Originally Posted by rowan

The low end Canon sensors have what is called a 1.6 times crop factor, which means that you have to multiply the focal length that is printed on the lens by 1.6. If you put the lens on a normal film body and then onto your 20D it's like you've zoomed in 1.6 times.

So it's not the telephoto (zoom) end of the range you have to worry about - you may find yourself in a situation where you can't "pull back" any further...

I found even 18mm wasn't wide enough on my 1.6X crop camera, so I went for a 12-24mm lens. That's a fairly extreme change though, if you're doing portraits you probably don't want to go that wide.