On paper, the Nikon D5 can go 7 stops beyond the Sony α7R II. The actual test tells a different story. Tony Northrup describes the testing:
"The $6,500 D5 (http://sdp.io/D5) is Nikon's first 4K video camera, and it offers up to ISO 3,276,800--which is *7* stops beyond what the Sony α7R II (http://sdp.io/a7r2) offers. It's not the low-light champ you'd hope, though.
"Note: We tested the α7R II in full-frame mode, even though it offers 1 stop better quality in Super 35 mode. We wanted to start testing with the worst case scenario for the Sony... but since it won even while handicapped, I don't see the need to continue testing.
"The α7S offers another stop beyond that, so it would be about 2 stops better than the D5. The α7S II offers still another stop improvement, so it should be about 3 stops cleaner than the D5.