Canon Speedlite 430EX User Manual page 32

The periphery or bottom of the picture looks dark.
When you set the flash coverage manually, the setting was a
higher number than the lens focal length, resulting in a dark
periphery.
Set the flash coverage that is a lower number than the lens focal
length or set it to auto zoom. (p.18)
If only the bottom of the picture looks dark, you were too
close to the subject.
Keep at least 0.7 m/2.3 ft away from the subject.
The flash exposure is underexposed or overexposed.
There was a highly reflective object (glass window, etc.) in
the picture.
Use FE lock. (p.15)
The subject has a very dark or light color.
Set flash exposure compensation. For a dark subject, set a
decreased flash exposure. And for a bright subject, set an
increased flash exposure. (p.14)
You used high-speed sync.
With high-speed sync, the effective flash range will be shorter.
Make sure the subject is within the effective flash range displayed.
(p.16)
The picture is really blurred.
The shooting mode was set to <W>, and the scene was
dark.
Use a tripod or set the shooting mode to <V>. (p.12)
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