Nikon D300 User Manual (en)

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 
51 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 70, the in-
focus indicator may sometimes be 
displayed when the subject is not in focus; 
confirm focus in the viewfinder before 
Focal Plane Position
To determine the distance between your subject 
and the camera, measure from the focal plane 
mark (
E) 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