Flash output compensation
Apart from the standard flash modes available, some cameras also feature flash output compensation. It is similar to exposure compensation in that it allows you to manually adjust the value of the flash output power.
Some enable you to vary compensation settings using the familiar EV range by 1 or 2 stops either way in 1/3 EV steps, while others have a “high, normal and low” setting.
This feature is very useful in situations where the camera's normal flash metering system might struggle to achieve a correct exposure.
A typical scene would be where flashlight is used to illuminate a small subject against a dominating dark background. To avoid overexposure or bleaching out of the subject, the flash would need compensating by –1 to –2 stops.
Also when you need just that tiny bit of extra light to soften shadows or to produce a catchlight in a person's eyes when taking a portrait, this feature allows you to fine tune flash exposure for a more subtle effect than is possible with the standard flash modes available on your camera. Since you can see the result of any compensation used immediately after capturing the image, it will be easy to obtain the kind of effect you want by simply taking several images at different settings.