Canon EOS 5Ds Instruction Manual page 273

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 dots of light.
Under fluorescent or LED lighting when the image flickers.
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 while 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 (spots, banding, etc.) appears on the screen during AF.
Using AF with any of the following lenses may take longer to achieve focus,
or the correct focus may not be achieved.
EF50mm f/1.4 USM, EF50mm f/1.8 II, EF50mm f/2.5 Compact Macro,
EF75-300mm f/4-5.6 III, EF75-300mm f/4-5.6 III USM
For information on discontinued lenses, refer to the Canon website.
Using AF to Focus (AF Method)
273