August 2015 - A little low light comparison of the Sony A7RII 4K Super35 crop mode to the Sony A7S in fullframe mode.
The A7RII was set to Super35 (APS-C) crop mode and internal XAVC-S 4K UHD recording with 100 Mbps, lens was the Zeiss FE 2.8/35 @ f/4.0.
The A7S was set to 4K mode with external recording to Atomos Shogun (ProRes HQ 4:2:2), lens was the Zeiss FE 1.8/55 @ f/4.0 (providing about the same viewing angle as the 35mm lens in crop mode).
In order to evaluate the details, please download the original file here: https://vimeo.com/135731718 (Download -> right-click on "Original 3840x2160 / 548 MB" - choose "save as...").
You will notice, that both cameras produce a very clean 4K image at ISO 2000. The image from the A7RII shows some more detail due to its downscaling from a 15 MP sensor readout. At 25600 ISO there is a slight advantage for the A7S. Regarding noise, the A7RII is not far behind the A7S but the A7S is able to keep up more details at very high ISO.
Picture profile used for that test was "Cine gamma 4" with "Pro" gamut.
The fllowing aerial video was recorded in 4K (full frame mode) at XAVC-S 100 Mbps. You can download the original 4K video file at Vimeo (Download -> right click on "orignal 3840x2160" -> "Save as..."):
It is available on YouTube as well, providing an option to stream the 2160p and 1440p version directly to the player: http://youtu.be/zAR5K3HZu0A
For evaluation purposes I recommend to download the original 4K 100 Mbps rendering from Vimeo due to the loss in detail after the reencoding in YouTube.