Olympus Camedia C-2 Reference Manual page 25

FOCUS LOCK
SUBJECTS THAT ARE DIFFICULT TO FOCUS ON USING THE AUTOFOCUS
The autofocus may not work properly for the following subjects.
If this happens, lock the focus on an object with high contrast that is at the
same distance from the camera as the subject, then aim at the subject to
take the picture. If the subject has no vertical lines, hold the camera
vertically to lock the focus, then return it to the horizontal position to take the
picture.
The green lamp on the right of the
viewfinder blinks and the subject
can not be brought into focus.
Subjects with low contrast at the
center of the image.
Subjects without vertical lines.
Subjects including an extremely bright
area at the center of the image.
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The green lamp on the right of the
viewfinder lights but the subject can
not be brought into focus.
Subjects at different distance.
Fast-moving subjects.
The subject to be focused is not
located at the center of the image.