Description by Canon:

An outstanding professional lens for bridging distances in sports, wildlife, and many other applications – including commercial and fashion for the distinct “compressed” telephoto character and totally out-of-focus backgrounds its images can possess. Fluorite and UD-glass are combined for incredible image quality, even wide open. An Image Stabilizer adds a new dimension in this long lens’s usefulness, even in marginal lighting. It works beautifully with both 1.4x II and 2x II extenders, and now focuses as close as 17.5 feet (5.4m). It has the same weather-resistant gaskets and sealed focusing and focus pre-set rings as Canon’s other Image Stabilizer super-telephoto lenses.