Air Sphere Coating (ASC) is a lens coating technology developed to enhance the anti-reflective properties of photographic lenses. Lens coating technologies developed by Canon include Super Spectra Coating (SSC) for reduced ghosting and flare and consistent color balance, and the groundbreaking Subwavelength Structure Coating (SWC) which prevents light from entering the lens at high angles of incidence and significantly enhances anti-reflective qualities. Used in conjunction with SWC, the new ASC technology further reduces the chances of lens flare and ghosting.
Ghosting and lens flare are caused when secondary reflections between the lens elements and the image sensor cause light distortion or effects that degrade image quality. When shooting into the sun, or in other strongly backlit conditions, part of the image may be obscured by lens flare, which appears as an overexposed white patch. Reflections on the surface of the lens may also appear as clutter in the image (ghosting). ASC is Canon’s most recent step in the evolution of coating technology. It ensures that light always strikes the surface of the image sensor at an ideal angle, thus greatly reducing the chances of lens flare and ghosting.
ASC is a technology used to add a thin film made of silicon dioxide and air on top of the vapor-deposition lens coating to suppress the reflection of light. The coating contains a precise ratio of air, which has a far lower refractive index than optical glass, to give this layer an ultra-low refractive index. This gives it exceptionally strong anti-reflective properties, particularly for incidental light that enters at an angle almost perpendicular to the lens. When combined with SWC, which suppresses the secondary reflection of light entering at high angles of incidence, ASC ensures that any extraneous light entering the lens can be suppressed, regardless of the incoming angle, to effectively prevent lens flare and ghosting. Since ASC can be used on lens surfaces with various curvatures, it places fewer limitations on lens design.
