Canon A580 - Manual (Page 98)

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z 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).
z 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 or is recommended. If the
flash fires when reading the white balance data with the
flash set to
, make sure that you also use the flash
when you shoot.
z The white balance data is retained even after the camera
power is turned off.