Update 2018-11-03: turns out that the 1.1mm figure from KolariVision for the sensor cover glass does not include the expoxied-on glass portion of the sensor. So the real figure is ~2.3mm total glass thickness, not 1.1mm (Nikon D850 is about ~2.5mm). Still 2.3mm is notably more thin than the 3+ mm for Sony mirrorless. I’ll have to see what actual testing shows.
Another unique aspect: the cover glass of the Z7’s sensor is about half a thick as the Sony’s, coming in at 1.1mm. This means the Nikon will have better performance with adapted lenses right out of the box. However, with our ultra-thin conversion measuring down to 0.2mm, the camera will still benefit a great deal from a modification service if you’re a Leica lens (or other legacy glass) enthusiast.
The thinner glass means that the MTF (micro contrast and astigmatism in particular) degradation will be much less than on the Sony A7R III, suggesting that M-mount lenses (Leica M and Zeiss ZM) may perform reasonably well. Though a high refractive index cover glass cannot be ruled out AFAIK.
See ray angle and Zeiss ZM 35mm f/1.4 MTF on Mirrorless Cameras for a full aperture set of MTF graphs for the Zeiss ZM 35mm f/1.4 Distagon. It would be wonderful if the ZM 35/1.4 and my Leica M lenses worked well on the Z7 (assuming an adapter materializes).
The graph below is a through-focus MTF graph which indicates the displacement of the blue-vs-red wavelengths for two different thicknesses of sensor cover glass. Without sensor cover glass (e.g., film), the red and blue MTF are close to alignment.
With 2.5mm sensor cover glass, the two diverge substantially, so that peak focus can be achieved for only red or blue, but not both. Moreover, MTF for red wavelengths is degraded slightly and MTF for blue increases slightly.