From the Manufacturer
Light. Portable. Excellent handling. Sharp. And amazingly fast autofocus. The 400mm f/5.6L is all of these things, and a premier choice for wildlife and nature photographers--one of the finest telephoto lenses in the world for fast-moving subjects such as birds in flight or motor sports. It uses UD-glass elements to provide outstanding optical quality, even wide-open, and image quality is preserved when used with either the EF 1.4x II or EF 2x II tele extenders. It accepts 77mm filters, and has a built-in removable tripod collar.
See a Different Dimension
A super telephoto lens can make an ordinary scene into an extraordinary one. Its ability to see surpasses that of the human eye. It can compress images and make them look as if they come from another dimension. It opens up new realms of photographic expression. All EF super telephoto lenses are L-series lenses to provide the highest quality. The USM provides quiet and high-speed autofocusing. These lenses promise outstanding delineation and put your imagination to the test.
This high-performance lens was designed with portability and handling ease in mind. One Super UD-glass element, whose characteristics are similar to fluorite, and one UD-glass element result in sharp pictures from corner-to-corner. The lens also has a built-in hood and a detachable tripod mount.