Canon POWERSHOT SD3500 IS Digital ELPH User Manual page 84

You can change the AF (Auto Focus) frame mode to match the conditions
you want to shoot.
Choose [AF Frame].
Touch FUNC. then
MENU
to choose the
tab. Touch [AF Frame], then touch 4_
to choose an option.
Detects people's faces and sets the focus, exposure (evaluative metering
only) and white balance (AWBonly).
When the camera is pointed at subjects, a white frame appears on the
face the camera judges to be the main subject, and gray frames (max. 2)
will appear on other faces.
A frame wil! follow a detected face that is moving within a certain range.
When the shutter button is pressed halfway, up to 9 green frames will
appear on faces on which the camera focuses. If a face is not detected
when Servo AF is set to [On], the AF frame will appear in the center of the
screen.
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