Is the Sigma 50-500mm f/4.5-6.3 APO DG OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 400mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Sigma 50-500mm f/4.5-6.3 APO DG OS HSM Lens set to 400mm f/6.3 sharper than the Sigma 50-500mm f/4.5-6.3 APO DG OS HSM Lens set to 135mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.