Hi Andrew, Very sorry to hear of your accident. Hope all goes well for you. I too like many others here have had numerous back operations with the latest one fusing three vertebrae. It takes time to heal but there is always light at the end of the tunnel. I too used a lightweight setup when my back was bad but now 2 years on I am swinging around a 600mm f4 with little problem. As far as the 5D3 is concerned there is a firmware update on the Canon site which allows the 5D3 too autofocus at f8 so the 400mm f5.6 with TC could still be an option if you haven't changed to Nikon yet.
Cheers and a speedy recovery Bryon

Hi and thanks Byron, no I haven't changed yet as I'm still trying to sell my big lens. The update for the 5D111sounds like a good option so I'll give that a go. I really like the 400mm/f5.6 even though it doesn't have image stabilisation. Will let you know how I go. Thanks & cheers Andrew

Hi Andrew forgot to mention, my wife uses the new Canon 100 to 400 ii on the 5D3 with great results even with a 1.4x extender giving very sharp images. Its a beautiful lens and the whole setup is very light and easy to use.

Hi Andrew forgot to mention, my wife uses the new Canon 100 to 400 ii on the 5D3 with great results even with a 1.4x extender giving very sharp images. Its a beautiful lens and the whole setup is very light and easy to use.
Cheers Bryon

Andrew, if you do try the 5DIII and firmware update, I have doubts about the 400 f5.6 mainly due to the lack of image stabilisation. It does make a BIG difference. I hadn't realised until I accidentally bumped mine off one day. I don't have personal experience with that lens though - just how much difference stabilisation makes. If you haven't sold your 600 yet, try some shots with the stabiliser turned off and you'll find out quite quickly, even though it will be a tougher test on the longer focal length lens. I can certainly recommend the 100-400 MkII if it is within your weight requirements. I also suspect the D500 + 300 + 1.4x would work well too.

Hi Andrew, Just went out the front of my place and took this shot with a Canon 5D3 Canon 100-400mm ii Canon 1.4x extender ii to show you of the quality of this lens with extender at 1/250 sec iso 1250 f8 at 560mm . The autofocus is still blistering fast with the extender.