Sorry for the long post, but it’s in case anyone is deliberating about lenses. I have embarras de richesse for my style of nature photography as I have more than enough gear to keep me more than happy, and would be only a little less happy without the redundancy. My priorities are first it has to be light enough for someone of my age to go out hiking with. Secondly it has to give adequately sharp, well-resolved and focussed images. And, thirdly, I can take distant birds and close up ones, and insects. To be honest, my 5DSR, which is now my Canon back up body (and my wife uses), and 100-400mm II plus 1.4xTC still meet our needs in the main and I would cheerfully take the pair on my next trip abroad. The R5 near enough matches the 5DSR for resolution and it is more versatile for BIF and pairs really well with the 100-400mm II and I can use it at 800mm. The Nikon D850 or 500 with the 500PF also pass the weight test. I get sharper images than with the zoom, especially close up. With the minimum focal distance of 3m, I usually get better images of dragonflies and butterflies with the 500PF than with 100-400 closer up, and apart from the tracking. It is as good as the R5 for BIF at 500mm but not at 700mm because the DSLR focusses slowly at f/8 when the 1.4xTC is on the f/5.6 500mm - the R5's AF is a killer at f/8. The 500PF has the edge on sharpness for distant small birds, especially with the 1.4xTC. So, I am very happy taking the Nikon pair on a days hike or on safari. The 400mm DO II catches up on the sharpness and focusses brilliantly at 800mm f/8 on the R5. However, I haven't decided yet on whether to keep the 400mm DO II. The difference in weight is just enough to make a difference. The R5 + 400 DO + lenscoat is 3.25 kg and 3.5 kg with the 1.4xTC. The D850 with the 500PF and lenscoat is 2.8 kg. Put the 2xTC on the 400 and the weight goes up to 3.6 kg, and with the 1.4TC on the 500, it's 3 kg. So, it's an extra 0.6 kg or so carrying the Canon prime pair vs the Nikon full frame for similar resolution. I am going to test the 400mm DO II for BIF on the R5. If it is much better the 100-400mm II, then I'll keep for special days out but not for general use.