Canon Xsi Instruction Manual page 106

Using AF to FocusN
With Live mode, AF can fail to achieve focus in the following
cases:
Low-contrast subject such as a blue sky and solid-color 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 strongly reflecting light.
When both near and far subjects are covering the AF points, 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 leaving the camera.
When AF is used for an extremely out-of-focus subject.
When shooting a soft-focus image with a soft-focus lens.
When using a special effect filter.
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