Canon EOS 70D Instruction Manual page 241

Using AF to Focus (AF Method)
Shooting Conditions that Make Focusing Difficult
Low-contrast subjects such as the blue sky, solid-color flat surfaces
or when highlight or shadow details are lost.
Subjects in low light.
Stripes and other patterns where there is contrast only in the
horizontal direction.
Subjects with repetitive patterns (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.
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 distant 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.
Noise (spots, banding, etc.) appears on the screen during AF.
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