Tokina AT-X PRO DX 11-20 mm f/2.8 review - Autofocus

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Tokina AT-X PRO DX 11-20 mm f/2.8

8 September 2015

Arkadiusz Olech

10. Autofocus

The autofocus mechanism can hardly be called silent and, what’s interesting, the volume of noise depends on the direction of the turn. If you go from the minimum focusing distance to infinity you can hear just quiet buzz but the movement in the opposite direction causes something that can be called already a whirr.

The speed is average – running through the whole range and confirming the focus takes about 0.7-0.8 of a second.

The accuracy of the Tokina’s 11-20 mm autofocus is, unfortunately, a huge problem. The first Nikon mount specimen which arrived to our editorial office had such a back-focus that there was not enough of scale for correcting it. We ordered a Canon mount specimen and the situation became better but still far from perfect. Almost always you could observe a huge back-focus effect which depended strongly both on the focal length and the distance from the object. For example while taking photos of the biggest testing chart at 11 mm we had to correct the autofocus by -5; at 16 mm the regulation value increased to -10, reaching -14 at 20 mm. When, for a change, we took photos of the autofocus chart with the lens positioned very close to the aim the correction had to be set at -20.

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It all makes the lens pretty unpredictable. It is very hard to remember all focal length combinations and their correction values so the performance of the autofocus seems to be rather chaotic.

It’s also worth checking whether the autofocus settings are repeated when you already find a proper regulation value at a given focal length and distance you need. In order to do that we took 50 photos of our resolution testing chart at the 20 mm focal length by f/2.8, setting the focus each time anew. The MTF values we got ranged from 1405 to 1532 LWPH with a median of 1490 LWPH. As the worst result differs from the best by less than 10% you can consider the autofocus performance as good.

50D, 20 mm, f/2.8