Sigma 17-70mm f/2.8-4 DC Macro OS HSM - £350
With its generous zoom range and fairly fast maximum aperture of f/2.8, decreasing to f/4 at 50-70mm focal lengths, the Sigma looks an attractive proposition at the price. The effective zoom range works out to 27-112mm, which gives plenty of scope, especially at the telephoto end.
It also focuses closer than other lenses in the group, giving a maximum magnification factor of 0.37x at 22cm and earning a 'Macro' label. Optical stabilisation lives up to its 4-stop claims.
Barrel distortion is very noticeable in the 17-24mm section of the zoom range but colour fringing is well controlled. Vignetting is apparent at f/2.8 but disappears at apertures of f/4 or smaller.
The sharpness of our review sample proved a hit and miss affair. Sometimes the ultrasonic autofocus system was very accurate whereas, on other occasions, it focused in front of or behind the target.
Some Canon bodies, such as the EOS 50D, have an autofocus fine-tune feature but this wouldn't cure the problem because of the lens's lack of consistency. It's a shame because, in other respects, the 17-70mm is a very good performer and costs a bit less than the Canon EF-S 17-85mm kit lens.