This is a Quick (non scientific) High ISO Test between Sony A7s, A7II, Nikon Df, and Sony A6000
Lenses used in the test : Sony/Zeiss 35/2.8ZA on A7s A7ii, Nikon 35/2.0 AF-d on Df and Sony/Zeiss 24/1.8ZA on A6000. All Shots taken handheld.
A7s-A7II-Df-A6000 ISO 25,600
For Some Reason, the A7s has some Focus Issues. It has the slowest AF among the rest. I would say that the A7s is usable up to 51,200, occasionally up to 80,000 ISO and even 102,400 ISO (check the real world Samples below).
the A7II could be usable up to 12,800 ISO
Nikon Df could be usable easily up to 51,200
Sony A6000 was positively surprised me in this test… as you can see, 12,800 is very usable.
A7s Real World Samples at high ISO
I will leave the conclusion to you …