Is the Sigma 12-24mm f/4 DG HSM Art Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 12mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 12-24mm f/4 DG HSM Art Lens set to 12mm f/8.0 sharper than the Sigma 12-24mm f/4.5-5.6 DG II HSM Lens set to 17mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.