Nikon D300 User Manual page 97

Manual focus is available for
lenses that do not support
autofocus (non-AF Nikkor lenses)
or when the autofocus does not
produce the desired results
(pg. 70). To focus manually, set
the focus-mode selector to M 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.
Focus-mode
selector
[] A-M Selection/Autofocus with Manual Priority
If the lens supports A-M selection, set the lens A-M switch to M (manual).
If the lens supports M/A (autofocus with manual override, MF priority) or
A/M (autofocus with manual override, AF priority), focus can be adjusted
manually, regardless of the mode selected with the lens. See the
documentation provided with your lens for details.
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