Sigma A 24-105 mm f/4 DG OS HSM
10. Autofocus
When it comes to the accuracy the first small reservations appeared during the sample shots session. It was a beautiful, sunny day and when we were taking photos of well-lit (so low contrast) fronts of some tenement buildings from time to time we experienced focusing problems. The lens could go past the required position and it returned to it only after a while – such a behaviour is rare when it comes to its brand-name competitors.
The accuracy of the mechanism wasn't perfect but we didn’t have any serious reservations here. The number of misses didn’t exceed 4% which we consider a good result; still it is a bit worse than that of the Canon and the Nikkor.
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While taking photos from typical distances we didn’t notice any front or back focus tendencies. When we moved to the studio and positioned the lens very close to the target, making the depth of field as shallow as it was only possible, we managed to notice them; while working on the EOS 1Ds MkIII the lens moves very slightly the depth towards the longer distance and on the 50D the situation is reversed. As we wrote that effect is not especially pronounced, rather difficult to perceive in real life photos; even if you take pictures from very close distances the photographed object is still situated within depth of field limits so there is no need to use the option of the AF microcalibration, available in the camera body.
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