Canon PowerShot SD870 IS DIGITAL Elph User Manual page 115

You are recommended to set the shooting mode to
and the exposure compensation setting to [±0] before
setting a custom white balance.
The correct white balance may not be obtained when the
exposure setting is incorrect (image appears completely
black or white).
Shoot with the same settings as when reading the white
balance data. If the settings differ, the optimal white
balance may not be set.
In particular, the following should not be changed.
- ISO Speed
- Flash
Setting the flash to [On] or [Off] is recommended. If
the flash fires when reading the white balance data
with the flash set to [Auto], make sure that you also
use the flash when you shoot.
Since the white balance data cannot be read in Stitch
Assist mode, preset the white balance in another
shooting mode beforehand.
White balance data is retained even if the camera's
power is turned off.
Various Shooting Methods
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