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The 17-55mm f/2.8 IS and 24-105mm f/4L IS are both excellent lenses. Do keep in mind that even though 7mm on the wide end doesn't sound like a lot, it's a substantial difference - 24mm is the FF equivalent of 38mm, which means when used on a 60D (or your 450D), the 24-105mm is really a 'normal to telephoto' zoom and lacks a wide angle focal range. You have an 18-55mm lens - look back over your shots and see how much you used the 18-24mm range of your kit lens. If you did not do much shooting at wide angles, and are willing to keep and use the kit lens for the times when you need that wide angle, then the 24-105mm makes sense (it also makes sense paired with an EF-S 10-22mm ultrawide zoom).Originally Posted by charlduplessis
For me, a 'general purpose zoom' is wide angle to short telephoto. For that use on a 1.6x crop body, IMO the EF-S 17-55mm is the best walkaround lens. Optically, it performs as well or slightly better than the 24-105mm. The EF-S 15-85mm f/3.5-5.6 IS is another option - similarly good optical quality, a broader focal range, but it's also got a slow and variable aperture which I personally don't want in a general purpose zoom.
Depends on your needs. There are a small number of EF-S lenses that offer optical quality equivalent to L lenses - the 17-55mm, 15-85mm, and 10-22mm are the three I'd judge to have L-level optics. The build quality of those lenses is not L-level, though (but they are still solid, well-constructed lenses and a definite step up from a kit lens). The other advantage to L-series lenses is that many of them are weather-sealed, including the 17-40mm and the 24-105mm (a filter on the front is needed to complete the sealing). Weather-sealing is the main reason I picked up a 24-105mm to supplement my 17-55mm, so I could shoot in the rain with my 7D. However, weather-sealing not a real advantage for you, since the 60D is not a weather-sealed body.
The 17-40mm does not perform quite as well on a 1.6x body, and it's also lacking at the long end - I don't think it's a great choice as a general purpose zoom.
My recommendation would be for the EF-S 17-55mm, and if your budget permits add a fast prime, either the 85mm f/1.8 for tight portraits, or the 50mm f/1.4 for more loosely-framed portraits. In both cases, the wide aperture helps isolate the subject of a portrait. The f/2.8 IS lens will also be better for shooting with ambient light indoors, especially for static subjects.
Here are a couple of portrait examples of the two lenses on a crop body (T1i, which I had prior to getting a 7D):
Rebel T1i, EF-S 17-55mm f/2.8 IS USM @ 50mm, 1/50 s, f/2.8, ISO 100 Rebel T1i, EF 85mm f/1.8 USM, 1/2000 s, f/1.8, ISO 100
Hope that helps...
--John
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