Bifocal Thin & Dark Glass LensesOur Bifocal Thin and Dark Glass Lenses are made using premium Corning Thin and Dark glass lens blanks, a material that can be surfaced to 1. 5 millimeters which is thinner than standard glass. Thin and Dark glass photochromic lenses are darker than PhotoGray Extra glass lenses when outdoors. Please note that these lenses are for full frames only, not semi rimless or drill mount frames.