Canon PowerShot SX10 IS User Manual page 121

The AF frame appears as follows when the shutter button is
pressed halfway.
- Green: Shooting preparations complete
- Yellow: Focusing difficulty ([Center] or [FlexiZone] option)
- No AF frame: Focusing difficulty ([Face Detect] option)
Face Detect Feature
When the camera detects faces, AF frames will be displayed on a
maximum of three faces. At this point, the frame the camera
judges to be the main subject appears in white while the others
appear in gray. When the shutter button is pressed halfway and
the camera focuses, up to 9 green AF frames may appear. You
can also select the person you wish to focus on (p. 122).
The camera will shoot using [Center] not the [Face Detect] function
if a white frame fails to appear and only gray frames display, or if a
face is not detected.
The camera may mistakenly identify non-human subjects as a
face.
In certain cases, faces may not be detected.
Examples:
- If faces are at the edge of the screen, or appear extremely small,
large, dark or bright, relative to the overall composition.
- Faces turned to the side or at a diagonal, or faces with a portion
hidden.
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