Is the Sigma 24-35mm f/2 DG HSM Art Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 28mm f/16.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 24-35mm f/2 DG HSM Art Lens set to 28mm f/16.0 sharper than the Canon EF 16-35mm f/2.8L III USM Lens set to 20mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.