Canon EOS 5D Mark III Instruction Manual (en)

Autofocus can fail to achieve focus (viewfinder’s focus confirmation light 
<o> blinks) with certain subjects such as the following:
Subjects difficult to focus
Very low-contrast subjects
(Example: Blue sky, solid-color walls, etc.)
Subjects in very low light
Extremely backlit or reflective subjects
(Example: Car with a highly 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 either of the following:
(1) With One-Shot AF, focus on an object at the same distance as the 
subject and lock the focus before recomposing (p.67).
(2) Set the lens focus mode switch to <MF> and focus manually (p.113).
When Autofocus Fails
For conditions where AF can fail to achieve focus with [Live mode]/[u Live 
] during Live View shooting, see page 213.