With the 100-400's there are some known isues with them. They occaisionally develop a 'ERROR' fault. To clear this you need to turn the camera off and take the lens off, and put it back on. I know one person who has one who has had the problem, the lens is at Canon at the mo, the other person has had no problems with his. They are a pretty good lens, and if I didnt have 'coverage' for those focal lengths, I wouldn't hesitate to get one.