This is a comparison of two lenses at 75mm: the Canon EF 70-200mm f/4L USM and the Tamron SP AF 28-75mm f/2.8 XR Di. The crops below were from images shot in raw using a 30D with sharpness set to 0, using the center focal point. They were converted using Canon DPP, then cropped from the center and corner. No other editing was performed. It is necessary to view the comparison charts at full size to see any differences. Both lenses seem to do quite nicely here at all aperatures and center to edge. The Tamron is slightly softer and less contrasty at f/2.8 (more noticeable at the edge), but that's to be expected.