICP Development Centre1: Hideki Sakai
ICP Development Centre 1: Tadanori Okada
An aspherical lens is placed in certain groups to minimise distortion and field curvature
- If we look at the sample photo, it seems like there is almost no distortion at the 11mm end. Which aspherical lens elements are responsible for the corrective effect?
Sakai The lens elements in front are highly effective for correcting distortion, particularly the first and second elements because of their aspherical shape.
A low-angle shot of a castle wall in Avila of Spain, a UNESCO World Heritage site. The entire image is so highly resolved, it is as if you could feel the texture of the rocks. Also, there is no noticeable chromatic aberration in areas such as the outline of the wall.
EOS 5D Mark III/ EF11-24mm f/4L USM/ FL: 11mm/ Aperture-priority AE (f/8, 1/400 sec., EV-0.3)/ ISO 100/ WB: Auto
- So it is important to reduce distortion at the time light enters the lens, am I right?
Sakai Basically, correction by the front lens elements is the main factor that helps the achievement of high image quality. One characteristic of the front lens group in super wide-angle lenses used in DSLR cameras is that it has the effect of a deeply-concave lens. Thus, it is important that any aberration that occurs is minimised here.
Lens construction of EF11-24mm f/4L USM
- Distortion correction and enhancement of peripheral image quality are generally understood to be properties with opposing effects. How did you address this issue on the EF11-24mm f/4L USM?
Sakai This understanding is correct. Although the aspherical lens elements in front correct distortion, in the case of wide-angle lens, it is likely that field curvature may still occur even with this. When this happens and you establish focus at the centre of an image, the peripheral areas of the image would appear out of focus, causing the resolution in these areas to deteriorate as a result. For the EF11-24mm f/4L USM, besides the first and second lens elements, a high-precision glass-moulded aspherical lens is also employed for the third and the last (sixteenth) elements. Each lens element has a different optical effect, all of which work together to achieve focal plane flatness, contributing to the correction of the different aberrations.
Light reflection within the lens allows us to tell the shape of the ground and polished (first element) and glass-moulded (second and third elements) aspherical lenses. The first lens element is characterised by the significant difference in thickness between the centre and surrounding area.
- Both SWC and ASC are employed on this lens. How did you utilise these two types of coating according to their unique properties?
Sakai Compared to conventional coatings, SWC and ASC have very low reflectivity. In particular, SWC is effective for light rays with a large angle of incidence. That's why we have employed it once again in the EF11-24mm f/4L USM, for the first and second lens elements where the incident angle of the light rays is relatively large. As for perpendicular incidence, both ASC and SWC demonstrate an equivalent level of effect against it, so we adopted ASC for the fourth element.
CHECK POINT 1: Peripheral image quality
Although peripheral light fall-off is observed at maximum aperture, the resolving power is good enough for practical use. Image quality reaches its peak at around f/8, but the maximum aperture can still be used without any problem. At the angle of view of 11mm, the high image quality of the peripheral area is amazing. While significant enhancements have been made to the performance of new EF wide-angle lenses in the past few years, this lens outclasses all of them.
As with the 11mm example, this image was taken and generated a few times at various apertures, starting from the maximum. The peripheral light fall-off that is apparent at maximum aperture does not constitute a problem, as it can be resolved once Canon releases the optical correction data. Image quality at the telephoto side of a wide-angle zoom lens is often not as good compared to the wide-angle side, but the EF11-24mm f/4L USM enables you to make full use of the entire zoom range without any deterioration in image quality.
Born in Aichi in 1960, Takahashi started his freelance career in 1987 after working with an advertising photo studio and a publishing house. Photographing for major magazines, he has travelled to many parts of the world from his bases in Japan and China. Takahashi is a member of the Japan Professional Photographers Society (JPS).
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