Nikon D3400 - Press the Shutter-Release Button Halfway (0

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4 Press the shutter-release
button halfway.
Viewfinder photography: Press
the shutter-release button
halfway to focus (if the
subject is poorly lit, the flash
may pop up and the AF-
assist illuminator may light).
When the focus operation is complete, a beep will sound (a
beep may not sound if the subject is moving) and the in-focus
indicator (I) will appear in the viewfinder.
Live view: The focus point flashes green
while the camera focuses. If the
camera is able to focus, the focus
point will be displayed in green;
otherwise, the focus point will flash
red.
In-focus indicator Description
I Subject in focus.
I
(flashes)
Camera unable to focus using autofocus.
See
page 91.
In-focus indicator