Many professional photographers will tell you that the worst flash results you can end up with occur when shooting with the external flash attached to the camera, because you often end up with a washed out image with too much light.
That's where flash compensation can help, as it can allow you to adjust the power of the flash unit to make the light from the flash complement the external light, rather than overpowering it.
Click the link to read some tips for controlling your DSLR's flash exposure compensation.

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