You tell your Nikon D3100 whether to focus automatically or manually through the Focus mode setting. If you choose to autofocus, you can pick one of two ways of autofocusing.
Here's how to work with each of the Focus mode options:
AF-S (single-servo autofocus): The camera locks focus when you press the shutter button halfway down. Generally speaking, this setting works best with still subjects. For shooting video, you can let go of the shutter button after focus is set.
AF-F (full-time servo autofocus): At this setting, the autofocus motor goes to work immediately, without waiting for you to press the button. For still photography, you press the shutter button halfway when you're ready to lock focus.
When shooting video, the camera adjusts focus continuously throughout the recording as long as you don't press the shutter button halfway. You still can lock focus at any time by pressing the shutter button halfway, though. Release the button, and continuous autofocusing begins again.
This feature has a downside: If you shoot a movie with sound recording enabled, the camera's microphone picks up the sound of the autofocus motor. So if pristine audio is your goal, use AF-S and set focus before you begin recording, or abandon autofocus altogether and focus manually. As another option, you can disable sound recording on the camera and then buy a separate audio recording device to capture sound. You then can combine the soundtrack and the video footage in a video-editing program. (Obviously, this option is for real video buffs only.)
MF (manual focus): Select this option to focus your camera manually, which is done by twisting the focusing ring on the lens.
With the D3100 kit lens and some other lenses, simply moving the switch on the lens from the A (autofocus) to M (manual focus) position automatically selects the MF Focus mode setting. But if your lens doesn't have such a switch, the camera many not choose the right Focus mode setting, so double-check to be sure.
To adjust the Focus mode setting, press the Info Edit button to bring up the Quick Settings screen. Highlight the Focus mode icon and press OK. Select your choice, press OK, and then press the Info Edit button again to return to the Live View display (or just press the shutter button halfway and release it).
