Scramble Messageboard

Hi guys, I recently switched to Canon as I always wanted to try the 400mm f5.6 prime. I bought a 50D with it but I am starting to wonder if the autofocus will be fast enough for airshows (F4 farewell and RIAT), so I was wondering if somebody has experience with both, preferably combined with the 400...

I used both 1.4 and 1.7 on my 300mm f4 (and d90), the 1.7 is really not bad at all but you will need more light for accurate focussing. As the difference between both (0.3x) is not that big I would go for the 1.4.

Arthur