Is the Tamron AF 28-75mm f/2.8 XR Di LD Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 75mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Tamron AF 28-75mm f/2.8 XR Di LD Lens set to 75mm f/4.0 sharper than the Canon EF 24-105mm f/4L IS USM Lens set to 50mm f/4.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.