Is the Sigma 17-50mm f/2.8 EX DC OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 21mm f/3.2 test image. Observe the contrast and resolution of these results.
Is the Sigma 17-50mm f/2.8 EX DC OS HSM Lens set to 21mm f/3.2 sharper than the Sigma 17-70mm f/2.8-4 DC Macro OS Contemporary Lens set to 70mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.