Canon REBEL T3I EOS 600D User Manual page 135

Changing the Autofocus Mode
Shooting conditions which 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 lighting 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 which 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.
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