AF Mode is a setting to assist you in focusing the subject. While you shooting might not be a still subject, AF Mode can "track" the subject and keep it focused.
The first thing you need to do is make sure your camera is in AF (Auto Focus) but not MF (Manual Focus). Basic Zone such as Auto Mode, Portrait Mode, Landscape Mode and etc are advised to use Auto Focus. While Creative Zone such as Shutter-Priority, Aperture-Priority and etc are advised to use Manual Focus, but you can select Auto Focus if you want to use other AF Mode to capture moving object.
There are three AF Mode in most DSLR, you can see the camera setting, shown in the green circle.
1) One Shot
- The camera will Focus once when you press Shutter Button halfway. If you need to re-Focus, you have to release the Shutter Button and press halfway again. This is suitable for still subject.
2) AI Focus
- This AF Mode is suitable for moving subject. The focus distance will keep changing while you hold down the Shutter Button halfway. The camera will first focused by the center AF Point. If the subject move away, the camera will track the subject with another AF Point.
3) AI Servo
- AI Servo got a high focusing speed. The focus distance will keep changing while you hold down the Shutter Button halfway as well. The AF Mode will automatically switch from One Shot or AI Focus to AI Servo once the subject move away from the selected AF Point.
However, If the focus cannot be achieved, the camera will not shoot even you press the Shutter Button completely. Unlike Manual Focus, you can shoot in any time even the subject is not focused. Of course, ALL Auto Focus AF Mode consume battery.
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