Is the Sigma 10-20mm f/4-5.6 EX DC HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 14mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Sigma 10-20mm f/4-5.6 EX DC HSM Lens set to 14mm f/5.6 sharper than the Sigma 10-20mm f/3.5 EX DC HSM Lens set to 12mm f/3.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.