Canon EOS Rebel XTI - Manual (Page 160)

160
The camera might not be able to focus the following types of subjects:
Low-contrast subjects
Example: Blue sky, solid-color walls, etc.
Subjects in low light.
Extremely backlit reflective subjects
Example: Car with a reflective body, etc.
Overlapping near and far objects
Example: Animal in a cage, etc.
Repetitive patterns
Example: Skyscraper windows, computer keyboards, etc.
In such cases, do one of the following:
1. Focus an object at the same distance as the subject and lock the
focus before recomposing. (p.42)
2. Set the lens focus mode switch to <MF> and focus manually.
When Autofocus Fails
If an Extender (optional) is attached and the maximum aperture of the lens
is f/5.6 or smaller (larger f/number), AF will not be possible. For details, see
the Extender’s instruction manual.