Sigma A 50-100 mm f/1.8 DC HSM review - Autofocus

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Sigma A 50-100 mm f/1.8 DC HSM

26 April 2016

Arkadiusz Olech

10. Autofocus

As its name indicates, the Sigma A 50–100 mm f/1.8 DC HSM is equipped with an HSM ultrasonic autofocus motor which is supposed to be an improved version of the devices used before. The mechanism is indeed completely noiseless – all you can hear is the swoosh of the shifting elements.

When it comes to the speed it is average – running through the whole range and finding the focus takes about 0.8 of a second.

In good conditions a properly calibrated lens shouldn’t have any accuracy problems and the performance in our testing studio is the proof of it. We set the lens several meters from the testing chart, chose a combination of 100 mm focal length and f/2.8 aperture and took several dozen of photos without any problems at all. The worst shot was less than 10% fuzzier than the best one and as many as 70% of photos had the deviation of 5% from the best shot. It is a brilliant result.

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In worse conditions, while photographing objects not so contrasted, the Sigma got lost more often but, after all the combination of f/1.8 aperture and focal lengths reaching 100 mm is very demanding indeed. From time to time the autofocus went over the proper position and it had to return which, of course, made the whole process longer.

When it comes to tendencies chosen by the autofocus the tested specimen featured all the spectrum so to speak. In the centre of the focal range it hit the bull’s eye. Near 50 mm we saw a slight back focus tendency and at 100 mm focal length – the front focus tendency. Still those shifts were slight and the microcalibration set at 4-5 units corrected them efficiently. What’s more, the effect was distinct only near the minimum focusing distance; at longer distances, even with the calibration set at 0, the depth of field included also the proper object. However it doesn’t change the fact that the complete calibration of the lens did demand a bit of play with the USB Dock.

Canon 50D, 50 mm, f/1.8