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SWC stands for Subwavelength structure coating
Canon developed SWC for its EF lens system in order to reduce unwanted reflections. It was first introduced in December 2008 with the EF 24mm f1.4L II USM lens.
SWC is a layer of material that varies its refractive index from air (1.033) to that of glass (approx. 1.6) and thereby minimises reflections.
The anti-reflective properties are not reliant on particular geometry and therefore are minimally affected by angle of incidence. Furthermore, because the coating removes the cause of a reflection rather than suppressing it, most of the light that hits the lens is transmitted, further improving lens performance.
