Thanks for your reply. I don't have much experience with manual focus at all let alone at high magnification, so this will be fun to try. I have used the 2x converter on my 70-200mm f/2.8 IS L II and love the reach it gives me. I can get decent shots with that combo. 400mm is more than enough for the shooting I do. Looking forward to trying a moon shot at 800mm providing the night is clear. I'll probably shoot at 400mm while I have it, just want to try it with the 2x converter. I am going to be trying my hand at shooting birds, which I have never done before, so it should be an interesting week. Thanks for the feedback.