Forum members have been offering some great advice to help me put together a small kit to cover my photography needs with my 5D. Based on your suggestions I've picked up the 24-105/4L, 70-200/4L and 50/1.8 lenses. To complete my kit I'd like to someday pick up a UW zoom lens to replace my 20-35/3.5-4.5 USM lens because I want something wider than 20mm. What ultra wide zoom would you choose? My budget would be in the $550 - $600 range.