Summary: On paper this lens look like a winner. The specification for the price is very good indeed. Although the resolution in the centre is usually good, the same cannot be said about the quality towards the edges, especially at wide apertures.
Pros: Excellent performance in the centre between 28 and 50mm when stopped down, Effective Optical Stabiliser, Compact and lightweight for an f/2.8 zoom
Cons: Lens really needs stopping down for best results, Sharpness towards the edges,, Focus ring cannot be disengaged
Conclusion: The distinctive rattle when we first lifted the lens up to the camera body made our hearts sink. Thankfully our initial fears were unfounded, as shooting at all focal lengths during our time at a local market showed impressive image quality throughout the frame, and reinforced the solid results we'd already found when we shot our test chart in the studio. With its near-silent focusing you could easily shoot away discreetly without anyone hearing a thing.
Summary: The Sigma 17-50 mm 2.8 EX DC OS HSM feels solid and is an alternative to the standard kit lens of Nikon. The zoom range of the Sigma is limited but the luminosity is much higher than of the kit lens. Using this lens on a 24 megapixel Nikon D3200 camera improved the resolution significantly in comparison with the Nikon D7000. The resolution in the center is high but is somewhat disappointing in the corners at 17 mm and 28 mm.
Pros: High resolution at the center, Fast, constant f/2.8, Limited vignetting, Low chromatic aberration, Feels solid
Cons: Low resolution in the corners at 17 mm and 28 mm, Sensitive to backlight, Limited zoom range, Steep purchase price