The Sigma 18-125mm f3.5-5.6 DC lens is roughly equivalent to the very popular 28-200mm range with 35mm film cameras, so although the numbers may seem unfamiliar, this lens covers pretty much all you’ll need in one lens. While that always sounds like a great idea it usually comes with fairly large performance drawbacks, as, in general, the wider the zoom range of a lens, the lower the image quality. However, over the last few years the difference, especially if you only take pictures in good light, has got much smaller. The use of moulded plastic aspherical elements that lens designers of 20 years ago could only dream of, has made lenses like this possible. So if you want something better than the kit lens, and with a wider range, is this a good alternative?
The Sigma 18-125mm f3.5-5.6 DC lens is designed for use solely on digital SLR cameras - it can’t be used with film cameras or full frame sensor DSLR’s. This lens is aimed at the Digital Rebel / Canon 450D / Pentax / Sony market. The lens feels very light and plastic - Similar to most kit lenses, and like them, although all the controls work smoothly, it doesn’t really ooze quality. On the plus side, as always, Sigma do give you a decent lens hood in the box.
Optically the performance is fine if you’re looking for something a step up from a compact and don’t want a bag full of lenses. In fact performance is similar to the Canon 18-55 EF-S with fairly good central sharpness but a very obvious fall in resolution away from the center of the field. Even in the middle of the range at f5.6 the corners are fairly soft. One other drawback shared by most lenses of this type is higher than average distortion levels - Barrel distortion is very noticeable at the wide end of the zoom so any buildings you photograph will have a few extra curves. All zoom lenses involve compromises but unless you really need the wide range the Sigma 17-70mm f2.8-4.5 is a much better lens all round for about the same money. This isn’t a bad lens really, but there are many better options out there.