Absorbs the ultraviolet rays which often makes outdoor photographs hazy and indistinct.  Also serves as a permanent lens protector.

HMC (Hoya Multi-Coating)
These popular filters are renowned for their ability to minimize reflection at the filter surfaces which reduces flare and ghosting. The result is an average light transmission of over 97%, giving sharp contrast and well balanced color. Hoya HMC filters are recommended for enhancing the performance of today's multicoated lenses.