Nikon D3100 Reference Manual page 79

The Electronic Rangefinder
If the lens has a maximum aperture of f/5.6 or faster, the
viewfinder focus indicator can be used to confirm whether the
subject in the selected focus point is in focus (the focus point
can be selected from any of the 11 focus points). After
positioning the subject in the selected focus point, press the
shutter-release button halfway and rotate the lens focus ring
until the in-focus indicator (I) is displayed. Note that with the
subjects listed on page 56, the in-focus indicator may
sometimes be displayed when the subject is not in focus;
confirm focus in the viewfinder before shooting.
A
Focal Plane Position
To determine the distance between your subject and the camera,
measure from the focal plane mark on the camera body. The
distance between the lens mounting flange and the focal plane
is 46.5 mm (1.83 in.).
A
The Exposure Indicator
If desired, the exposure indicator can be used to determine whether the focus point for
manual focus is in front of or behind the subject (0 144).
46.5 mm
Focal plane mark
z
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