Sigma 120-400 mm f/4.5-5.6 APO DG OS HSM
10. Autofocus
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Sigma 120-400 mm OS is equipped with an ultrasonic HSM motor, which makes its autofocus work really silently. In terms of speed, it’s hard to complain here, as it’s not the worst. The mechanism goes through the entire scale in less than 1 second. It is slower than in the case of Canon 100-400 mm IS, though. It’s too bad, then, that Sigma didn’t decide to build an autofocus limiter in, e.g. for the range from 3 meters to infinity, what would definitely accelerate its work.
In terms of accuracy, we do have some reasons to complain. First of all, the lens has a tendency to focus just behind the correct object and, what’s interesting, this tendency is visible for the range 120-200 mm and disappears at 300-400 mm. Accuracy doesn’t satisfy either, as the number of misses in studio conditions totaled almost 11%, which is a medium result at the most.
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