Nikon D90 - Manual (Page 80)

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❚❚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 focusing
ring until the in-focus indicator (
I) is displayed. Note that with
the subjects listed on page 55, 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.).
Focal plane mark
46.5mm