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Conclusions (Google Translated)
It seems that Nikon has managed to improve image quality with high ISO settings. This is especially noticeable with ISO 12800 and 25600 (Nikon D810 Hi1). However, the digital noise pattern is quite different than that of the Nikon D810. Despite the less color noise, the Nikon D850 has a slightly more pronounced Luminance noise, probably due to a completely new light sensor and a different way of processing and sharpening the image. We will be able to get the right comparison as it will be possible to compare the photos in RAW format. At ISO 6400, the Nikon D850’s image was slightly sharper, which was because of my error, it was fiercely slightly out of the comparison point.
You may also take a look at D850 Vs. D810 features & specs comparison here.