Your Canon 1100D - Rebel T3 or T3i offers three different autofocusing schemes, which you select through the AF mode control. The three choices work like so:
One Shot: In this mode, which is geared to shooting stationary subjects, the camera locks focuswhen you press the shutter button halfway. Focus remains locked as long as you hold the shutter button at that halfway position. This autofocus setting is always used for Portrait, Landscape, Night Portrait, and Close-up modes.
AI Servo: In this mode (the AI stands for artificial intelligence, if you care), the camera adjusts focus continually as needed from when you press the shutter button halfway to the time you take the picture. This mode is designed to make focusing on moving subjects easier, and it’s the one the camera uses when you shoot in Sports exposure mode.
For AI Servo to work properly, you must reframe as needed to keep your subject under the active autofocus point if you’re working in Manual AF Point Selection mode. If the camera is set to Automatic AF Point Selection, the camera initially bases focus on the center focus point. If the subject moves away from the point, focus should still be okay as long as you keep the subject within the area covered by one of the other nine autofocus points.
AI Focus: This mode automatically switches the camera from One Shot to AI Servo as needed. When you first press the shutter button halfway, focus is locked on the active autofocus point (or points), as usual in One Shot mode. But if the subject moves, the camera shifts into AI Servo mode and adjusts focus as it thinks is warranted. AI Focus is the only setting available when you shoot in the Scene Intelligent Auto, Flash Off, and Creative Auto modes.
One way to remember which mode is which: For still subjects, you only need one shot at setting focus. For moving subjects, think of a tennis or volleyball player serving the ball — so AI Servo for action shots.
You can set the AF mode in two ways:
AF mode button (right cross key): Your fastest move is to press this button. Select a setting and press Set.
Quick Control screen: Display the Shooting Settings screen. (Just press the shutter button halfway and release it.) Then press the Quick Control button and use the cross keys to highlight the icon. The selected AF mode setting appears at the bottom of the screen. Rotate the Main dial to cycle through the three mode options.
If you prefer, you can press Set after highlighting the AF mode icon to display the screen where all the mode choices appear. Then use the cross keys or Main dial to highlight your choice and press Set again.