Nikon COOLPIX S9 - Manual (Page 69)

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The Movie Menu
Mo
vi
es
Auto-Focus Mode
Choose how the camera focuses in movie mode.
Z Single AF (default)
Camera focuses when shutter-release button is pressed halfway and locks at
this position during shooting, preventing sound of camera focusing from
interfering with recording.
a Full-time AF
Camera focuses continuously, even when shutter-release button is not
pressed halfway.