No. 3 Series II anastigmatic patent lens F.5.4 6.5inch f.l. in a heliocal focusing mount and ring type iris diaphragm. Double Anastigmat with a field of approximately 85 degrees at the maximum aperture of F.5.4. Either front or back lens can be used on its own as a single landscape lens. Using the front combination the focal length is 1.5 x that of the entire lens while the rear combination gives a single lens with a focal length double of that of the whole lens. First introduced in 1897. Dallmeyer