Nikon D3100 - Manual (Page 56)

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Focus.
Press the shutter-release button halfway to focus.
Face-priority AF : The camera sets focus for the face in the
double yellow border while the shutter-release button is
pressed halfway; if the camera can no longer detect the
subject (because, for example, the subject has looked
away), borders will no longer be displayed.
Wide- and normal-area AF : The camera focuses on the subject
in the selected focus point while the shutter-release
button is pressed halfway.
Subject-tracking AF : The camera focuses on the current
subject. To end tracking, press J.
The focus point will blink green while the camera focuses. If the camera is able
to focus, the focus point will be displayed in green; if the camera is unable to
focus, the focus point will blink red. Note that pictures can be taken even when
the focus point blinks red. Check focus in the monitor before shooting. Except
in
i and j modes, exposure can be locked by pressing the AE-L/AF-L button
( 0 80). Focus can be locked by pressing the shutter-release button halfway.
D
Subject Tracking
The camera may be unable to track subjects if they are small,
moving quickly, or a similar color to the background, or if both
the subject and the background are very bright or very dark,
the brightness and color of the subject or background varies
greatly, the subject leaves the frame, or the subject visibly
changes size.