Sharpness at 28mm

For this test, the Sigma 17-70mm F2.8-4 DC Macro OS HSM C lens was attached to a Canon EOS 70D body, which in turn was mounted on a sturdy tripod. Exposure delay mode was activated. Tonal and colour variances across the crops are due to changes in natural light during the session.

The full frame at 28mm

Centre sharpness is high through from f/4 to f/11, with f/16 and f/22 being adversely affected by diffraction. The edges aren't quite as sharp as the centre, with f/5.6-11 producing the sharpest results.

Aperture	Centre Crop	Edge Crop
f/3.5
f/4
f/5.6
f/8
f/11
f/16
f/22