Hi.

I am looking to buy a zoom lens for new 40 D, roughly no wider than 15/17 mm and no longer than 55 mm. I am therefore considering the likes of Canon 17-40 L, 17-55 IS(2.8) or are there any independent brands to consider, the likes of Sigma EX or Tamron 17-50 di II. ?
Optical quality is my primary consideration.
Thanks !
Keith

Could you be more specific about what you mean by "optical quality"? What kind of shooting do you do?

Even the 18-55 IS USM is getting pretty good reviews for sharpness, between say f/5.6 and f/8. Of course, distortion can be pretty bad at the wide end with that one, or is that something that doesn't concern you? Are you bothered by CA and fringing?

Perosnally, I'd go for the 17-55 2.8, but many feel that crop frame cameras are a dead end, and do not want to invest heavily in lenses for them.

Hi, yes I would like to buy the best overall lens. Optical quality, resolution etc.would be more important , than say, build quality, as I am not a professional user, but the 17-55 seems expensive when compared to the 17-40 for example. My photography is varied, so f4 wouldnt be a deal breaker when compared to f2.8.
Keith

The 17-55is is the best lens in that range for a 40D. Photozone and slrgear should both have reviews.

Unless you're extremely cash limited, in which case the new 18-55 EFS IS USM is a heck of a good deal, at 1/5th the price. Supposedly it has an extra element over the old kit version that was intended to tame the CA. Apparently, it drastically imroves sharpness wide open too. Go figure.

I've always said the old 18-55 was surprisingly useful at mid aperture, so this thing ought to be very good for the money. Of course, if you can afford a $1400 camera, you can probably afford some better glass too.

I'm considering getting both the 17-55 and the new 18-55. The 17-55 for serious work, and the 18-55 for walking around, bad weather, etc.

A few weeks ago, when I purchased the 40D, I ordered the EF 16-35mm f/2.8L Make II and the EF-S 15-55mm f/2.8, with the declared intention that I keep only one of them. The return option was for one week, so I tested them a week long against each other and against the Sigma 20mm f/1.8, the Tamron 28-75mm and the Canon 50mm f/1.4, always equal focal lengths and apertures, on the 40D.

After several hundred shots dedicated to the comparison and pixel-peeped the not demosaiced images, I concluded, that the EF 16-35mm is a tiny bit better than the 17-55mm in some situations only. The 17-55mm is an unbelievable lens, it gave a hard time even the 50mm f/1.4. So, I kept it.

Note: CA is as bad on both the 16-35mm and the 17-55mm as on the Sigma. Flaring too was the same between the two contenders.

Add the IS and the longer range to the virtues of the 17-55mm, and no question remains (and a small addition: the hood of the 17-55mm is designed for the cropping camera, it is more effective than the hood of the 16-35mm.)