I have the Canon 100-400 f/5,6 L IS lense.
Can you comment on your experience with the combination of this lense with a Canon converter 1,4x II, regarding the quality of the image, autofocus, etc ?
Any other suggestion welcome.
I was at Farnborough and chatting to a guy who was using a 1Ds with a 100-400 and 1.4 extender. As mentioned above the EOS 1 range will autofocus with this combo and he claimed he was getting good results.
My experiences with the 100-400 + 1.4 Canon converter (on a 5D) are not satifying for A-net use. MF is not fast enough for flying aircraft, unless you are very trained. So I think it would bring you more quality to crop than to make your lens more "powerful" by help of a converter.
We fly to find out that the birds are more elegant than us ...
I've used the 1.4x on the 100-400 IS and I was not satisfied with the results overall. The shot shown above by Javier Guerrero is fantastic, but for a long exposure he may very well have stopped it down on a tripod for the 6 second exposure, in which case the image quality would be much improved over the wide-open shooting you'd typically be doing while hand-holding the lens.
It's ok to try it sometimes, but I would never ever count on it as something I'd shoot with all day. You just lose to many shots using MF, which is a pain on the 100-400 especially when extended at 400mm.
Like Javier said, it's ok to try different things which will most likely not make in onto a.net.