Sigma 18-250mm f3.5/6.3 DC OS HSM Lens Review

Sigma introduced this lens in 2009, as an upgrade to its 18-200mm lens. The lens comes in Sigma, Canon, Nikon, and Pentax mounts, and it includes optical stabilization (as denoted by the "OS" in the lens' title). It is slightly cheaper than most of its rivals, but this is a lens that is trying to cover a huge focal length. Does it succeed? 

Pros

• Quiet and speedy autofocus
• Low levels of distortion for this kind of lens

Cons

• Soft corners at nearly all settings
• Ineffectual image stabilization at telephoto end

Sigma's 18-250mm lens is made from fairly high quality plastic, with a metal lens mount. The plastic helps to keep the lens fairly lightweight, and it's also pretty compact, especially when you consider its large focal length range.

The lens comes with a very useful zoom lock button, which does an excellent job of preventing the zoom from creeping forward. The filter thread doesn't rotate during focusing, which is very useful, and both the zoom ring and manual focus ring are easy to grip and use. It's a nicely made lens.

Autofocus

This lens benefits from Sigma's Hypersonic Motor technology. It's nearly silent, and it's extremely quick and accurate in its focusing, even at full telephoto. It's certainly an improvement on Canon's rival offering, the EF-S 18-200mm, and the Sigma 18-250mm lens is perfect for grabbing quick photos of moving subjects.

There's a major issue with this lens, and that's the significant drop off in sharpness at the telephoto end of the lens, particularly at 135mm. Additionally, at almost all focal lengths, it has softness in the corners. Inexplicably, it's at its best at around 24mm, with an optimal aperture of f5.6.

The lens has some chromatic aberration, but no more than should be expected in a zoom lens of this class. Impressively, there are very low levels of distortion. The optical stabilization only seems to work effectively at wide-angles, and it lets the lens down at the telephoto end (where it is needed most). Coupled with a maximum aperture of f6.3, you may find yourself struggling for enough available light at telephoto. 

In Conclusion

If you're after pin sharp images, this isn't the lens for you, as other super-zooms perform better in terms of sharpness. But the Sigma lens has impressively fast autofocus and very low distortion. For those not wanting to produce enormous prints, this could be a valuable addition to a kit bag. As with all super-zooms, the big draw is the ability to carry only one lens to perform a multitude of tasks. In this, the Sigma is an ideal choice.