A robin was flitting around a tree in my back garden a few minutes ago, back lit in not so good light. As I am having some problems carrying my heavier lens I thought I'd have a quick comparison of the 5DSR +100-400mm II vs 5DIV + 400mm DO II + 2xTC. It's not a scientific test as they conditions were different, but they show I can get satisfactory results with either as the bird was relatively close. The pose with the 5DSR, more sideways with peak pointing at a good angle towards me, has more of the bird in complete focus (top, size of bird = 1360x1040 px, 1/250s, f/5.6 iso640) than with the 5DIV, which has the front somewhat better resolved than the 5DSR but the head pointing way is less well-focussed because of dof (bottom, size of bird 2270x1320 px 1/400s, f/8 iso1250).