Selecting Focusing Pairs in Live View with a Nikon D5100
The way the Nikon D5100 sets focus during Live View and movie shooting depends on your Focus mode and AF-Area mode settings. If you use the kit lens (or a similar lens), you also need to set the switch on the lens barrel to either A for autofocusing or M for manual focusing. For other lenses, check the lens instruction manual for information about this step.
Until you get fully acquainted with all the various combinations of Focus mode and AF-Area mode and can make your own decisions about which pairings you like best, try the following settings:
For moving subjects: Set the Focus mode to AF-F and the AF-Area mode to Wide Area. You also can try the Subject Tracking AF-Area mode, but some subjects may not be well suited to that mode. Either way, remember that in AF-F mode, you don't press the shutter button halfway until you're ready to lock focus and take the picture. Focusing begins immediately after you switch the Autofocus mode to AF-F and continues until you press the shutter button halfway.
For movie recording, keep your finger off the shutter button if you want the camera to continuously adjust focus during the recording. Just remember that if you use the camera's internal microphone, the sound of the autofocus motor may be audible in the movie. Attach an external microphone or record audio using a separate device to avoid this problem.
For stationary subjects: Set the Focus mode to AF-S and the AF-Area mode to Wide Area. Or, if you're shooting a portrait, give the Face Priority AF-Area option a try.
Press the shutter button halfway to initiate focusing; after the camera finds the focusing point, focus is locked. For movie recording, you can then release the shutter button. For still photography, keep your finger on the button — otherwise, focus will be reset when you press the button to take the picture.
For difficult-to-focus subjects: If the camera has trouble finding the right focusing point when you use autofocus, don't spend too much time fiddling with the different autofocus settings. Just set the camera to manual focusing and twist the focusing ring to set focus yourself.