The ES-2 is mounted on the lens using an adapter, right? So the critical parameter is the working distance (i.e. distance between front of the lens and subject) and not the distance between the sensor and the subject, right?
Correct. The only helpful information I have seen is on the Nikon USA site in the form of Nikon's answers in the Product Forum for the ES-2 (
http://www.nikonusa.com/en/nikon-products/product/miscellaneous/es-2-film-digitizing-adapter-set.html#tab-ProductDetail-ProductTabs-Forum). That does not include physical specifications, but assuming it is true that the ES-2 mounts on the 40 DX without an adapter it itself must be about 16cm long, closest focus for the 40 DX. The Apo Rodagon 75mm is said to have a working distance of 11.8 cm at 1:1 (
http://coinimaging.com/aporod75-4var.html) so you won't get 1:1 with the ES-2 unless you take a slice out of it and re-join the ends.
Another issue would be the filter threads, which are 62mm on all three of Nikon's designated lenses, so presumably the ES-2 is also 62mm. Adding step-up rings would add length.
If you don't get 1:1, all that means is wasted pixels, which in the case of the D850 is not likely to be a major problem. You might lose some of the speed advantage getting accurate focus. Of course, the accuracy of focus with the ES-2 depends on how well made Nikon's holders are and how well they fit the slot in the ES-2, which we don't yet know.
Nikon's price for a plastic tube with a threaded end is outrageous, so I am going to wait for the $5 copies to appear. With a bit of luck an enterprising person might even make one to digitise medium format negatives.