Even at f/5.6, in daylight the light reaching the sensor/meter is much greater than when shooting indoors or twilight conditions. Unlike the DOF preview of SLRs, the stopped-down lens does not need to result in a dim viewfinder, the EVF can compensate and give a bright view. Stop-down metering in theory could be more accurate since there is no discrepancy between the preset aperture and the actual aperture. Stop-down shooting also eliminates focus-shift which may occur between full-aperture and stopped down.
The disadvantages of stop-down shooting is that the DOF becomes greater so it is harder to distinguish between perfectly-in-focus and nearly-in-focus. When shooting in dim conditions with small apertures, the amount of light may be too small for accurate metering, and the EVF becomes noisy.
Pardon me for saying this bluntly, but: No way. Unfortunately things are not that easy. During the couple of hours I was able to try out the Z6 and Z7 models, I naturally also tried what happened when I mounted an AIS lens - by the way the Nikkor 35mm 2.0 AIS. The lens of course mounted fine and it would meter - so far so good.
However, it turned out to be far from good, precisely because of the stop down metering. When I stopped down to F/5.6 and further down, it was disspiriting to see how the electronic viewfinder image lost in dynamic range and quality. In other words, if the system fails to provide for metering at full aperture the compensation by the electronic finder necessarily leads to reduced quality. As a result, a stopped down image is in no way superior to an optical image through the D750 and D850, even in available light conditions. It is inferior, and this reality should be spoken about openly so that owners of the many good to brilliant manual Nikkors can make an informed choice. I now have first hand experience from both, and it simply is mistaken to believe that the electronic viewfinder's compensation for darker conditions means that all is well.
It would seem it is a matter of psychological perception rather than a realistic assessment if we decide that the EVF is generally superior to the OVF. What is true, is that the OVF becomes darker in dark conditions. But that does not imply that it is inferior. Darker, but still clear - and the DOF preview which requires the system to offer metering wide open is an advantage that in my opinion is as least as important as the focus shift. Focus shift is a real problem in macro shooting, but the thing is that the optical finder has been completed today by Live View which solves and eliminates the focus shift problem without the need for abandoning the optical viewfinder.
Modern cameras have been increasingly able to focus in dim light. Just think about the focus peaking feature of the D850, which is a present pinnacle of camera performance in available light.
But to conclude with the original point: There is no way around it: Stop down metering is not progress, it means a factual regression back to the (darker) days before metering at full aperture was invented. Metering at full aperture should be considered a matter of course, and was and is true progress.
