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Your camera's default autofocus system is very good, and should be able to provide you with accurate focus for most of the subjects that you shoot. However, focusing is a complex process. Your camera's autofocus can be stymied by low light situations, moving subjects, solid colors, and more. For that reason, the camera includes several different focus modes. Your Mark III has three different focusing modes that change a little bit of the behavior of how the autofocus system works. By default, you'll be in One Shot mode, unless you are shooting in Scene Intelligent Auto mode.
The rest of the modes default to One Shot mode. This is pretty much probably how you expect autofocus to work. You half-press the shutter button, the camera calculates focus, and then locks focus, beeps; shows you your focus lock indicator in the viewfinder. If you press the AF-Drive button, and then turn the main dial, you can see the other two modes: AI Focus, and AI Servo. AI Servo also tries to find focus when you half-press the shutter button. The difference is, if there is a moving subject in your frame, it will track it, or it will try to track it, and keep it in focus.
So there is a difference here in AI Servo mode; when I half-press the button, I never actually hear a beep. I can still shoot at any time. It's just I'm not hearing a beep, because the camera is trying to track a moving subject. Now, I don't have a moving subject here, but for shooting sports, or wildlife, or any other situation where you've got something moving around, this might be an easier way to autofocus. Let's go back and take a look at AI Focus. This does either One Shot, or AI Servo. The camera tries to decide on its own which mode it should be in.
So if something is not moving, it will go to the One Shot autofocus. If something is moving, then it will start tracking it. You are never going to see this display up here change. You will just feel the behavior of the camera change. So nothing is moving in my scene right now; if I half-press the shutter I hear a beep, because it's basically treating this as a One Shot autofocus situation. If something was moving, I wouldn't hear that beep; instead, I would hear a subtle, quieter, continuous beeping to indicate that it was in the middle of focus tracking. Again, I could shoot at any time.
I'm going to switch back to One Shot. My other focus control has to do with where the camera is choosing to focus in the scene, and that's what we're going to spend the rest of this chapter looking at.
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