Originally Posted by
rpcrowe
The 17-55mm f/2.8 IS lens will let you do things which cannot be done with your 17-85mm. It is faster, a lot faster - both in aperture and in focus speed. Additionally, it has a later and more sophisticated IS system.
The constant f/2.8 along with the later generation IS, make this lens a very good low light glass.
As far as image quality, it will beat your 17-85mm hands down; both in sharpness and in contrast. The 17-55mm f/2.8 IS is a totally superb lens.
Now, would I like a longer focal length - yes. However, I do very well with the 17-55mm,
I combine this lens with the 70-200mm f/4L IS lens on two cameras (30D and 40d hanging from an OPTECH Dual Harness). So in effect, I have a 17-200mm range available at my finger tips. I don't miss the 55-70mm gap.
Yes, you will probably get at least equivalent quality from cropping your 55mm images to what you would get from your 85mm lens. But, the great advantage will be the increased quality of imagery and the added versatility from 17-55mm.
If you need a longer lens; the 55-250mm IS lens might be an inexpensive fix.