Is the Tokina 16-28mm f/2.8 AT-X Pro FX Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 20mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Tokina 16-28mm f/2.8 AT-X Pro FX Lens set to 20mm f/5.6 sharper than the Nikon 14-24mm f/2.8G AF-S Nikkor Lens set to 15mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.