I'm ready for my closeup Mr. Stephens...
, on Flickr
That was taken at 1000mm, hand-held in a snow storm, with no cropping except to take to a square format. It's from my first outing after installing Canon's new firmware update for the 5D3, allow AF at f/8. I used the EF 500mm f/4L IS combined with the EF 2.0X TC-III.
I must say that I think that the firmware works like a charm with my lens/TC combination. Generally, it would grab AF quickly and very accurately. This little bluebird would come up close and then bounce away, so I was switching the lens' limiting mode from full range to long range. At full range AF the lens would hunt unsuccessfully when I'd go from near minimum focus distance to maybe 10-yards. Snow (it was snowing...again) reduces contrast and light, so the conditions were a tough test for the new setup.
The sun is out today and I'll try birds in flight. I think it's going to be very usable. Hand holding 1000mm seems no tougher than hand holding 700mm, at least for me. Of course, I've taken over 50,000 hand held super-tele shots, so YMMV.
I'm reading reports of guys putting a 1.4X TC on their 400/5.6 lenses. That lens has super quick AF with most bodies. They're reporting that it only loses a little speed at f/8 now. For under $2000 you can have a really effective 540mm.