Canon EOS REBEL XS Manual Do Utilizador

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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.
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