Canon PowerShot V10 Advanced User's Manual page 78

Face Tracking AF
The camera detects and focuses on people's faces.
When no face is detected, focusing areas are determined automatically.
Caution
An object other than a human face may be detected as a face.
The camera may not detect faces that are very small or large in the picture, too
bright or dark, or partly hidden.
AF may not detect subjects or people's faces at the edges of the screen.
Recompose the shot to center the subject or bring the subject closer to the center.
78