FujiFilm Discovery S600 ZOOM Date Owner's Manual page 24

Under certain unusual conditions, the camera's autofocus system may not be able to function accurately. At these times, you
should use the "Autofocus Lock" setting ( p.25). Conditions which may require these settings include:
A bright light source with the picture area such as the sun near your subject or a strong reflection from water or the windshield of a vehicle;
A mirror or other strongly reflective surface near the center of your picture;
Photographing black or very dark light absorbing subjects such as hair;
Presence of flames, smoke or gaseous substances;
Photographing through glass.
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