Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 357)

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 light sources with constantly changing brightness, colors, or patterns.
Night scenes or points of light.
The image flickers under fluorescent or LED lighting.
Extremely small subjects.
Subjects at the edge of the screen.
Strongly backlit or reflective subjects (Example: Car with a highly reflective surfaces,
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.
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.
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