Yes a lot of vignetting, what would be a good replacement for the Sigma?That Sigma Lens is a DC lens. DC = Crop sensor, and DG = Full frame in Sigma lingo... I would imagine that at the wide end of that lens on a D850 you would expect to see a lot of vignetting.
Bear in mind your current DC Sigma is, roughly, a $300 lens. Finding an equivalent at that price point probably isn't going to happen.... the Sigma 12-24 DG is a very highly rated full frame lens.
Guys thanks for your help, on the DX body i just found the 17/50mm a great walk about lens, i now know that to get something to suit the D850 in around that range is gonna cost a lot of money. i need a wide angle lens for landscapes first, so i will use away at the Sigma for a while.