The Sigma 85mm F1.4 DG HSM lens has a really wide focus ring. There are hard stops at both ends of the range, making it easier to set focus at infinity. Circular Polariser users will be pleased that the 86mm filter thread doesn't rotate when focusing.
When it comes to auto-focusing, the lens was a great performer, it was fast, consistent, and reliable on both my D800 and D810 bodies. I didn't experience any "hunting" when focusing in both good and bad light, with the lens focusing accurately on the majority of occasions. The main issue I had shooting at such wide apertures was actually subject movement. I also encountered a few occasions where the AF double faulted but it was actually due to error on my part; I had become so engrossed that I had moved too close to the subject. We shot in a really low light situation when Paul was at work and as you can see, the 85mm Art performed excellently.
The 85mm Art is also a super quiet performer, thanks to the built-in Hyper Sonic Motor. As a Pro the last thing I need when trying to build a rapport whilst shooting portraits of personalities (I hate the term celebrity) is a really noisy AF motor on the lens I'm using. I had to stop using my Sony 85mm G Master in that situation because it makes a terrible crunching noise when focusing. In saying that, I'm not missing my Sony 85mm GM at all as the Sigma 85mm Art kills it for sharpness too. To avoid the noise issue on my A7RII, I'm using the 85mm Art in Sigma fit via the MC11 adapter. Another bonus here is that the lens is very well-suited to video recording.
Chromatic aberration also known as “color fringing” or “purple fringing”, is a common optical problem that occurs when a lens is either unable to bring all wavelengths of color to the same focal plane, and/or when wavelengths of color are focused at different positions in the focal plane. This is typically seen as purple or blue fringes along high contrast edges in a photo. Chromatic aberration is conspicuous by its (almost complete) absence in the shots I've taken with the Sigma 85mm Art lens.
With the lens set to its maximum aperture of f/1.4, there is fair amount of light fall-off in the corners. Stopping-down to f/4 virtually eliminates this. Whilst some people might find issue with this, I actually quite like it as I often add a fairly heavy vignette to my portrait shots in post.