My expectation is that the Nikon D810 sensor will be of the same generation and quality as the Pentax 645Z sensor, both being made by Sony is my understanding. With only a slightly smaller photosite size (4.88 microns vs 5.3 microns, ~20% in area), I’d expect the D810 to offer per-pixel quality not too far off from the 645Z. Elecronics matter too, as does the quality grade of the sensor, so that is all a little rough. But what really does matter is lenses, and there, Zeiss Otus on Nikon has few if any equivalents on Pentax. Lenses are big deal for any system.
So I hope to get out in the field with both by the end of this month, and do some field work side by side. But first, I’ll nail down some stuff on my temporary home-grown test scene, which offers an effective proving ground. The main issue is how to match field of view; there is no good equivalent focal length match for a 90mm on the Pentax 645Z. Reader Andrej K points to the Sigma 70mm f/2.8 EX DG Macro, but it’s no match either (66mm needed for a D810 crop to 4:3 or 75mm needed for a width match). Testing for a 4:3 match shows an 8% difference—too much.