A fellow photographer was nice enough to let me use his Mark 3 for an hour or so at Palo Alto Baylands on a cloudy morning, not the greatest conditions for good bird photography. This short tryout convinced me to order the camera, which provided excellent acquisition of auto-focus, including flying birds against a busy background, and a higher rate of sharp images compared to my 40D, which is an excellent camera itself. Pictures taken with 500/4 + 1.4x (the avocet) and 400 DO (the others).