Fastest aperture for the XF 100-400mm + 1.4x at 560mm is F8. When shooting birds I usually stop down to F9 or F10 for a little bump in contrast and sharpness and ISO is usually between 200 to 2500. In daylight F11 at 800mm can be fast enough and still provide nice out of focus backgrounds. You will find decent zoom lenses that go to 600mm without spending a fortune, but not 800mm.
Lars,
That is a very good post. Yes, I have used the 1.4x tc with that lens many times and it pushes it to 840 at F8 right? I think I almost always shot it at F8 but looked at a bunch of shots I took with it and I did shoot some at F11 on a monopod!
So I guess your post made me realize that poking fun at a locked F11 in order to get that kind of price size weight is not such a bad idea. Hmmmm. Canon will have a way to get that long long glass into the hands of more people that can make it work in many situations.
But it also makes you think of one of the big advantages of APSC. Reach.
Now, time for Fuji to make some long primes. That will happen. But will they be super high end and fast? Like that 200? I think so, but I know they were freaked out by that 200.