When Autofocus Fails - Canon EOS 5D Mark III Instruction Manual

When Autofocus Fails

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.111).
For conditions where AF can fail to achieve focus with [Live mode]/[u Live
mode] during Live View shooting, see page 211.
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