I've been really curious about the Sigma 50mm F1.4 DG HSM, aka: the 50 Art lens. I've read a lot of reports about autofocus problems, but wanted to check it out for myself.
I rented a copy of the Sigma 50 Art from LensRentals.com. Before I ordered, I emailed their customer service to ask if they used the USB dock to do any kind of calibration with the lens. They replied that they take every 50 Art that comes back into the shop, put in on the USB dock and reset it to the default values. That sounded good to me, as I planned to use the microfocus adjust feature on my 5D3 to make any needed changes. I was planning to do an outdoor photoshoot where my models would always be roughly ten feet from the camera, so I did not feel like I'd need the USB dock to adjust the focus for different distances.
Upon receiving the lens, I did my usual autofocus calibration. This time only at ten feet. Using the center focus point, the Art lens was pretty consistent on my 5D3. I settled on +3 MFA value and was pretty pleased with the stable focus from shot to shot.
Then I tried using other focus points. I found that as you move away from the center point, the lens will front focus more and more. Using the outermost horizontal points, the front focus was about two feet or a 20% error. The next pair in toward the center front focused about one foot or 10%. Using the top and bottom points, the front focus error was about 1 foot or 10%.
When the lens was in focus, it was very sharp, so I went ahead and did the planned outdoor photoshoot. I used mostly the center point, but occasionally moved the focus two spots left or right. Used within this restriction, I was very happy with the sharpness, bokeh and autofocus accuracy.
I did a few shots with my old Sigma 50 Classic, which was autofocusing pretty well that day, since I fed it a shot of cheap gin before leaving the house and said some magic words over it. The Art lens was definitely sharper, but not stunningly better. They both created very nice bokeh of the distant trees when used wide open.
Here is a shot with the Art lens at F/2.0:
Here's a shot with the Sigma 50 Classic at F/1.4:
These are just fun shots to give you an idea of what kind of photoshoot I was doing, they don't really prove anything.
One thing I learned on this shoot was that the DOF at 1.4 is even more shallow than I thought. Shooting two models standing side by side you have to make sure they are both exactly the same distance from the camera. Unfortunately couples naturally want to move around. I had a lot of shots where one model was in perfect focus and the other was slightly out of focus. I was kind of expecting that and shifted focus back and forth between the two models, so I got plenty of usable photos.
In the future, if I'm not going for the absolute maximum bokeh, I plan to use my Sigma 50 Classic primarily at F/2 on these kinds of photoshoots.
One more item... Before sending back the Art lens, I attached it to my old 7D backup body and ran it through my usual autofocus test. It was difficult to come up with a precise AFMA number because the focus was not very consistent. I settled on +12. I tried the outer focus points to see if the 7D showed the same error pattern as the 5D3. It did not, but the general accuracy and repeatability was poor enough that I would not venture a guess as to what the real pattern is. Basically, I would not want to use the Art lens on my 7D unless I stopped it down to about F/4 or so. As a control, I put my 40 pancake lens on the 7D. It focused better than the Art lens, but still not as well as I wished. I don't know about your 7D, but mine has never had a very accurate AF system, which is one reason I rarely use it now that I have a 5D3.
As a result of my experience, I have decided not to buy a new 50mm lens. I will keep using my Sigma 50 Classic and wait and see if Canon comes out with a new 50mm lens in the 1.8 to 2.0 range. If it is sharp wide open, it would be the perfect lens for me.
Update: I turned this photoshoot into a blog post that includes some additional photos taken with the 50 Art lens. Some are mildly naughty, so consider yourself warned.
Check it out here: http://www.beyondboudoirphoto.com/blog/2014/10/outdoor-photoshoot-for-a-portland-couple