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This is a settable option - custom function 7. I will explain.
By default, you initiate autofocusing on a Canon Rebel T3 or T3i by pressing the shutter button halfway and lock autoexposure by pressing the AE (autoexposure) Lock button, which lives on the top-right side of the camera back. After you’re feeling more comfortable with your T3 or T3i, you may want to customize the locking behaviors of the two buttons.
To configure the buttons for still photography, set the Mode dial to one of the advanced exposure modes. You can enjoy full button control only in the P, Tv, Av, M, or A-DEP modes. Then head for Custom Function 7 (on the T3) or 9 (on the T3i). You can choose from the following configuration options. (The first part of the setting name indicates what happens with a half-press of the shutter button; the second part indicates the function of the AE Lock button.)
AF/AE Lock: This is the default setting. Pressing the shutter button halfway initiates autofocus; pressing the AE Lock button locks autoexposure.
AE Lock/AF: With this option, pressing the shutter button halfway locks autoexposure. To initiate autofocusing, you instead press the AE Lock button. In other words, this mode is the exact opposite of the default setup.
AF/AF Lock, no AE Lock: Pressing the shutter button halfway initiates autofocusing, and pressing the AE Lock button locks focus. Autoexposure lock isn’t possible.
This option is designed to prevent focusing mishaps when you use AI Servo autofocusing. In AI Servo mode, the autofocus motor continually adjusts focus from the time you press the shutter button halfway until the time you take the image. This feature helps you keep moving objects sharply focused. But if something moves in front of your subject, the camera may mistakenly focus on that object, which may leave your subject blurry.
To cope with that possibility, this locking option enables you to initiate autofocusing as usual, by pressing the shutter button halfway. But at any time before you take the picture, you can press the AE Lock button to temporarily stop the autofocusing motor from adjusting focus if an intruder moves into the frame. When you release the button, the autofocusing mechanism starts up again.
AE/AF, no AE Lock: Similar to the preceding mode, this one also is designed to help you capture moving subjects in AI Servo mode. Pressing the shutter button halfway initiates autoexposure metering. Pressing the AE Lock button starts the autofocusing servo system; releasing the button stops it.