If you had to pick one of these 2 lenses for an every day walk around lens which one would you pick? The Canon 17-40 L f/4 USM or the Sigma 17-50 f/2.8 EX DC OS HSM FLD?

i have gotten a lot more use out of my 17-40 than i expected. it was a nice walkabout lens for the wife on my 30D (crop). and on my full-frame bodies it provides some nice wide angle shooting.

i've shot almost the entire wedding reception with a 17-40 (full frame), not to mention wedding formals and also some other types of events like graduations (where i will suddenly need a wide angle option for a group).

with my 6D i can crank ISO so i don't miss f2.8 so much. and i'm not a big fan of image stabilization. but i haven't tried the other lens you mentioned.

Several years ago, i would have said buy the Sigma. Today, not so much.

For starters, f/2.8 doesn't mean as much as it used to. It's still better than 4, but it's not the gulf it once was, now that every camera made in the last several years makes a good 5x7 at 800 ISO or better.

Second, the rising number of FF shooters means that even if you never buy one yourself, the 17-40 is going to hold value much better, and the difference will only increase as time goes on. It's going to be a LOT easier to swap the Canon for the Sigma in a year if you change your mind than the other way round.

I wouldn't be surprised if a few years from now you could swap the Canon for two Sigmas, and even get them to throw in a little cash as well.

The only reason I'd pick the Sigma today is if you wanted shallow DOF and multiple focal lengths, and you didn't want to change lenses.

Or if they were both used. You can buy a used copy of that lens damn cheap.

I haven't used the Sigma, but I have the found the 17-40 to be incredibly sharp. It's certainly a bargain in the "L" series.

I've used the Sigma 17-50 for about 2 years and I'm pleased with it.

Depends on which Canon lens you are talking about. IMHO, the Canon EF 24-70mm f/2.8L II USM Lens is the best but if you are talking about the initial version of this Canon lens, I'd go with the Sigma.