Canon EOS 5D MARK II Instruction Manual page 34

Using AF to Focus
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If the focus is way off. face detection will not be possible. If the lens
enables manual focusing even while the lens focus mode switch is set to
<AF>. turn the focusing ring to attain rough focus. The face will then be
detected and <: :> will be displayed.
An object other than a human face might be detected as a face.
Face detection will not work if the face is very small or large in the
picture_ too bright or too dark. titled horizontally or diagonally, or partially
hidden
The <: :> focusing frame might cover only part of [he face.
When you press <,:_:,> straight down. the AF mode will switch to the Live
mode (p. 117). You can tilt <,:_ > to move the AF point. If you press <,:_, >
straight down again, it will switch back to Live _ (face detection) mode.
Since AF is not possible with a face detected near the edge of the
picture, the <_ _> will be grayed out. Then if you press the < AF-ON >
button, the center AF point <[_> will be used to focus.
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