This super telephoto lens has the longest focal length of an interchangeable lens for AF SLR cameras at the time. Silent and fast AF is realized using a ring USM (Ultrasonic Motor) and rear focusing system. Extremely sharp, professional-quality images are realized through use of two large-diameter artificial crystal fluorite lens elements (3rd & 6th) that thoroughly eliminate secondary spectrum. Although the optical design is the same as FD1200mm f/5.6L (prototype), this lens is optimized for the EF system with a nonlinear focus cam for moving the focusing lens group, a focus preset function for quick focusing at a predetermined point, and power manual focusing with three focusing speeds. A one-touch revolving mechanism for quick switching between vertical and horizontal formats is possible. Extenders EF1.4X or EF2X transforms the lens into a manual focusing super-super-telephoto 1700mm f/8 or 2400mm f/11 lens.