Is the Sigma 28-70mm F2.8 DG DN Contemporary Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 28mm f/11.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 28-70mm F2.8 DG DN Contemporary Lens set to 28mm f/11.0 sharper than the Tamron 28-75mm f/2.8 Di III RXD Lens set to 35mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.