Lens review

Sigma A 20 mm f/1.4 DG HSM

3 November 2015
Arkadiusz Olech

10. Autofocus

The Sigma A 20 mm f/1.4 DG HSM, as its name indicates, was equipped with an ultrasonic autofocus motor. That’s why the focusing mechanism works noiselessly – the only thing you can hear is a quiet whirr of moving elements.

The Art line Sigmas so far haven’t impressed us with their autofocus speed. For example the A 24 mm f/1.4 DG HSM model needed over one second to move through the whole scale and set the focus even in favourable conditions. In the case of the new 20 mm lens that time got shortened noticeably, amounting to just 0.4-0.5 of a second. It is a good result and we are happy to see it.

When it comes to the accuracy of the autofocus our impressions are mixed. With the lens attached to the EOS 5D Mark III we had no serious reservations. A significant miss happened 1-2 times every 100 shots. In the case of the older EOD 50D the situation seemed to be worse. It is true we didn’t take as many photos as with the 5D Mark III but when photographing the autofocus testing chart we found out almost every shot was set a bit differently. The differences weren’t big and in most of cases the object fit the depth of field but you couldn’t speak of the full stability of work.

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When it comes to the front or back focus tendencies there were no big problems there. Taking shots from a minimum focusing distance and using both bodies, with the depth of field being at its shallowest, the object still remained near the centre of the depth. When we took photos from a bigger distance a slight back-focus could be observed. With the autofocus calibration set at 4 units the lens worked properly well practically across the whole range of useful distances.

50D, f/1.4
Sigma A 20 mm f/1.4 DG HSM - Autofocus
5D Mk III, f/1.4, 0.28 metre
Sigma A 20 mm f/1.4 DG HSM - Autofocus
5D Mk III, f/1.4, 0.6 metre
Sigma A 20 mm f/1.4 DG HSM - Autofocus