Canon PowerShot S5 IS User Manual page 92

You are recommended to set the shooting mode to
and the exposure compensation setting to zero (±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.
Since the white balance data cannot be read in
mode, preset the white balance in another shooting
mode beforehand.
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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.