A large aperture super telephoto lens employing rear focusing system for the first time for Canon. Lenses which carry out focusing by moving one or more lens groups behind the diaphragm position are called rear focusing lenses. As the lens length does not change during focusing and a compact, lightweight design is possible with rear focusing system, hand-held shooting is possible even with such a super telephoto lens of 400mm. A cam system is applied for moving focusing lens group instead of conventional helicoid and this system enables adjustment of focusing speed according to focusing distance; quick movement of focusing lens group for shorter focusing distance and slower movement for longer focusing distance. This vari-pitch system gives better focusing operation.