Large-aperture telephoto L lens with a camera shake correction function, which is best suited to shooting by holding the camera with hands Developed as a successor to the EF300mm f/2.8L USM. A fluorite lens and two UD lenses are used in the optical system to assure very high picture quality. This lens has also a protection glass and a rear filter as standard, which are included in the number of lenses. The camera shake correction has an effect of approx. two stops* in terms of shutter speed. This lens has also "camera shake correction mode 2" which corrects shaking of the viewfinder image when, for example, shooting a moving object, and a tripod can be used with the camera shake correction function ON. High-speed autofocusing is achieved by the weight reduction of the group of focusing lenses and improvement of the autofocusing drive algorithm. A new AF stop function, which stops autofocusing temporarily, is adopted. Manual focusing is performed mechanically to save power. Main components are made of a magnesium alloy to reduce the weight of the lens, including the optical system, making it much lighter than conventional lenses. Rubber parts are used as mount and switch components to achieve excellent dust- and drip-proofness.