It's not often that an independently manufactured lens costs more than some camera manufacturers' own-brand rivals, but this Sigma is a bit special. Unlike the Canon and Nikon lenses on test, the Sigma 17-50mm boasts a fast, constant aperture of f/2.8 that remains fixed throughout the zoom range.
That's quite an achievement for a lens that also includes optical image stabilisation. Other finery includes two of Sigma's FLD glass elements, which the company claims provides the very best low-dispersion performance to reduce chromatic aberration while enabling maximum light transmission.
The HSM (HyperSonic Motor) autofocus system fitted to this lens isn't Sigma's best, because there's no full-time manual override. Unlike many Sigma HSM lenses, the focus ring rotates during autofocus, so you have to be careful not to foul the action of the focus ring in handheld shooting.
A drawback of many fast lenses is that images lack a little sharpness and contrast when shooting at the maximum aperture, but the Sigma proved amazingly crisp at f/2.8 throughout.
Distortion is well controlled, and while there's a little colour fringing, it's only really noticeable around very high-contrast edges. There's also a distinct lack of vignetting, with excellent peripheral illumination in the corners of even wide-angle images.
The optical stabiliser works well with most Pentax and Sony bodies, but you need to turn off the camera's own sensor-shift stabiliser to avoid the two systems conflicting with each other.
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