Pentax K-X Operating Manual page 11

When you adjust the focus manually, you can use either the focus
indicator or the matte field in the viewfinder.
The focus indicator O appears in the viewfinder when the subject is in
focus even during manual focus.
You can manually adjust the focus using the focus indicator O.
1 Set the focus mode lever to MF.
2
Look through the viewfinder,
press the shutter release button
halfway and turn the focusing
ring.
The focus indicator • appears and you
will hear a beep when the subject comes
into focus.
Focuslndicator