« on: June 01, 2012, 09:01:17 PM »
IQ is almost identical at 400mm. I basically use my 100-400L at 400mm like most. I kept the 100-400 mostly for the following reasons:
It is much shorter and stores in the same length as the 70-200L. You will not like trying to store the long 400mm f/5.6 L
IS comes in handy, don't underestimate it, I've used it at 1/20 sec handheld.
Close focus. When you want to actually fill the frame with a small bird or object, you can get close enough to do it, the 400 5.6L does not focus closely.
Ocassionally, the zoom comes in handy.
If it matters, resale value is a little better as well.
For your purpose, the 400mm f/5.6 L will be fine, but the other advantages of the 100-400L make it a lot more versatile and easy to use. Thats why you see it recommended so often.