Lens review

Sigma A 24 mm f/1.4 DG HSM

8 March 2015
Arkadiusz Olech

10. Autofocus

The Sigma A 24 mm f/1.4 DG HSM, as its name indicates, is equipped with an ultrasonic autofocus motor. Because of it the focusing is noiseless – you hear just the delicate swoosh of moved elements.

Still the speed of the mechanism is something you can complain about. Even in good lighting conditions running through the whole distance scale and setting the focus can take over one second. It also seems that running from the infinity to the minimum focusing distance takes a bit less than the other way round. Both the Canon and the Nikkor were almost two times faster and in the case of the Canon its ring turns through as much as 160 degrees, having definitely a bigger range than 90 degrees of the Sigma ring.

When it comes to the accuracy of the mechanism we don’t have any serious reservations. The average number of misses in studio conditions didn’t exceed 5%. During the outdoor photo session the Sigma coupled with the 5D MkIII missed more often so we decided to perform another test. We put the lens on a tripod in our studio, in front of our biggest resolution testing chart and took 50 photos by f/2.8 changing the focus each time. The highest vertical MTF50 value amounted to 2112 LWPH and the lowest was 1968 LWPH; the median amounted to 2048 LWPH. The worst result we got was just 7% or so worse than the best so you can assume the repeatability of the autofocus settings is very good.

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When it comes to the front or back focus tendency the situation wasn’t perfect. On the EOS 50D the lens set focus more often than not behind the mark. In the case of the EOS 5D MkIII there was no such a problem but the measurements performed on our resolution testing charts showed a tad higher results for the microregulation AF settings around +5 units than in the centre of the scale.

50D, f/1.4
Sigma A 24 mm f/1.4 DG HSM - Autofocus
5D MkIII, f/1.4
Sigma A 24 mm f/1.4 DG HSM - Autofocus