A photographic filter installed on a lens becomes an optical element in that lens system. This filter by Sigma is multicoated like all the other elements in the lens, and does not introduce any reflections into the lens system.It also has the best and most practical finish I've ever seen on a filter. The metal band surrounding the glass is TEXTURED. Really grippy. Wow, does this ever help in screwing the thing on and off. Very helpful.Given the anti-reflective coatings on the glass and the grippy texture of the band, I would always look for Sigma filters in the future.