1Dx is at least 1 stop better for noise compared to 5D3.
Is that true - DxO rates the 1DX only 0.28 stops better (http://www.dxomark.com/Cameras/Compare/Side-by-side/Canon-EOS-5D-Mark-III-versus-Canon-EOS-1Dx___795_753 )? And Ken, bless his heart, Rockwell, rates both very similar ( http://www.kenrockwell.com/canon/comparisons/5d-mkiii-vs-1dx.htm#iso )? Perhaps someone who has both could enlighten.
I think Ken Rockwell says the 1D X is half a stop better than the 5D III, although someone with both cameras like FEBS might know better.
He doesn't say that the 1DX is half a stop better, he says that Canon is cheating by having the real iso on the 1DX half a stop lower than the reading it is set at, relative to the 5DIII, so you think it is half a stop higher than it really is.
My 5D Mark III is about a half-stop faster at any given ISO setting than my 1D X.
In other words, at any given ISO, my 5D Mark III gives the same image with a half-stop less exposure. In other other words, my 5D Mark III actually runs at a half-stop higher ISO than marked, or my 1D X is actually running a half-stop slower than marked, or somewhere in between.
It's common for camera makers to cheat a little here, since Canon needs to make the 1D X look like it has better high ISO performance than the 5D Mark III. In this case, it's stacked the deck a little, since when anyone compares at the same indicated ISOs, the 1D X appear to have a half-stop advantage since it's really only operating at a half-stop less ISO than the 5D Mark III at any given ISO. When comparing images, you won't notice unless you shoot at the same manual exposure, in which case my 1D X is a little darker than my 5D Mark III, or if you get both to match, you'll notice that your 5D Mark III only needed about a half-stop less exposure."