Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 289)

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Focusing with AF (AF Method)
Shooting Conditions that Make Focusing Difficult
Subject with low-contrast such as the blue sky, solid-color flat
surfaces or when highlight or shadow details are clipped.
Subjects in low light.
Stripes and other patterns where there is contrast only in the
horizontal direction.
Subjects with repetitive patterns (Example: Skyscraper windows,
computer keyboards, etc.).
Fine lines and subject outlines.
Under a light source whose brightness, color, or pattern keeps
changing.
Night scenes or points of light.
The image flickers under fluorescent or LED lighting.
Extremely small subjects.
Subjects at the edge of the picture.
Strongly backlit or reflective subjects (Example: Car with a highly
reflective body, etc.).
Near and distant subjects covered by an AF point (Example: Animal
in a cage, etc.).
Subjects that keep moving within the AF point and will not stay still
due to camera shake or subject blur.
A subject approaching or moving away from the camera.
Performing AF when the subject is very far out of focus.
Soft focus effect is applied with a soft focus lens.
A special effect filter is used.
Noise (dots of light, banding, etc.) appears on the screen during AF.