Sigma 18-50mm f/2.8-4.5 DC OS HSM - £175
Remarkably inexpensive, the Sigma 18-50mm barely costs more than the humble Canon EF-S 18-55mm kit lens supplied with the Canon EOS 1100D and 600D. Build quality is better, with a more robust feel and a metal rather than plastic mounting plate.
You get a 4-stop optical stabiliser, which matches the Canon for performance, plus ultrasonic autofocus rather than a basic micro-motor.
Autofocus is quieter than with the Canon kit lens, but no faster. Neither lens has full-time manual focus override; however, at least the Sigma has a front element that doesn't rotate during focusing.
Rotation-specific filters such as a circular polariser are therefore much easier to use. During our tests, this lens suffered none of the autofocus inconsistencies of the Sigma 17-70mm.
Sharpness is pretty reasonable at the centre of the frame but a little lacking towards the edges and corners. Performance is respectable in terms of vignetting, colour fringing and distortions but, for overall image quality, we couldn't see any improvement compared with the Canon EF-S 18-55mm lens.
Ultimately, it's not really worth considering as an 'upgrade' but it's still a good buy if you need a standard zoom and you're on a very tight budget.
Read the full Sigma 18-50mm f/2.8-4.5 DC OS HSM review