Canon makes two distinct lens ranges. The EF lenses will fit any of the EOS bodies, but the EF-S lenses will only fit Canons with the smaller APS-C sized sensor. That includes the EOS 1000D, 400D, 450D, 40D and new 50D. You can't use EF-S lenses on the pro-level 1D Mk III or 1Ds Mk III because they have bigger sensors and a slightly different lens mount.

The EF-S 17-55mm f2.8 IS USM lens being reviewed here, then, is designed for these smaller-sensor EOS models. It's the biggest and most expensive in a range of three 'standard' zooms you can use on these cameras.

The cheapest is the 18-55mm f3.5-5.6 IS, which is the kit lens for the EOS 1000D and the EOS 450D. It works well enough, but its optical quality is average. The next lens up is the 17-85mm f4-5.6 IS USM, which makes a good alternative kit lens for these two cameras, with an extended zoom range and superior optical quality. It's also ideal for the semi-pro EOS 40D or 50D. This 17-55mm f2.8 is the third and most expensive. It doesn't have the wide zoom range of the 17-85mm but it does have a constant f2.8 maximum aperture, and this is why it's both more expensive and heavier. This lens would also be an ideal kit lens for the EOS 40D or new EOS 50D.

So why is it so expensive? It's because of that constant f2.8 maximum aperture. It costs a great deal more to design and manufacture a zoom lens of this type, and the extra glass required makes it bigger and heavier too.

At its minimum focal length, the 17-55mm f2.8 has barely a half-stop aperture advantage over the other standard zooms in the Canon range, but by its maximum focal length of 55mm, it has a two-stop advantage.

It makes a big difference in two sets of conditions: (a) when you're shooting in low light and need a wide aperture to minimise the risk of camera shake and (b) when you want to isolate a foreground subject with shallow depth of field.

This lens's low-light capabilities are enhanced even further by the built-in IS (image stabilisation) system. Canon claims this offers a three-stop gain in shutter speeds, and this is borne out in testing, delivering a satisfactorily sharp shot at 1/8sec at maximum focal length, three shutter speeds slower than the 1/80sec you'd normally consider the safe minimum at this zoom setting.

Optically, it's very good. You shouldn't expect a clear jump in definition over the standard zoom, though. That's because kit zooms are generally very sharp, at least in the centre of the frame and at their optimum focal lengths.

The 17-55mm is a lot more consistent. Cheaper lenses lose definition at their longer focal lengths, but the 17-55mm delivered some of the most consistent figures we've seen in our resolution tests. What this means in practice is that you can shoot at any focal length without worrying about whether you're using the lens's 'sweet spot'. And while the definition does drop off at maximum aperture, by f4 this lens is already properly sharp.

You're paying for other, less obvious characteristics too. Chromatic aberration, the scourge of cheap lenses, is minimal, and there's less barrel distortion in wide-angle shots than you'd normally expect to see (though there is a fair bit of pincushion distortion at longer focal lengths).

Considering the price, the handling and build quality are faintly disappointing. There's a plasticky feel to this lens at full extension, and there's a little play in the extending section by this point too. The zoom action is generally light and smooth, though there is a slight tight spot near the wide-angle end of the range. The autofocus is fast and quiet, a particular characteristic of Canon's USM lenses.

The constant maximum aperture is this lens's big selling point, and there's no doubt that it does give you much more scope when shooting in poor light or creating differential focus effects. But if it's outright versatility you want, Canon's 17-85mm f4-5.6 is smaller, cheaper and lighter and optically not far behind.

Final Verdict This is the best standard zoom you can get for your EF-S format EOS SLR, but it's not cheap and it will only balance well on the heavier 20/30/40/50D. OVERALL

Our lens reviewer, and technical expert, Rod is a veritable photographic encyclopaedia. His illustrious CV has seen him write for many mags, websites and journals.