Flash Exposure Compensation
An offset applied to TTL flash output, in stops, independent of the ambient exposure compensation.
Measured in
stops (EV)
In depth
Because it moves only the flash, it is the control that sets the balance between flash and ambient light. Dialling it down produces subtle fill that lifts shadows without announcing itself; dialling it up makes flash the dominant source. It is the single most useful control on an on-camera flash.
Examples in practice
Setting −1⅔ EV of flash compensation turns obvious fill flash into a discreet lift under a subject's eyes in daylight.