Your 70-200 not long enough? Add the 1.4x extender for that extra reach. Adding extender reduces the f/stop on your lens by one full stop. I.E.: f/2.8 to f/4.0
Designed to extend a lenses focal length by a factor of 2x, the redesigned Canon Extender EF 2X III delivers optical performance on par with the finest Canon super telephoto lenses. It features a built-in microcomputer for seamless communication among camera body, lens and extender, features careful lens placement and coatings to minimize ghosting, flare and chromatic aberration, and even has a new Fluorine coating that keeps soiling, smears, fingerprints to a minimum. Extender EF 2X III meets necessary standards in terms of rugged, dustproof and water resistant construction, and should be in every professional's camera bag.