This is just a quick test between the Canon 85L f/1.2 and the Sigma 85mm f/1.4. The images were done in jpg and cropped. I used ISO 800, with ambient lighting. The camera was setup on a tripod and triggered remotely.
I found that the Sigma performed admirably against one of the holy trinity, the fabled 85L f/1.2 II. The Sigma to me was sharper at all f stops. I just tested it to f/2.2, because you buy these lenses for their wide aperture. For $899 vs the Canon’s $1800-2000, it gets the job done. I can live without that extra 1/2 of a stop. Again this is just IMHO.
PDF (small file, compressed images)
MS Word Document (Larger file)