Click images for larger, and metadata.
All images shot on Canon Rebel T1i
18mm - 18-55 (Canon kit Lens) vs a Tamron ultrawide 10-24.
The first set is from the side of the frame. The Tamron shows a little more sharpness, but shows a lot of chromatic aberration where the kit lens has none.
The second set is from the center of the image. Neither are particularly good, but again the kit lens outperforms the Tamron. The full size image also shows significantly more distortion on the Tammy. I still like the lens, this kind of shooting isn't its strength though.