Canon 850 IS - Manual (Page 52)

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You can lock the flash exposure so that the exposure settings are
correctly set regardless of the composition of your subject.
To Release the FE Lock
Press the
button.
Locking the Flash Exposure Setting
(FE Lock)
Shooting Mode
1 Turn on the LCD monitor.
2 Press the button and select (flash on).
3 Focus on the part of the subject on which
you wish to lock the flash exposure setting.
4
Press the shutter button halfway and press
the button.
The flash will pre-fire and will display.
5
Re-aim the camera to compose the shot as
desired and press the shutter button fully.