Canon SPEEDLITE EL-1 Advanced User's Manual page 61

Note
If the ceiling or wall to bounce the flash light on is too far away, shooting with the
appropriate exposure may not be possible since the bounced flash may be too
weak.
If the picture appears dark, use a larger aperture opening (smaller f/number) and
try again. You can also increase the ISO speed when using a digital camera.
The ceiling or wall to bounce the flash light on should be plain white for high
reflectance. If the bounce surface is not white, shooting with the appropriate
exposure may not be possible, since a color cast may result in the picture or the
bounced flash may be too weak.
When Quick flash is fired with bounce flash, underexposure may occur since the
flash output decreases.
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