Canon EOS 6D Instruction Manual page 196

Using AF to Focus (AF Method)
Shooting conditions that 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 and when the image flickers.
Extremely small subjects.
Subjects at the edge of the picture.
Subjects strongly reflecting light.
The AF point covers both near and faraway subjects (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 very far out of focus.
Soft focus effect is applied with a soft focus lens.
A special effect filter is used.
If you use AF with any of the following lenses, using [Quick mode] is
recommended. If you use the [FlexiZoneAF (Single)] 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.
EF28mm f/2.8, EF35mm f/2, EF50mm f/1.4 USM, EF50mm f/1.8 II,
EF50mm f/2.5 Compact Macro, EF135mm f/2.8 (Softfocus),
EF75-300mm f/4-5.6 III, EF75-300mm f/4-5.6 III USM
For information on discontinued lenses, refer to your local Canon Web site.
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