Nikon D90 User Manual page 332

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Live View
Face-priorityautofocus: A double
yellow border will be displayed
when the camera detects a
portrait subject facing the
camera (if multiple faces, up to
a maximum of five, are
detected, the camera will focus
on the closest subject). The camera automatically sets
focus and exposure for the subject indicated by the
double yellow border. If the camera can no longer
detect the subject (because, for example, the subject
has looked away), borders will no longer be displayed.
Focus.
I
n
wide and normal autofocus,
the camera will focus while the
shutter-release button is pressed
halfway. In face-priority autofocus, the camera will
focus on the face in the double yellow border while the
shutter-release button is pressed halfway. The focus
point will blink green and the monitor may brighten or
darken 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, and that single-servo autofocus
is used regardless of the option selected for autofocus
mode. Check focus in the monitor before shooting.
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