Nikon D40 - Manual (Page 40)

28 Reference: More on Photography (All Modes)/Focus
Manual Focus
Use this option when you wish to adjust focus manually or when
using a manual focus lens. To focus manually, select manual focus
mode and adjust the lens focusing ring until the image displayed
on the clear matte field in the viewfinder is in focus. Photographs
can be taken at any time, even when the image is not in focus.
When using a lens that offers A-M selection, select M when focusing manually. With lenses
that support M/A (autofocus with manual priority), focus can be adjusted manually with the
lens set to M or M/A. See the documentation provided with your lens for details.
Controls used
The Electronic Range Finder
If the lens has a maximum aperture of f/5.6 or faster, the viewfinder focus
indicator can be used to confirm whether the portion of the subject in
the selected focus area is in focus. After positioning the subject in the
active focus area, press the shutter-release button halfway and rotate the
lens focusing ring until the in-focus indicator (z) is displayed.
Focal Plane Position
To determine the distance between your subject and the camera, mea-
sure from the focal plane mark on the camera body. The distance
between the lens mounting flange and the focal plane is 46.5mm
(1.83in.).