First of all, could you please explain the roles and functions of an anti-reflective coating?
MakinoAir and glass differ greatly in their refractive indices. Therefore, at the point where the air and glass meet on the lens surface, light is reflected and causes light transmittance to decline. Also, there can occur phenomena like ghosting (light that, after repeating reflection on the lens surface, results in an image containing artifacts that aren't actually there) and flare (intense light like backlight, which is reflected or scattered by a lens or lens barrel, that causes a part of the photo image to become whitish). To prevent such phenomena, light reflection needs to be minimized by coating the lens surface with an extremely thin film. Even so, there were cases that anti-reflective coating failed to work well, depending on such conditions as the incident angle of light.
Ghosting and flare
- Example with ghosting and flare
- Example without ghosting and flare
The structure of SWC
To enhance the anti-reflective performance of our lenses even further, we took up the challenge of developing a cutting-edge lens coating technology using nanotechnology.
The key to preventing the reflection of light is how to control the boundary surface between the outside air and the thin film. In 2008, we began to apply SWC (Subwavelength Structure Coating) technology to our lens products for the first time. SWC is a ground-breaking technology in that it utilizes countless wedge-shaped structures of nanometer size (even smaller than the visible light wavelength: 380-780nm*) arranged on the lens surface so that light does not react to the boundary surface in effect. This coating also exhibited stellar results for types of light whose reflections were previously difficult to control, such as light entering at an angle, significantly reducing the occurrence of ghosting and flaring.
However, the making of nano-sized wedge-like structures particular to SWC requires highly sophisticated, complex processes. Accordingly, our next target project was to develop a high-performance anti-reflective film, the production method of which would be much simpler and require no such difficult-to-make structures as SWC’s. This is how the development of ASC (Air Sphere Coating) technology began.
* 1nm (nanometer) = one billionth of a meter
The structure of SWC
What kind of coating technology is ASC?
MakinoDeveloped in 2014, ASC is our latest lens coating technology. It is made up of a film containing air on top of vapor-deopsition films on the lens surface. It significantly reduces light reflection by sandwiching a film containing air between vapor-deopsition films and the outside air, which is highly effective for controlling incident light that comes in at a nearly vertical angle. Since it doesn’t involve any special structures, ASC can be applied to many more types of lens surfaces than SWC. In fact, expanding the degree of freedom in terms of applicable lens surfaces was one of the important objectives of its development.
The structure of ASC
Glass (no coating)
If the lens only has a glass surface with high reflectance the lens reflects much of the incident light.
ASC-coated lens (layer of ultra-low refraction index + vapor-deposition film layer)
With the application of ASC coating, the lens can reduce light reflection and realize a clear image.
(Illustration) The layer of ultra-low refraction index made of air and silicon dioxide
I understand that SWC is effective for incident light coming in diagonally while ASC is effective for light coming in almost vertically. Is that correct?
MakinoIn principle, wedge structures don’t form a boundary at all whether light comes in from above or sideways. If there is no boundary, then the lens can demonstrate outstanding anti-reflective performance. As such, SWC delivers an exceptional performance not only for light coming in diagonally, but also for light coming in almost vertically. This coating is not specifically for light coming from a certain angle but can effectively deal with light coming from any direction.
On the other hand, ASC has a boundary surface. This means the ASC-coated film produces a superior effect on light coming from a near-vertical angle while SWC exceeds ASC in anti-reflective performance for light coming in diagonally.
Then, how do you decide whether to use SWC or ASC?
MakinoMakino: Basically, it depends on the type of lens and the particular technologies needed for eliminating flare and ghosting. To put it simply, we mainly use SWC for lenses that require superior anti-reflective performances for incident light coming in diagonally whereas we use ASC for other types of lenses.
Please tell us a little bit more about ASC.
MakinoWe developed the layer with an ultra-low refractive index for the purpose of minimizing reflection on the boundary between the outside air and the film. In the case of films made of ordinary materials, the average refractive index for films with low refractive indices is around 1.37. Given the refractive index of air is 1, in order to minimize light reflection as close to that of air as possible, we came to the conclusion that having the film contain air inside it would be the only solution, save for the idea of using wedge structures to eliminate the boundary between outside air and the film, as is the case with SWC. Then comes the problem of how we can have the film contain a good amount of air within it. In the case of ASC, we adopted the method of having the film contain countless fine particles of air.
If the size of each air particle is too large or if air particles are distributed unevenly, air itself could become a cause of diffused reflection, degrading the transmittance of light and making the film useless as an anti-reflective film. On the other hand, too large an amount of air contained can make the film vulnerable.
So, you could say that the key point of the ASC coating technology is to control the size and quantity of air particles by giving full consideration to the materials in terms of both their composition and the manufacturing method used. As a developer involved in both materials and process development, I went to great lengths to hammer out a solution from both angles while taking a good balance between performance capabilities and how the lenses would handle into consideration.
What have you done that you feel is really worthwhile?
MakinoIn a sense, the development of an anti-reflective film is a seemingly less-exciting job because it concerns an invisible nanometer world; the better performance the film delivers, the less conspicuous its existence becomes. However, let’s take the commercialization of the recently launched “EF11-24mm f/4L USM” lens for example - a lens with a super-wide-angle focal length. Making possible image capture that was previously impossible, to be taken now, the ground-breaking technology behind this fantastic lens is sure to have a real impact on the culture of photography. I was especially happy when I saw how satisfied users of this superb lens were.
At this point*, Canon is the only manufacturer in its industry with two proprietary high-performance anti-reflective film technologies like SWC and ASC that totally differ in properties. Our development efforts continue towards making lenses with even higher performance through the clever application of these two lens coatings.
* As of June 2018
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Behind such impressive achievements, there has always been an invincible pioneer spirit shared by all Canon developers who would never give up, overcoming obstacles through trial and error. In fact, these developers take great pleasure in facing new challenges. This undaunted willingness to take on a challenge must be the driving force behind expanding the scope of their creativity and motivating them to create things that were once considered impossible. Supporting the evolution of optical technology, Canon’s superb production technologies will continue to fulfill photographers’ desires to capture images just as they see them.