I shoot with a Nikon 850 and have mostly nikon lenses, but after much looking, I purchased the Sigma 12-24 2.8. It is an excellent lens and the reivews for this len are beter than the comparable Nikon len
The extra mega pixels with the 850 let me crop and picture is still sharp. There a lot of times I can not get in the right positon for the picture, but croping helps.