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I;ve been doing lots of practice on BIF. Maybe this is a stupid question, but.... anyway, It seems often when shooting BIF with my 7D and 100 400 I have to zoom out to about 200 to get it to focus on the bird far away, then I zoom back to 400 to follow and take the picture. I have been seriously considering the 400 prime. My question is, on BIF far away, does it just focus better since I would no longer have the option to zoom out.
I suspect that the reason why you are finding it easier to lock in focus when you start at 200 mm than when you are at 400 mm is because, at 200 mm, with a wider field of view, initially finding the subject in the viewfinder is easier than it is at 400 mm, and, also because, at 400 mm, the bird is moving through the field of view, relatively speaking, faster than it is at 200 mm. Thus, you need to have a higher level of eye to hand coordination and quicker ability to react with the narrower field of view at 400 mm.
That is why it is much more difficult to shoot birds in flight with, say, an 800 mm lens than it is with a 400 or 500 mm lens. The longer the lens, the higher the level of challenge with regard to initial acquisition of the subject and locking in focus. I'd suggest that the solution is merely lots of practice. While I know that there are some who do, with the 100-400, successfully acquire focus at a shorter focal lengths and then zoom in once focus is acquired, using that technique brings with it its own set of challenges.