I ran a similar series of bookcase pictures as the ones I posted earlier today, only with 3 stops higher ISO settings.
I put a Zeiss 35 mm f/2 Biogon ZM on the Sony a7S, using a Novoflex adapter. I put the camera on a big RRS carbon fiber tripod. I set the ISO to 25K, the aperture to f/8, focused with live view, and set the shutter speed for an ETTR exposure. I took a picture:
Then I changed the ISO to 12K. I did not change the f-stop or the shutter speed. I took another picture. I continued that pattern all the way to ISO 800.
I developed the images in Lightroom 5.5 with default settings except for all noise reduction off. Then I set the Exposure control on the ISO 800image to an extra +5, the Exposure control on the ISO 1600 image to an extra +4, the Exposure control on the ISO 3200 image to an extra +3, the Exposure control on the ISO 6400 image to an extra +2, the Exposure control on the ISO 12K image to an extra +1, and didn’t adjust the Exposure of the ISO 25K image. I white balanced the ISO 25K picture and set all the others to that WB.
Here are the overall pictures, less the ISO 25K one above:
Here are the crops:
The binding of the black book is noisier and brighter at ISO 1600 and ISO 800 than in the other images. I think this is because of lower sensor-refered read noise at ISO 2000 and above. I attribute this to the conversion gain change that I think is happening as the ISO is changed from 1600 to 2000.
If, as I suspect, Sony is using the Aptina switched capacitor conversion efficiency change technique, it’s working well for them.