I should explain a bit more about the above setup, and why I didn't just purchase a factory made unit.
In the picture above, you see a white tube connecting the camera/lens to the slide-holder (in this case). In use, the translucent white tube would be illuminated to add "bias light" to the front of the slide being copied. Bias-light is important in controlling the ratio between the dark and light parts of the image, essentially how you control contrast in the system. (Similarly you can control contrast of a TV or Monitor screen by adding lights in front of the screen.)
The Nikon ES-2 Film Digitizing Adapter lacks any method for introducing bias light, so you'll need to rely on some post-processing to control contrast from the capture.
This may partially explain why PetaPixel said; "Sadly, the digitizer mode in the D850 has some real problems. The biggest problem is that the curve applied to the files when converting to positive is way too aggressive in setting white and black points. It clips one or the other (or both) in nearly every frame that I tested. In some images, this was not a huge problem. While this is annoying to lose some shadow detail, I could live with blocked up shadows in many instances."