therwol Loc: USA

I did some research into how this ring is attached. It is attached to a thin ring that goes under the barrel of a lens. The focusing ring doesn't just slip off of something underneath that you can turn easily. The screws that attach the ring can only be accessed from under the barrel, and this would require considerable disassembly of the lens. If you still have the pieces shown in your photo, do you feel comfortable gluing them back in place, maybe with super glue? I think that paying someone to take this lens apart and then put on a new ring, which may not even exist at this point, would be more expensive than buying a new (used) lens on eBay. I suspect that the focusing ring is the Achilles heel of this lens and that it's going to be very hard to find one.

But for 30 bucks, you got a very sharp lens for general photography.

JohnBoy5562 Loc: Alabama

that is what I wanted do but it is broken into three pieces and when I bought I didn't not know it was broken until I got home and I only have two parts of the ring. I email the person I bought it from to see if she could find it. so far she hasn't replied.

therwol Loc: USA

If you can't get the third part of the ring, you might consider gluing the other two pieces opposite each other. There would be gaps, but you might have enough to make manual focus viable. By the way, I believe that the D850 should be able to focus stack with this lens if you can set the initial focus.

Thinking about this, I'll bet the person who sold you this lens knew that the focusing ring was broken and figured that at the price you paid, you'd let it go.