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Lens review

Sigma C 17-70 mm f/2.8-4.0 DC Macro OS HSM

13 March 2013
Szymon Starczewski

10. Autofocus

The tested lens is equipped with an ultrasonic autofocus motor (HSM) so it works noiselessly. When it comes to the speed the results are average – running through the whole distance scale takes a bit under one second. The 17-70 mm model without image stabilization (so the oldest of the series), in the same working conditions set the focus roughly as fast, although it whirred because it didn’t feature an HSM motor. Taking into account a very narrow working range of the manual focus ring the results in this category don’t impress at all.

We have no reservations concerning the accuracy of the autofocus. The lens, attached to the Canon 50D, in studio conditions hit the proper target 99 times out of 100.

When it comes to the front or back focus tendency the photo below shows that the depth of field is slightly shifted towards the objects positioned further than the proper target. Such an effect is not big, though, because the depth of field itself still includes the object you focus on. it’s also worth adding that that particular photo was taken near the minimum focus distance which always creates the most demanding conditions.

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Sigma C 17-70 mm f/2.8-4.0 DC Macro OS HSM - Autofocus