It reduces the glare and reflections from glass and water and other non-metallic surfaces. It deepens blue skies, and saturates the colors. The Sigma circular PL filter is designed to give best performance with AF Cameras. The Sigma wide multi coated circular polarizing filter is very thin, so it does not cause vignetting with wide-angle lenses. You can monitor the changes at different angles through the viewfinder as you rotate this filter.