Is the Sigma 24-70mm f/2.8 DG OS HSM Art Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 50mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 24-70mm f/2.8 DG OS HSM Art Lens set to 50mm f/4.0 sharper than the Tokina 24-70mm f/2.8 AT-X Pro FX Lens set to 28mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.