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Old Apr 27, 2010, 6:03 PM   #1
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Whats the deal with the different types of auto focusing. Which one should I use for a moving person and how do I use it?
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Old Apr 27, 2010, 7:12 PM   #2
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In general, if the subject is moving, you should use AF-C (Continuous Autofocus.) It focuses on a subject, and keeps the subject in focus even as it moves toward or away from you.

The alternatives are usually AF-S and AF-A. AF-S focuses on a subject and stops. You can take the picture or not, but the focus won't change if the subject moves. AF-A starts off like AF-S, but if the subject moves, the camera will keep it in focus.

Another difference is that AF-S and AF-A will only let you take the photo is the subject is in focus. With AF-C, you can take a photo even if the AF system loses the subject. This is useful because, while a subject may not be in focus, it may be within the depth of field, so you can still get a good photo of it. With AF-S and AF-A, you can't take a photo if the subject isn't in focus even if it is within the depth of field.
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Old Apr 28, 2010, 10:05 AM   #3
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What camera? I see in another post here that you just bought a Canon 7d. If you're talking about that camera then the specifics of TCAVs response do not apply as Canon does not have AF-A and AF-C modes.

So what camera are we talking about?
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Old Apr 28, 2010, 12:54 PM   #4
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JohnG is absolutly correct. If you're using the Canon 7D, it has One-Shot AF for still subjects, AI Servo AF for moving subjects, and AI Focus AF which works like One-Shot AF until the subject moves and then it works like AI Servo AF.
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Old Apr 28, 2010, 1:45 PM   #5
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Additionally the 7d is a bit more complex. There are custom functions which control the behavior of the focus system.

For what it's worth, my recommendation is to stay out of AI FOCUS mode. Don't know about the 7d, but in other canon models, it's fairly unreliable.
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Old Apr 28, 2010, 5:33 PM   #6
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I never use my 7D, or any other of my Canon bodies in AI Focus there is no way that the camera knows well enough if I want to track something or not and John is right, if you are shooting with a 7D it is worth making a tweak to the settings to get it optimised.
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Old Apr 28, 2010, 6:45 PM   #7
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Without a doubt, the 7D does not appear to be a camera for the faint of heart.
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