Through the AF-area mode available to you on your Nikon D3100, you tell the camera's autofocusing system what part of the frame contains your subject so that it can set the focusing distance correctly.
The Live View AF-area mode options are different than the ones available for viewfinder photography. For Live View photography and video recording, you can choose from the following settings:
Face Priority: Designed for portrait shooting, this mode attempts to hunt down and lock focus on faces when you press the shutter button halfway. This setting is the only one available if you use the Auto or Auto Flash Off exposure modes, and is the default setting in Portrait, Child, Landscape, or Night Portrait mode.
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. When you press the shutter button halfway, the camera locks focus on objects within the 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. It’s the default mode for pictures you take in the Close Up exposure mode.
Subject Tracking: This mode tracks a subject as it moves through the frame and is designed for focusing on a moving subject. But note that subject tracking isn't always as successful as you might hope. When the conditions are right, it works well, but otherwise, the Wide Area setting gives you a better chance of keeping a moving subject in focus.
Again, Auto and Auto Flash Off modes don't permit you to change the AF-area setting. In other modes, use either of these two techniques:
Quick Settings display: Press the Info Edit button to shift to Quick Settings mode. Then highlight the AF-area mode icon. Press OK to access a screen containing the four focusing options. Highlight your choice, press OK, and then press the Info Edit button again to exit the Quick Settings screen.
Shooting menu: As an alternative, you can change the setting via the Shooting menu. Select AF-Area Mode from the menu and then press OK. Then choose Live View to display the list of options.
Any time you press the Menu button, the camera exits Live View mode. So after you change the menu setting, you must flip the Live View switch to get back to the Live View display.
