Large industry binoculars normally benefit from some sort of Erfle configuration, patented in 1921. These have 5-6 aspects in three groups. The teams may be two achromatic doublets which has a double convex singlet among them or may perhaps all be achromatic doublets. The entry guidelines of the web-site http://lorenzosrpmj.get-blogging.com/15405777/binoculars-an-overview