dhofmann said:
I don't think I would replace the 15-85 with the 17-55, and here's why:
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=398&Camera=474&Sample=0&FLI=3&API=0&LensComp=675&CameraComp=474&SampleComp=0&FLIComp=2&APIComp=0
To summarize, at 35mm focal length, the 15-85 is much sharper wide open (f/4.5) than the 17-55 (f/2.8). You have to stop down the 17-55 to f/4 to get the same sharpness, but then it's only 1/3 of a stop faster. And you lose 2mm at the short end and 30mm at the long end. So the 17-55 is just not a very useful lens.
The other issue with getting a faster lens is that focusing becomes more difficult.
If you read the TDP review of the 17-55mm, there's a statement that the ISO 12233 crops of that lens seem less sharp than Bryan thinks they should. Also, you're selecting one focal length. At other parts of the range, the 17-55mm is sharper. Overall, across the zoom and aperture range, basically it's a wash between the two lenses in terms of sharpness. Even if it weren't, sharpness is just one element of lens performance, and offer not the most important (but it is one of the easy ones to measure and gives a number for comparison, so people focus on that, sometimes exclusively). Consider - the $125 50/1.8 II, when stopped down, is sharper that L-series prime lenses costing 20x as much - if that's your main criterion, you should never spend more than $125 on a lens.
Regardless, to say the 17-55mm is just not very useful simply makes no sense. At the long end of the 15-85, the 17-55 is letting in
4 times more light. Also, the DoF with the 17-55mm f/2.8 is shallower than you can achieve with the 15-85mm f/3.5-5.6, for the same framing. The 15-85mm also has much worse vignetting and barrel distortion. So, overall, the 17-55mm has slightly better IQ, and f/2.8, whereas the 15-85mm has a larger zoom range. Both are useful, for partly overlapping uses. The 15-85 is a good outdoor walkaround lens, the 17-55 is better indoors, for action where you need a fast shutter, and for portraits.
Finally, I really don't get your last comment. How, exactly, does an f/2.8 lens make focusing more difficult? For AF, an f/2.8 lens activates the high-precision center AF point found on most Canon bodies, resulting in more accurate and more precise focusing than you'd get with a slower lens like the 15-85mm. With MF, f/2.8 means a thinner DoF which makes it easier to tell what's in focus and what's not, even if the shot is taken with a narrower aperture. How is that 'more difficult'? I suspect what you mean is that f/2.8 can result in a shallow DoF, meaning parts of the scene you wanted to be in focus are outside the DoF - but that's not the fault of the lens, that's the fault of the person holding the camera not choosing an appropriate aperture. That thin DoF of a wider aperture doesn't have to be used all the time, but having it available is useful.
