Getting AF focus locked on a small bird against a bright clear sky is difficult for any setup. For long telephoto shots, a lot of practice is needed to be able to point the camera so as to get the bird in the frame immediately because of the magnification and a narrow 5-8 degree view. If you use continuous AF, it will just keep hunting to infinity and back and forth, thus making it difficult to lock on when you have the bird in your focal point. When you use AF, you need to use spot focusing and ensure the centre focusing point is on the bird (to give it a target) before you half-press the shutter. Primes will make it easier to use hyperfocal focusing first followed by fine-tuning manually.
You should be able to use your lens with practice. If you want something faster and reasonably priced, try the Tamron SP AF200-500. It's inner focusing mechanism only takes a 1/4 turn to go from its min. focal distance of 8ft to infinity and thus is faster to regain focus if the bird keeps flying out of the frame. It is also light enough to be hand-held which would be what you will be doing mostly for inflight shots. Here's some pics:
http://www.putup-or-shutup.us/portal/phpBB2/viewtopic.php?t=105
Except for the one where I indicated I used the beercan, all were taken with the Tamron 200-500mm.
If you are prepared to accept 3rd party, Geno was thinking of selling his Tamron SP AF 300 f2.8. No ssm etc but it comes with a focus limiter which would be useful.
thanks bro, yup i use spot focussing, the only thing that irritates me is the AF hunting, takes forever... any idea on Tamron 200-500 pricing and availability?
