Is the Sigma 18-35mm f/1.8 DC HSM Art Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 20mm f/1.8 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-35mm f/1.8 DC HSM Art Lens set to 20mm f/1.8 sharper than the Rokinon (Samyang) 16mm f/2 ED AS UMC CS Lens set to 16mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.