The 70-200mm F2.8 OS is somewhat unconventional in design for a lens of this type. Most notably, the zoom ring is positioned towards the front of the barrel, with the unusually slim manual focus ring immediately behind it (all other fast telezooms place the zoom ring towards the center of the barrel, with the focus ring up front). The switches that control the autofocus and stabilization systems are in the usual place, though, inhabiting a prominent panel on the left side of the lens.
Cosmetically, the lens also represents a departure from Sigma's EX designs of the past decade. The relatively slimline barrel mainly eschews the familiar crinkle-effect coating in favor of an understated matte-black paint, although the old finish is still found on the zoom and focus rings and the tripod collar. The overall affect isn't quite as harmonious as before, although it's not unattractive.
The construction quality is difficult to fault, given that the price is substantially lower than the Canon and Nikon equivalents. Those matte-black barrel shell components are plastic, not metal, but really little the worse for it - the lens feels robust with a high standard of fit and finish. Both the zoom and focus rings are smooth in operation, and the large autofocus and optical stabilization switches operate in a positive fashion.
Last but not least, the tripod collar design is inherited from the old unstabilized 70-200mm F2.8, which is welcome because as far as we're concerned it's pretty much the best in class.
On the camera
Like all lenses of this type the Sigma is a sizeable beast, and handles best on larger bodies. It's perhaps most at home on professional DSLRs such as the Canon EOS-1D(s) or Nikon D3 series, and it positively dwarfs the current breed of compact bodies, illustrated here by the EOS 550D. Handling on this kind of small camera is hugely improved if you add a vertical grip, especially if you do a lot of portrait-format shooting.
The most unusual aspect of the handling is the zoom ring, which is situated right at the front of the lens. In practice we've found this works perfectly well, and effectively eliminates any need to rotate the tripod foot out for the way for hand-held shooting. But if you're used to grabbing the ring at the front to tweak focus, you'll have to re-train yourself for this lens.
However, there is one small caveat. The 70-200mm comes with a huge hood, and when reversed this obstructs all of the controls; most importantly the zoom ring becomes completely inaccessible, however both the focus ring and the AF and OS mode switches are partially blocked too. So if you pull the lens out for your bag for a quick grab shot with the hood reversed, you won't be able to zoom (uniquely for a 70-200mm F2.8). For many users this will be a complete non-issue, but for a few it could be a concern.
This lens features Sigma's ultrasonic-type hypersonic motor for autofocus, which performs extremely well. It's practically silent in operation, and we found it to be fast and accurate in everyday use with no evidence of any systematic focusing errors using a number of Canon test bodies (including the EOS 5D Mark II and EOS 7D). We also tried out the lens's continuous AF capability by shooting fast-moving track cyclists using the EOS 7D at 7fps, and found AF tracking to work very well, with an impressive return of in-focus 'keepers'.
As usual, though, it must be noted that focus speed and accuracy is dependent upon a number of variables, including (perhaps most importantly) the camera body used, subject contrast, and light levels.
Change in angle of view on focusing ('focus breathing')
The Sigma 70-200mm F2.8 OS gives a wider angle of view on focusing closer, in a somewhat similar fashion to Nikon's 70-200mm F2.8 VR II, although to a lesser extent. By our reckoning, at closest focus the image field is about 1.1x larger in each direction, making the effective focal length roughly 180mm at 1.4m. A side-effect of this is the maximum magnification of 0.13x, which is slightly low for this type of lens.
Lens body elements
Reported aperture vs focal length
This lens allows an aperture range from F2.8 to F22 at all focal lengths.