Lens review

Sigma A 105 mm f/1.4 DG HSM

10 July 2018
Arkadiusz Olech

10. Autofocus

As its name indicates, the Sigma A 105 mm f/1.4 DG HSM was equipped with an ultrasonic autofocus motor called HSM so the focusing process is very quiet. When it comes to speed, while attached to the Canon EOS 5D Mark III the lens’s performance is hardly impressive. In average lighting conditions running through the whole range and setting the focus takes about 1 second; the problem is, from this kind of equipment we expect half of that time. When you focus on a well-illuminated, properly contrasted object you might reduce the focusing time to 0.7-0.8 of a second.

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In the case of the EOS 50D camera the lens’s performance remains similar. Momentarily the focusing time can be reduced but a difference of 0.1 of a second is hardly a significant improvement.

When it comes to accuracy, already while shooting sample shots we managed to spot one alarming thing. The autofocus performance is quite reliable when you use the central point and its closest areas but it worsens notably when you use the outermost points of autofocus. In that case you can experience distinct misses in even about 30% of shots.

In order to check the autofocus performance in a more controllable manner we decided to test it on our resolution charts. We chose very demanding conditions, using the f/1.4, not the usual f/2.8 aperture, because we wanted to see whether the central point provides you a reliable performance even in the area close to the maximum relative aperture. It seems the Sigma fared well enough – it hit the target spot-on in half of cases, with the quality of photos not worse than 10% from the best shot. There were 28% of photos a bit worse but still completely acceptable (decrease of resolution ranging from 10 to 20%). In the next range, from 20-30% we placed about 22% of all photos. We didn’t find misses greater than 30% at all.

The tested lens didn’t have any front or back focus problems while attached to any of two bodies used in our test. It didn’t need any calibration either from the camera level or done with the help of the USB Dock.

Canon 50D, f/1.4
Sigma A 105 mm f/1.4 DG HSM - Autofocus
Canon 5D MkIII, f/1.4
Sigma A 105 mm f/1.4 DG HSM - Autofocus