Nikon d3s 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 focus ring until the in-focus 
indicator (
I) is displayed. Note that with 
the subjects listed on page 98, the in-focus 
indicator may sometimes be displayed 
when the subject is not in focus; confirm 
focus in the viewfinder before shooting.
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