What's the difference between a 17-85mm lense and a 18-55mm. How much more zoom does one have than the other?

Answer 1

The 17-85 (5x zoom) obviously has more zoom range than the 18-55mm (3x zoom).

But I'll bet it its less bright (smaller maximum aperture).

Lenses with more zoom range, also tend to have a larger depth of field. Depending on your photographic needs, this can be a good thing, or a bad thing.

Answer 2

These lenses both start with the same amount of wide angle coverage (sufficient for just about everything) but the 17-85mm zoom gives you a bit more flexibility by letting you zoom in tighter at the tele end. It's an appreciable difference but not enough to make the 17-85mm vastly more suitable for different things.
More important than the zoom range, is that the Canon 17-85mm lens provides much better image quality. This is why it costs over 3x as much.
Both of these lenses provide plenty of zoom range for landscapes, street photography, and portraits, etc. and neither one will let you zoom in close enough for sports or wildlife.
If you want an all-in-one lens, you should look at an 18-125mm zoom or even an 18-200mm zoom. The drawback with these mega-zoom lenses is that they provide poor image quality.
For better image quality, stick with a standard zoom (such as the 18-55mm or the 17-85mm) and add a second lens to cover the 70-200mm range.
The obvious companion lens for the (inexpensive) 18-55mm is the (also inexpensive) Canon 55-200mm zoom. You could also get a 70-300mm lens without really missing the 55 to 70mm gap.
The obvious companion lenses for the 17-85mm zoom are either the 70-200 f/4 (if you prefer image quality over zoom range) or the 70-300mm zoom (if your priorities are the oher way around). Both of these cost around $550.

Answer 3

Well, "zoom" is simple math.

85/17 = 5X
55/18 = 3X

What matters for real is the focal length at the end of the telephoto range. As Morey says, 85 mm is more powerful than 55 mm when it comes to magnification.

On a digital camera, which I will assume from the focal lengths in your question, a 55 mm lens is about the equivalent of a 1.5 power telescope, which means barely any "zoom" at all. An 85 mm lens is about the equivalent of a 2.5 power telescope, which is about like some nice "opera glasses." So I guess we would say that the 85 mm lens has about 1.67 times more "zoom" than the 55 mm lens or about 67% (two-thirds) more "zoom."