Sigma 18-250mm f/3.5-6.3 DC OS HSM - £400
Not quite identical twins, the Sigma 18-250mm has almost exactly the same dimensions and the same 72mm filter thread, zoom lock and general finish as the Sigma 18-200mm f/3.5-6.3 DC OS. There are some important differences, however, apart from the extra 50mm length at the telephoto end. For starters, the 18-200mm is available only in Canon, Nikon and Sigma mount options, whereas the 18-250mm adds Pentax and Sony options to the list.
The 18-250mm also has a quieter HSM autofocus system, although it's only the basic version, which isn't particularly fast and doesn't have manual focus override. This also means that the focus ring rotates during autofocus, but it's smaller and further away, at the front end of the lens barrel, so there's less chance of your fingers fouling its action.
The 18-250mm lens features Sigma's latest-generation optical stabiliser, which gave between three and four stops of anti-shake benefit in our tests. It was almost as good as the Canon, Nikon and Tamron lenses, which all gave a slightly more consistent four-stop advantage.
Distortion is quite well controlled, which is impressive considering the extra-large zoom range. Sharpness is less so, being mediocre at 18mm and going downhill all the way to 250mm. Colour fringing is quite pronounced towards the edges of the frame, particularly at the shortest and longest focal lengths.
Centre sharpness across the focal range is good compared with the other Canon-fit lenses, but it drops off steeply towards the edges.
Fringing across the frame at all focal lengths is minimal, but at 18mm there's some mild blue fringing towards the edges.
The Sigma lens has less barrel distortion at 18mm than all the other lenses on test and is very easy to correct.
Image test verdict
The Sigma 18-250mm has the least barrel distortion in the test and only minimal fringing. Sharpness is good, but it does suffer towards the edges.