OK, so since I've stumbled onto the "ducks-forever" page, here's my duck story. Overcast day. Grey geese doing take-offs and landings. (OK, a goose isn't a duck. Stick with me.) A fozen lake. Snow on shore. 95% missed pictures. Apparently, there was not enough contrast in the scene to allow my slow but image-stabilized lens to focus at 200mm and a maximum f/stop of 6.3. Let me repeat: The lens would not auto focus at all. Lesson here: Image stabilization is no substitute for speed. A good, fast, constant f/2.8 rules!