Canon PowerShot SX10 IS User Manual page 8

1141
_ The AF frame cannot be zoomed under the following
circumstances:
- When [AF Frame] hsset to [Face Detect] and a face is not
detected, or when the face is extremely large in relation to
the overall composition.
- When the camera cannot focus.
- When the digital zoom is unuse.
- When a TV is being used as the display.
- When [Servo AF] (p. 124) is set to [On].