Is the Tokina 14-20mm f/2 AT-X Pro DX Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 18mm f/2.0 test image. Observe the contrast and resolution of these results.
Is the Tokina 14-20mm f/2 AT-X Pro DX Lens set to 18mm f/2.0 sharper than the Rokinon (Samyang) 16mm f/2 ED AS UMC CS Lens set to 16mm f/2.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.