Canon PowerShot SX10 IS User Manual page 15

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Changing the AF Frame Size ([Flex_Zon÷] selected)
You can change the size of the AF frame to match the size of the
subject. When your target subject is small or when you want to focus
on a specific portion of the subject, you can decrease the size of the
AF frame to shrink the area of focus.
<:
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I_] he AF frame size is set to normal when the digital zoom,
digital teleoconverter or manual focus is used.