Canon EOS 5D Mark III Instruction Manual page 213

Shooting conditions that can make focusing difficult
Low-contrast subjects such as the blue sky and solid-color, flat
surfaces.
Subjects in low light.
Stripes and other patterns where there is contrast only in the
horizontal direction.
Under a light source whose brightness, color, or pattern keeps
changing.
Night scenes or points of light.
Under fluorescent or LED light sources or when the image flickers.
Extremely small subjects.
Subjects at the edge of the picture.
Subjects strongly reflecting light.
The AF point covers both a near and faraway subject (such as an
animal in a cage).
Subjects that keep moving within the AF point and cannot keep still
due to camera shake or subject blur.
A subject approaching or moving away from the camera.
Autofocusing while the subject is way out of focus.
Soft focus effect is applied with a soft focus lens.
A special effects filter is used.
If you use AF with any of the following lenses, using [Quick mode] is
recommended. If you use the [Live mode] or [u Live mode] for AF, it may
take a longer time to achieve focus or the camera may not be able to
achieve correct focus.
EF 50mm f/1.4 USM, EF 50mm f/1.8 II, EF 50mm f/2.5 Compact Macro,
EF 75-300mm f/4-5.6 III, EF 75-300mm f/4-5.6 III USM
For information on discontinued lenses, refer to your local Canon website.
Using AF to Focus
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