I'll second that Tamron 28-75. The range is perfect for a walk-around. I switch to my 50mm f1.8 indoors a lot. It's perfect inside restaurants with low lighting.
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Canon 40D | Canon XTi | Canon T2 | Tamron 28-75mm f/2.8 | Canon 70-200mm f/4L | Canon 50mm f/1.8 II | Canon 18-55 f/3.5-5.6 IS
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