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