Short and sweet: Flash exposure compensation – your control to balance flash light with ambient light.
A flash exposure compensation increases or decreases the amount of light emitted by the flash compared to the amount determined by the camera. The adjustment is made as with ”normal“ exposure compensation with numerical values of e.g. +/- 1.0, whereby each change by a whole step doubles or halves the amount of light.
The left of the two photos of Freddy is as in the introducing article The flash – what can it do to your photos? – fully automatic fill flash, the shadows in the face have almost disappeared. To the right, another shot with flash exposure compensation -1.0, the shadows are more pronounced, more natural this way.
A second example shows all three variants in comparison, taken on a lost places tour.
My experience is that when brightening shadows, a negative flash exposure compensation of e.g. -1.0 is usually advantageous. This way you preserve some of the shadows and the photo looks more lifelike.
The same trick applied to the example photo for long flash sync: Here a flash exposure compensation of -1.0 reduces the very harsh contrast between the main subject and the background.
Finally, two more tips on flash exposure compensation: