Balanced Fill-Flash automatically balances the output of the Nikon Speedlight and the scene’s ambient light.
Immediately before the main flash goes off, the Speedlight fires a series of monitor pre-flashes, which convey Information about the scene’s lighting to the camera. Combined with information from the camera’s 3D Color Matrix metering system, the information is analyzed to adjust flash output to balance the scene’s ambient light.
All of this complex processing happens in a fraction of a second, before each exposure, to provide unprecedented levels of flash precision and performance.
Now you Know!