Sigma 20mm f/1.8 EX DG
£520, $630 Faster than any of the zoom lenses on test, this Sigma has a widest available aperture of f/1.8
All three of Sigma’s DX offerings gave a strong showing in last month’s big test, but this FX lens feels more basic. Rather than ring-type ultrasonic autofocus, there’s no built-in actuator at all. Instead, autofocus needs to be driven from in-camera motors, as are featured in all Nikon FX bodies. Annoyingly, switching between autofocus and manual focus is a two-stage process, requiring you to operate the focus ring’s pushpull mechanism, in addition to flipping the AF/M switch on the camera body.
On the plus side, build quality is good and the lens features a fast widest aperture of f/1.8. This enables a relatively tight depth of field at short focus distances, as well as giving the option of faster shutter speeds in low lighting conditions.
In our tests, sharpness at the centre of the frame proved better at f/1.8 than from the Samyang at its widest f/2.8 aperture. However, the Sigma’s sharpness towards the edges and corners of the frame is very poor indeed, even when stopping down to f/8 or f/11. This can be a major problem for architectural and landscape photography, and any other situations where you want good levels of sharpness across the whole image frame.