It doesn't matter which lens you are using. All you have to do is press this button a couple times.
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I use 1pt AF 95% of the time.
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The rest I use zone AF which is a 9pt cluster.
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You can also change it from the back of the screen here.
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If you are photographing anything that can move you need to make sure you are in AI Servo and not in One Shot too...
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Also you need to go to the blue menu, page 3, and make sure "AF point display" is enabled. When you are reviewing your photos there will be a red box where the camera focused. This way you can check and see if the box is on your subject or if it focused somewhere else.
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