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A lot depends from the composition of the picture you make. With wide angles it's easy to have a subject at 1/3 of the image at a distance below a meter while at center point the image is at infinity for example. Imagine you shoot an urban scene where you take a wall at 30 cm in the left 1/3rd part of your image, while the rest of the image is at infinity. Than a slight move of the AF sensor over the image subject can make the camera focus wrong, because the focus point shifts from 30 cm by a slight move over a great range to infinity. Even though depth of field compensates for a part, and even though field curvature can be both helpful and counterproductive, one always has to carefully compose and keep the simple rules for DOF control and the AF points in mind. In general, unless you are mainly shooting landscapes at infinity, I find the wider a lens, the more precisely you have to set your focus point. That is a pitfall many wide angle photographers seem to overlook. This is also true for the wide angle zoom the 16-35mm is. For everything else I have never noticed this lens has a better or worse AF than other great L lenses. It's just a matter of carefully composing the picture and set the AF point where you want the main point of sharpness.