3D Multi-Sensor Balanced Fill-Flash
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The camera is using the lens internal D chip which provides it with additional subject to distance information. The Multi Sensor Balanced Fill-Flash (without the 3D) on the other hand does not need the distance input to work.
In this mode, the camera fires off a series of small, so called "monitor pre flashes" and measures the light coming back in using multiple TTL sensors. Combined with the distance-to-subject information it adjusts the light for the main flash.
Using your camera-lens-flash in this combination should give you natural fill-flash light. It is fill flash so you should not use it when you need more light than just for filling.
This mode has now been superseded by the the i-TTL mode's of later camera/flash combinations.
You probably also want to see our information on CLS, Nikon's Creative Lighting System.















