Nikon has a single Nano Crystal Coating, while Canon has not only an equivalent coating (Sub Wavelength Coating), but they also have an Air Sphere coating which seems to do much the same thing. Why does Canon have two kinds of similar coatings? Is one cheaper but not as good? Do they do significantly different things? Is one of them weaker to such an extent that it needs to be combined with traditional lens coatings on the same surface to get best results?
It seems Canon has developed different coating technologies for different applications instead of trying to force a "one size fits all" solution on all of their premium lenses.
As far as I can tell, Canon does not use both ASC (first released with the EF 100-400mm f/4.5-6.3L IS II in 2014) and SWC (first released with the EF 24mm f/1.4L II in 2008) on the same lens elements. It's either one or the other.
- ASC seems to be used for fairly long focal length lenses
- SWC shows up in wider focal length lenses
As with many things related to lens design, what is "better" than something else depends upon how it is being applied and what one intends to do with it. Canon seems to have decided that ASC works better (or works just as well at a reduced cost) for lenses with narrower angles of view while SWC works better for lenses with wider angles of view. There's also a hint (included in the quotes below) that ASC might not work as well for lens surfaces with significant curvature, while SWC can be especially effective when applied to lens elements with significant curvature.
The following quotes are from Canon's article Evolution of "L"
Air Sphere Coating (ASC) technology creates a film containing microspheres of air over a vapor deposition lens coating. Systematic lining of the inside of the coating with spheres of air forms an ultra-low refractive index layer. This results in extreme anti-reflective performance, particularly for incidental light that enters at nearly a vertical angle, effectively preventing flare and ghosting. The layer of air microspheres is covered by an interfacial layer, so the ASC coating is highly durable.
Anti-reflective Subwavelength Structure Coating (SWC) suppresses light reflection with countless wedge-shaped structures more minute than the wavelength of visible light on the surface of a camera lens. This coating layer enables continuous variation of the refractive index, resulting in full-scale suppression of reflection. SWC realizes excellent anti-reflective effect even with a large angle of incidence, and dramatically reduces flare and ghosting in peripheral areas of the lens, which were difficult to suppress with conventional coatings.
Comparison of the two technologies from Canon's article ASC: Reduction of Flare and Ghosting
ASC utilizes the anti-reflection principle from destructive interference of light, and it has excellent anti-reflective effects particularly with respect to incident light that enters almost vertically. However, even with increased angle of incidence, ASC still can significantly reduces flare and ghosting that cannot be prevented with conventional vapor-deposited multi-layer coatings. ASC can also be applied to a variety of lenses, making it extremely versatile.
The Subwavelength Structure Coating (SWC), on the other hand, applies a different anti-reflective principle. The advanced Canon developed technology uses a special process to arrange a countless number of nano-scaled (200-400nm) pyramidal structures, smaller than the wave length of visible light, on the lens surface. Since there are no surface boundaries where refraction indices change significantly, making it possible to prevent the reflection of light by implementing gradual refractive index modifications between air and lens. SWC has excellent angle of incidence characteristics, exhibiting excellent anti-reflective effects even for light with particularly large angles of incidence, even better than ASC. As SWC can also applied to surfaces with significant curvature, it is now possible to reduce ghosting and flare that can occur around the periphery of even lens elements with a large curvature in wide angle lenses.
From these descriptions, it sounds like SWC may be more expensive to implement but is also more effective with lenses that have surfaces with a lot of curvature. ASC, on the other hand, sounds like it is cheaper to do but works just as well (or even better) with lenses where most of the light entering them is at lower angles with respect to the lens' optical axis.
At the end of the same article is this summary:
From the initial design stage of a lens’ optical design, the importance of reducing flaring and ghosting has already been taken into consideration. Base on the lens types and specifications, Canon engineers utilize SWC or ASC on the most appropriate lens surface to reduce flare and ghosting in order to raise image quality. This is same as the utilization of fluorite, UD glass, ground aspherical lens or glass-molded aspherical lens, consideration is made for the total performance by using the right element in the right place.
The English optician Harold Taylor, in 1892 observed that old lenses transmitted 4 to 6% more light than new ones. He figured out why. Seems old lenses were blemished with soot. This was during the industrial revolution and the air was laden with smoke and soot from the coal fires that powered the steam engines and gave warmth. This coating of atmospheric pollution settled on lenses on the shelf and etched them. He discovered that this thin transparent coat somehow reduced surface reflections allowing more light to transverse the lens.
Taylor experimented and found a way to artificially bloom (age) lenses. This truly was an important discovery because new lenses suffer a 4 to 6% loss in light due to light being reflected from their polished (mirror like) surfaces. Now lenses used in cameras and telescopes are complex systems with many lens elements sandwiched together. Thus multi-lens element systems can suffer a light loss of 40 – 50%.
This discovery and remedy is important as modern lenses often use many elements and groups. Losing 4 to 6% at each junction translates to quite a high loss. Most loss is from internal junctions (glass to air and glass to glass) within the barrel. Each internal reflection caused light rays to go astray and many misdirected rays bathed the film/chip with light scatter called flare. Flare is devastating; it degrades the image by reducing contrast. Gross reflections cause glare spots.
Many coating methods are used. One method is to place the lens to be coated in a vacuum chamber. The air is evacuated and the mineral that will be the coat is heated causing it to vaporize. This vapor condenses on the glass lens and coats and etches. It is the thickness of the coat plus the material that does the trick. Each coat is optimized for just one color of light. A modern lens has multiple coats applied. Each coat is different in thickness. A high quality lens can have as many as 7 thru 11 coats.
As time passes, coating technology advances. Likely each discovery will give small improvements.
A tip of the hat to Harold Taylor.