Your camera has different auto focus modes. AI Servo mode tracks a moving object, continuously focusing on it. This can help you out. Also, I would suggest a better framing of your subject. In the photo you posted, there's a lot of space above the runner that isn't interesting or part of the subject. In the lower part of the image, you have the legs cut off in an awkward manner. I think just aiming down a little, getting the whole body in the shot, would have helped. As for an aperture suggestion, that would be a matter of taste, whether you wanted a shallow or deep depth of field. Filling the frame with more of the runner, less backround, shooting in the portrait position, is another suggestion.
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Bryan
flickr
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