On paper, the Sigma 24-70mm F2.8 IF EX DG HSM ($899 direct)($899.00 at Amazon) is an appealing alternative to similar full-frame zoom lenses. Available for Sigma, Sony, Pentax, Canon, and Nikon cameras, it's roughly half the price of the Canon, Nikon, and Sony takes on the same design. Unfortunately it's not as sharp as comparable lenses when shot at f/2.8, although it is a solid performer at f/4. If you're not looking for stellar performance at maximum aperture it's worth considering, but if you're an event shooter and outstanding performance at f/2.8 is a requirement, you'll want to take a pass on this zoom.

The lens is fairly squat, measuring 3.7 by 3.5 inches (HD), but it's a bit heavy for its size at 1.7 pounds. Compare this to the elongated design of the Nikon AF-S Nikkor 24-70mm f/2.8G ED, which measures 5.2 by 3.3 inches and weighs just under 2 pounds. The Sigma lens has a front element that accepts 82mm filters, there's no rotation, so using a polarizing filter is an option. It can focus as close as 15 inches and ships with a carrying case an a lens hood. There's no optical stabilization, but this is also true of the Nikon and Canon 24-70mm f/2.8 zooms. Sony and Pentax provide in-body stabilization, so you'll benefit from that system regardless of which lens you use.

I used Imatest to check the sharpness of the lens when paired with the 36-megapixel Nikon D800. At 24mm f/2.8 the lens is impressively sharp in the center, it exceeds the 1,800 lines per picture height we require for a sharp photo. Its center-weighted score is 2,346 lines, but its edges are a disappointment, the mean score is a downright fuzzy 745 lines, and there's significant purple and green color fringing at the edges of the frame. Stopping down to f/4 goes a long way to improve image quality at 24mm. It increases the overall sharpness to 3,050 lines, and edge performance is much better. Regardless of aperture, distortion is noticeable, but it's not a huge issue at 24mm. The lens shows 1.4 percent barrel distortion, which will make straight lines appear to curve outward. This can be corrected with ease in software like Adobe Photoshop Lightroom.

Image quality drops as you zoom in. At 50mm f/2.8 the lens only manages 1,432 lines. It's another story entirely at f/4; the score improves to 2,690 lines there and edges are sharp. There's 1.3 percent pincushion distortion, which makes lines curve in rather than out, but that's also manageable via Lightroom and won't be evident to the eye in a lot of shots. At 70mm the lens is soft again at f/2.8, scoring only 1,495 lines. Narrowing the aperture to f/4 improves the score to 2,599 lines. There's a little more pincushion distortion, about 1.7 percent. The Sony Carl Zeiss 24-70mm f/2.8($649.99 at Amazon) shows similar distortion characteristics and also loses some resolution as it zooms, but it manages to keep its score above 1,800 lines at f/2.8 throughout its zoom range.

If you're in the market for a fast zoom lens for your full-frame SLR, the Sigma 24-70mm F2.8 IF EX DG HSM is likely on your radar. It's priced aggressively when you compare it with the $2,000 lenses that Canon, Nikon, and Sony have released to cover the same range. If you stop down to f/4 you'll find the lens to be quite sharp and a good performer. But it doesn't offer that same performance at f/2.8. If you're an event shooter and need the lens to deliver sharp photos in all kinds of light, skip this one. But if you're willing to accept the softness at f/2.8 and use the lens at f/4 and smaller apertures, you can save a good deal of money.

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