Comparison of Tamron 28-75 f/2.8 with Canon 17-55 IS, using a Canon 20D body. Taken on tripod, with mirror lockup and remote shutter release. The tripod/camera wasn't moved for any of these shots, so the longer the focal length, the higher the resolution (because more of the subject filled the frame.) All shots are f/2.8 or f/8.0. They were cropped (with no resampling) in PhotoShop CS2, and no additional sharpening was applied. These are in-camera JPEGs, with saturation, sharpening, and contrast all set at "0".

I used the center focus sensor. The focus point was on the "windows" of the small boat, approx. two windows in from the left and two windows down from the top. So you are seeing the center and the top of the frame.

Click on a thumbnail and then on "original" to see them at 100%.