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XD

Extra Low dispersion - usually refer to glass type (refer page C2).  Glass with XD properties indicating special rare earth glass or special formulated glass that limiting or correcting of light rays passing through the lens elements to achieve all spectrum of colours to falls on the same plane of focus - especially the Red and Blue spectrum and is usually more apply to longer focal length lenses where the problem is more serious.  Kowa lens line - with multi-coating of dielectrics (C3 Coating) allows for a much higher reflectivity than with coatings of silver or optically-enhanced aluminum, offering a reflectance of 99% or higher across the entire visible spectrum.