Selecting a Focusing Target in Live View with a Nikon D5100
Through the AF-Area (Autofocus Area) mode, you give the Nikon D5100's autofocusing system instructions on what part of the frame contains your subject so that it can set the focusing distance correctly.
As with the Focus mode, the Live View AF-Area mode options are different than the ones available for viewfinder photography. For Live View photography and movie recording, you can choose from the following settings:
Wide Area: In this mode, you use the Multi Selector to move a little rectangular focusing frame around the screen to specify your desired focusing spot. The red rectangle you see is the Wide Area focusing frame.
Normal Area: This mode works the same way as Wide Area autofocusing but uses a smaller focusing frame. The idea is to enable you to base focus on a very specific area. With such a small focusing frame, however, you can easily miss your focus target when handholding the camera. If you move the camera slightly as you’re setting focus and the focusing frame shifts off your subject as a result, focus will be incorrect. So for best results, use a tripod in this mode.
Face Priority: Designed for portrait shooting, this mode attempts to hunt down and focus on faces. Face Detection typically works only when your subjects are facing the camera, however. If the camera can't detect a face, you see a plain red focus frame, and things work as they do in Wide Area mode. In a group shot, the camera typically focuses on the closest face.
Subject Tracking: This mode tracks a subject as it moves through the frame and is designed for focusing on a moving subject. But subject tracking isn't always as successful as you might hope. For a subject that occupies only a small part of the frame, autofocus may lose its way. Ditto for subjects moving at a face pace, subjects getting larger or smaller in the frame (when moving toward you and then away from you, for example), or scenes in which not much contrast exists between the subject or the background. Oh, and scenes in which there's a great deal of contrast can create problems, too.
In the Auto and Auto Flash Off exposure modes or the Miniature Effects mode, you have no control over this setting; the camera uses the Face Priority setting for Auto and Auto Flash Off and Wide Area for the Miniature mode. In other exposure modes, adjust the setting by pressing the Info Edit button to shift to Quick Settings mode. Then highlight the AF-Area mode icon. The icon represents the currently selected setting. To change to a different setting, press OK. You then see the four focusing options; again, the currently selected one is highlighted initially. Use the Multi Selector to highlight your choice, press OK, and then press the Info Edit button again or give the shutter button a quick half-press to exit the Quick Settings screen.