See my growing review of the Nikon D850—to expand over time.
My Nikon D850 has been back for a while from Nikon service, but because of the Fujifilm GFX + 45/2.8 + Sony A7R III plus a miserably stressful and enervating 3 days of evaluating health care for 2018*, I have not had time to verify all aspects of its flange focal distance behavior, nor to carefully evaluate the symmetry corner to corner.
* That health-care plan evaluation time cut out my field work by 3 days, a great source of frustration being perfectly positioned with ideal weather conditions, but it could not be put off due to the Dec 15 “open enrollment” deadline.
I have nothing but terse notes on the repair slip from Nikon, which turned around my D850 in one day, which I appreciate as excellent service. As for the issues I raised, I do not expect that any comment or clarification from Nikon on any of these concerns will be forthcoming, as I predicted last month—unfortunate, but I’m “moving on”.
But here is one thing I can state clearly: I see no change in the flange focal distance at 24°F; the Zeiss 18/2.8 has scarcely 0.5mm of focus throw left at that frigidity (way past the infinity mark), just as I reported last month, and, also, an authority of impeccable credentials confirms my findings but I cannot name or quote that expert here, regrettably.
Things are what they are. As the D850 has so far not prevented me from getting to infinity focus (except perhaps in the mid zones for the 55/1.4 Otus), I am going to move on and accept the behavioral change. (which is not to say I will let the matter rest if and when I gain more insight).
The D850 is a very fine camera clearly a step up from the Nikon D810 in most all respects. As I’ve said before, I consider the D850 the finest DSLR ever produced, in haptics/ergonomics and image quality, excepting the too-small 4-way controller which makes use with medium-weight gloves difficult.
One more point for those shooting old Nikkors and the like: have no concern.
The infinity focus issue (flange focal offset distance) appears to be provoked by lenses having outstanding correction for color, which means special types of glass that are sensitive in refraction characteristics to temperature. Older lenses have little or no such glass, so breathe easy if that is the concern. Wider focal lengths with high degree of color correction seem particularly susceptible, longer ones less (e.g., 100mm or 135mm).