can someone help me understand the need or why you would need a $20000 600 f4 over a 200-600 f5.6-6.3 . I realise the f4 allows a lot more light in but with the technology of the mirrorless bodies and the high iso numbers on these new cameras wouldnt this over ride the need for such an expensive lens. I hope this is not a stupid question and to add a bit more to this i use the first edition 5D with a sigma 150-600c for bird photos and if i was to put my sigma lens onto a sony a1 body would this improve the quality of my photos ?