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Infobank

Lenses: SubWavelength structure Coating

SubWavelength structure Coating (SWC) technology from Canon was introduced with the EF24mm f/1.4L II USM lens and is appearing in other lenses where appropriate. It aims to minimise lens flare and ghosting.

Since the early 1970s, Canon has used the Super Spectra Coating to help to reduce lens flare. However, the vapour deposition method of lens coating used for the Super Spectra Coating has been superseded, for some uses, by SubWavelength structure Coating.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 lens coatings are to reduce this reflection and refraction and to pass as much light as possible through the lens elements to the camera's 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, thereby reducing ghosting or flare in images.

 

The EF24mm f/1.4L II USM was the first Canon lens to feature SWC when it was announced in September 2008.

SWC was created from a discovery made in the 1960s while observing moth's eyes. The structure of a moth's eye means it doesn't reflect light at all due to unevenly arranged structures that create a very low refractive index.

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.

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.

When shooting with a lens without Canon's SubWavelength structure Coating, wideangle lenses can exhibit flare and ghosting as shown (above, left). This same scene, taken with a lens using the SubWavelength structure Coating (above, right), shows much reduced flare across the frame.