Your camera offers three different autofocusing schemes, which you select through a control called AF mode. The three choices work like so:
1 One-Shot: In this mode, which is geared to shooting stationary subjects, the camera locks focus when you depress the shutter button halfway. Focus remains locked as long as you hold the shutter button at that halfway position.
1 AI Servo: In this mode, the camera adjusts focus continually as needed from the time you press the shutter button halfway to the time you take the picture. This mode is designed to make it easier to focus on moving objects.
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, keep the subject within the diamond-shape area covered by the nine autofocus points. (The preceding section explains these two modes.)
In either case, the green focus dot in the viewfinder blinks rapidly if the camera isn't tracking focus successfully. If all is going well, the focus dot does not light up at all.
1 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.
Which of these three modes are available to you, however, depends on the exposure mode, as follows:
1 In P, Tv, Av, and M modes, you can select any of the three Focus mode options.
i In A-DEP mode as well as in Portrait, Landscape, Night Portrait, and Close Up modes, the camera restricts you to One-Shot mode.
i Sports mode always uses AI Servo autofocus.
i Full Auto and No Flash modes always use AI Focus.
So, assuming that you can choose from all three, which mode is best? Well, here's my take: One-Shot mode works best for shooting still subjects, and AI Servo is the right choice for moving subjects. But frankly, AI Focus does a good job in most cases of making that shift for you and saves you the trouble of having to change the mode each time you go from shooting still to moving subjects. So, in my mind, there's no real reason to fiddle with the setting unless you're shooting moving objects and want to be able to lock focus at a specific position — in which case, my recommendation would be to simply switch to manual focusing anyway.
Whatever you decide, you can set the Focus mode as follows:
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