Canon EOS rebel xs - Manual (Page 162)

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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 AF). For details, see the Extender’s instruction
manual.
For the cases AF can fail to achieve focus with Live mode AF, see
page 106.