Thomas, If you stick to 100 ISO, you will soon find that f/5.6 is too slow in many situations, especially with a tele lens. That's why I personally switched from a 70-210 slow telezoom to a 180 2.8 fixed lens. You may ask yourself how often you would need a 400mm or so over a 200mm, for instance. 400mm is pretty long and the gap between it and your other lenses would be huge. I recommend considering the option of getting a 180 2.8 plus a TC for the cases you really need a long lens. If you can justify the 400mm, go for it, but forget about using a TC with it (other than for special applications like moon shots). You will probably feel the need for a shorter and/or faster lens, anyway. Don't forget that you can crop a print after enlargement, and that the loss in sharpness will be more limited that the one induced by shake, when going from 1/250s @ 180mm, f/2.8 to 1/60s @ 400mm, f5.6. Hope this helps, Nicolas.
