Right I'm vexed something rotten. I've decided that I need a new lens in the standard zoom length kind of area - currently got a 17-70mm 2.8-4.5 DC Sigma. It's very good but the autofocus motor can't keep up with the autofocus on the 7D. So new lens time. Narrowed it down to:
Canon 17-55 F2.8
or
Canon 15-85 3.5-5.6
Tried both in the shop, both have quick autofocus (point number 1 satisfied). Both weigh roughly the same (600g ish). More or less the same size...
The 2.8 is nice, but then I worry I'll miss the extra length of the 15-85. Having compared the DOF of 55mm @ F2.8 vs the DOF of 85mm @ F5.6 is pretty much the same (at 20ft) so at the kind of distances i tend to shoot at they'd give the same about of subject isolation. Having also looked into the 7Ds autofocus, all the focus points are cross up to 5.6, so that doesn't help... but i assume in theory the 2.8 should maintain focus easier in low light (i.e. UK MTBing in the trees). And it does give an extra 2 stops...
So which shall I get? Anyone acutally use the 17-55 for MTB photography?
). There'd be a difference in the image quality between the 17-55 and the 15-85mm, whether you'd notice it or not is another question... the 2 stops of aperture at the long end might be important to you, though with modern camera's high ISO ability, maybe not... personally I always go for the fastest glass I can afford, so I have the 17-55 f2.8, my favourite lens (though closely rivalled by the 10-22mm 
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