Sharpness at 20mm

For this test, the Sigma 18-35mm F1.8 DC HSM lens was attached to a Canon EOS 700D 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 20mm

Centre sharpness remains high through from f2.8 to f/11, with f/16 being adversely affected by diffraction. The edges aren't nearly as sharp as the centre, with f4-11 producing the sharpest results.

Aperture	Centre Crop	Edge Crop
f/1.8
f/2.8
f/4
f/5.6
f/8
f/11
f/16