Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 144)

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Autofocus can fail to achieve focus (viewfinder’s focus indicator < o >
blinks) with certain subjects such as the following:
Subjects Difficult to Focus
Subjects with very low contrast
(Example: Blue skies, solid-color flat surfaces, etc.)
Subjects in very low light
Strongly backlit or reflective subjects
(Example: Cars with highly reflective bodies, etc.)
Near and distant subjects framed close to an AF point
(Example: Animals in cages, etc.)
Subjects such as dots of light framed close to an AF point
(Example: Night scenes, etc.)
Subjects with repetitive patterns
(Example: Skyscraper windows, computer keyboards, etc.)
Subjects with finer patterns than an AF point
(Example: Faces or flowers as small as, or smaller than an AF point,
etc.)
In such cases, focus by doing either of the following:
(1) With One-Shot AF, focus on an object at the same distance as the
subject and lock the focus, then recompose the shot (p.89).
(2) Set the lens’s focus mode switch to < MF > and focus manually
(p.145).
When Autofocus Fails
Depending on the subject, focus may be achieved by slightly
recomposing the shot and performing AF operation again.
For conditions that make focusing difficult with AF during Live View
shooting or movie shooting, see page 289.