Originally posted by nater Where are you getting this from? Everything I read says 3dB = 1 f-stop. For instance:
I'm reading the graph. The horizontal axis grid markers are one stop. Look up the A7S at a given fidelity - for example 30 dB. I think you'll find that the ISO value is 3702. Do the same for the A7R, and I think you'll find that fidelity at 2746.
3702/2746 = 1.348, or about a third of a stop.
(I chose 30 dB because I think that's how DxO evaluates it's high-ISO capability of a camera, and it also seems, to me, to be a reasonable limit for a 8x12 print).
The slope you've quoted - 3dB / f-stop - is just an approximation based on lots of cameras. It might've been more true a few years back. Right now the A7S has 45.1 dB at ISO 80 and 39.9 at ISO 329. Let's call that two stops, and 5.2 dB of difference. You might call that close enough but that's decibels are a logarithmic scale, so it's quite a difference.