Although (as at April 2018) it’s out of stock everywhere in the Nikon mount, I managed to pick up a Tamron SP 15-30mm f/2.8 from the ever reliable Samy’s Cameras.
Look, this is a very good lens, but so are the Nikon 12mm-24mm 2.8 and the Sigma 14mm-24mm 2.8.
All 3 have excellent colour rendition, are built like Swiss watches / German tanks and are as sharp as photographers (as opposed to techy geeks) need in the corners of full frame cameras.
Remember, that as they’re all zooms, at virtually all apertures you’ll inevitably get smearing and distortion in the corner depending on the subject (far off landscapes tend to be the worse offenders).
If you need really really clean corners you have to go prime.
Some may crave a few extra mm’s on the wide end or want to easily use filters, but apart from that they’re mostly identical.
Except for vibration control.
As camera resolution increases, the latitude for errors introduced by hand holding decreases exponentially. In the good old film days, the formula (1/focal length) gave a rough guide as to the shutter speed needed to give a sharp picture.
This is no longer a safe rule of thumb, especially for high resolution cameras.
For the D850 think 3x or even 4x the focal length. I would suspect that 75% of D850 users waste the huge resolution potential of their beasts by hand holding them at too low shutter speeds.
Only one of the above mentioned lenses have vibration control. So unless your wide angle lens is going to spend most of its time on a tripod with filters attached, there is only one sensible choice – the Tamron SP 15-30mm f/2.8.