Canon eos rebel t1i - Manual (Page 192)

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Autofocus can fail to achieve focus (the focus confirmation light <o>
blinks) with certain subjects such as the following:
Subjects difficult to focus
Low-contrast subjects
(Example: Blue sky, solid-color walls, etc.)
Subjects in low light
Extremely backlit or reflective subjects
(Example: Car with a reflective body, etc.)
Near and far subjects covered by an AF point
(Example: Animal in a cage, etc.)
Repetitive patterns
(Example: Skyscraper windows, computer keyboards, etc.)
In such cases, do one of the following:
(1) With One-Shot AF, focus an object at the same distance as the
subject and lock the focus before recomposing (p.46).
(2) Set the lens focus mode switch to <MF> and focus manually.
When Autofocus Fails
If an Extender (sold separately) is attached and the maximum aperture of
the lens is f/5.6 or smaller (larger f/number), AF will not be possible
(except Live mode/u Live mode AF). For details, see the Extender’s
instruction manual.
For the situations where AF can fail to achieve focus with Live mode/u
Live mode AF, see page 115.