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Technique

Le présent article n'est pas disponible en Français
February 2009

The new SubWavelength Structure Coating (SWC) technology from Canon is currently employed in three lenses - the EF24mm f/1.4L II USM and the recently announced TS-E17mm f/4L and TS-E24mm f/3.5L II tilt and shift optics - to minimise lens flare and ghosting. CPN technical editor David Newton explores how it works and what are the benefits for photographers.

For many years, since the early 1970s, Canon has used the Super Spectra Coating to help to minimise lens flare. There is some evidence that it was in use as far back as the 1960s in some form or other. However, the vapour deposition method of lens coating used for the Super Spectra Coating has recently been superseded, for some uses, by the new SubWavelength structure Coating.

 

The EF24mm f/1.4L II USM was the first Canon lens to employ SubWavelength structure Coating.

To understand SubWavelength structure Coating, you need to understand something about optical physics. Fortunately, it can be kept pretty simple. When light passes through a surface, in this case glass, there is both reflection and refraction at the air/glass interface. At each surface it passes, some light is lost. The principal aims of coatings are to reduce this reflection and refraction and to pass as much light as possible through the lens elements to the sensor where the photons of light can be recorded.

The air-coating interface with Super Spectra Coating has a refractive index lower than that of the air-glass interface and so there is less reflection. This is a good thing because reflection causes both flare and ghosting which can adversely affect the image quality.

Back in the 1960s, scientists discovered that the eyes of moths didn’t reflect light because there are thousands of microscopic structures on the surface that are unevenly arranged and distributed. This functions as a very low refractive index layer that cuts out the reflections. It is this discovery that has been harnessed and progressed to help to produce the new Canon SubWavelength structure Coating.

 

Two lens elements showing the difference between SWC coating (left) and Vapour-Deposited film coating (right). The difference in surface reflection is clearly visible, with the SWC coated lens exhibiting much lower reflectivity.

With a SubWavelength structure Coating, there is no single air-surface boundary and therefore there is no sudden change in the refractive index. Instead, the surface of the coating is made up of tiny wedge-shaped nanostructures that are of a similar scale to that of the wavelength of visible light. This causes a continuously changing refractive index as light travels through the coating and allows a smoother, and smaller change in the overall refractive index. Because of this lower refractive index, more light is transmitted and substantially less light is reflected within the SubWavelength structure. This leads to more light passing through to the sensor and lower levels of flare and ghosting.

The key to harnessing this technology is in ensuring that the tiny structures are aligned on the surface so that the points of the wedge point out away from the glass. This creates an effect that’s almost like channelling the light from the tips of the wedges down towards the lens.

Without the SubWavelength structure Coating, wideangle lenses can exhibit flare and ghosting as shown here (above, left). This same image, taken with a lens using the SubWavelength structure Coating (above, right) shows much reduced flare across the frame.

SubWavelength structure Coating is especially effective when used in wideangle lenses, such as the EF24mm f/1.4L II USM lens and the ultra wideangle TS-E17mm f/4L, where the curve of the lens elements is very large. Having such a large curve means that the angle of light hitting the lens can be very great and this could result in the issues of flare and ghosting. By the use of this SubWavelength Coating, more light is drawn into the lens along the nanostructures and the potential issues are greatly reduced.

Overall, the SubWavelength structure Coating technology marks a significant step forwards for Canon wideangle lenses that will improve the image quality that is achievable when using lenses of this type.