Making of Canon L Series 500mm F4L IS USM. Part 1 of 3 in the production of expensive camera gear. L series is Canon's Premium lens line featuring fluorite lenses. IS refers to Image Stabilzation build into the lens, and USM refers to Ultrasonic Motor drive that makes focusing faster than older systems. Currently I'm using a Rebel XT, although I'm thinking of upgrading to an XTi or 5DMk2. This 500 with the New 5D would be a sweet combination. www.razorfantasygirls.com