This lens is part of Sigma’s EX “pro” series and the DG means it’s optimised for digital SLRs (but can be used with film bodies) and the HSM is Sigma’s answer to Nikon’s AFS and Canon’s USM high speed focusing.
At 1260g this is quite a heavy lens and is finished in a very slightly speckled matt black with a smart gold ring near the front of the lens. I’d have to say the black ribbing on the focus and zoom rings isn’t to my taste - it’s not the sort of thing you usually even notice on a lens but it was the first thing I noticed when I picked this lens up. The rings have a highly ribbed rubberised covering which although fine to hold and use, is just a bit more noticeable than average. The lens feels substantial and well made although it doesn’t have the smooth silky feel you might expect at this sort of price. The zoom ring is fairly stiff - possibly it will loosen off with use. A large removable tripod mount is fitted just to the rear of the zoom ring and Sigma supply a soft case and an excellent petal shaped lens hood in the box. In testing optical performance was found to be very good at full aperture and excellent even one stop down. By f 5.6 all results were superb.
The Sigma’s weaknesses are in it’s handling - the stiff zoom ring right next to the tripod mount meant my, admittedly very large, hands kept hitting the tripod mount to the extent that I eventually took it off to cure the problem. With the mount removed handling was fine but really you should have a tripod mount on this sort of lens so not an ideal solution.
Sigma’s HSM is a good answer to Canon’s USM high speed focusing but I’d have to say that it’s not quite as fast or accurate as the Canon system. At one stage during testing I was suspicious that the lens wasn’t focusing absolutely accurately so I shot some manual v autofocus tests. The manually focused results were slightly better but to see the difference - which was tiny - I had to look at the images at 100% or more on screen. As a comparison I then carried out the same test with my Nikon D70 and Nikon 80-200 2.8 and found that it was less accurate than the Sigma ! So I think the lesson in this is that even the best autofocus systems aren’t perfect - They’re now very good but they haven’t quite replaced humans yet.