Polaroid 140 - Manual (Page 30)

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The camera may not be able to focus in certain situations. At this time, the
focus-lock function (p. 28, 41) can be used to focus on an object at the same
distance as your subject, and then the image can be recomposed to take the
picture.
When your subject is
near a very bright object
or area.
When two subjects
overlap in the focus
frame.
SPECIAL FOCUSING SITUATIONS