The-Digital-Picture have published a review of the Tamron 28-75mm f/2.8 XR Di AF lens.

” The most notable feature of this lens is the impressive center sharpness it delivers. With good center sharpness at f/2.8, very good center sharpness at f/3.2 and excellent center sharpness at f/4, the Tamron 28-75mm F/2.8 XR Di Lens is weakest in the center at 75mm where it needs an extra stop to be at its best. I make the center designation clear as corner sharpness is a completely different story. The Tamron 28-75mm F/2.8 XR Di Lens is soft in the corners and my copy is dreadfully soft in the upper left portion of the frame. Corner sharpness improves as the lens is stopped down and zoomed out (toward 75mm). But, some corner softness is still present at 75mm and f/8. Fortunately, users of 1.5x/1.6x FOVCF digital SLR camera bodies will avoid most of this pain. “

Website: The-Digital-Picture - Tamron 28-75mm f/2.8 XR Di AF Lens Review