Thanks for that link. I'm going from of this website
http://pdnpulse.pdnonline.com/tag/medium-format that says "
According to Canon, the cameras will offer a 4.14 micron pixel pitch, giving them roughly the same pixel density as the new 7D Mark II. However, the new 5Ds and 5Ds R won’t offer the low light performance of either the 7D Mark II or the 5D Mark III — instead, they’ll top out at a native ISO of 6400, with a high setting of 12,800." Camera makers used to tout their low lux rating for their cameras, it would be nice if they did that using a standard lens at a standard ISO as proof for comparison. Unfortunately I don't own both these cameras, so that's why I posted. That would be an easy comparison for dpreview to do. But what's in theory is not always observed in practice.
Also from another site,
Where the 5D Mark III has a 24.2-megapixel sensor, the two newer models have 50.6-megapixel sensors. Shots taken with good lighting at low ISO settings will offer a staggering amount of detail.
However, it’s not an entirely positive change. Both cameras have a full-frame sensor, meaning that the photosites of the 5DS and 5DS R are much, much smaller than those of the Canon EOS 5D Mark III. You get 6.25 micron sensor pixels in the older camera, and just 4.14 micron pixels in the new ones.
Canon doesn’t even try to pretend that performance will be the same when those sensor pixels are stretched with poor lighting conditions and high ISO settings. This is pretty clear when we look at their respective ISO ranges. The standard ISO range of the older Canon EOS 5D Mark III goes up to 25,600 while there are ‘non recommended’ 51,200 and 102,400 modes for when high sensitivity is paramount. Like the Nikon D810, the 5D MKIII is a superb low-light performer, letting you shoot handheld even at dusk or night without ending up with blurry or terribly noisy results.
The Canon EOS 5DS and 5DS R are nowhere near as capable in lower light, despite being three years ahead. Their upper standard ISO range takes a huge dive from 25,600 to 6400, which can be extended to 12,800.