RICHARD BUTLER, DPREVIEW:
One of the D810's most significant features was its ability to shoot at ISO 64. There are two main factors that influence the Raw (saturation-based) ISO rating: efficiency and full-well capacity. Higher efficiency (a higher proportion of photons being registered) would push base ISO upwards, while an increased full-well capacity (capacity for electronic charge) pushes it down.
The D810 offered a lower ISO by increasing its full well capacity (or, at least, finding a way to squeeze a bit more out of it). This meant it was able to capture just as much of the light it was exposed to as the D800 but could tolerate more light before it started to clip. This meant it could be given greater exposure, which improves the signal-to-noise ratio and gives cleaner tones across the image: an improvement often interpreted as improved tonality.
This ability to tolerate an extra 2/3 EV exposure is why we say that the D850 is able to compete with the latest batch of medium format cameras, whose 44 x 33mm sensors would capture 2/3EV more light at the same F-number and shutter speed.
So does the D850 manage to repeat this trick? Yes, it certainly looks like it.
Read more, here.