Ii-TTL Flash Control
Nikon's i-TTL (intelligent through-the-lens) flash system meters every exposure, setting the Nikon Speedlight's output to the precise level needed to balance the scene's ambient light.
Milliseconds before the main flash, Nikon Speedlights emit a monitor pre-flash, which reflects off every object in the frame, sending to the five-segment i-TTL flash sensor and RGB sensor the data about the scene's available light and shadow areas, subject distance, reflectance and color temperature. This information, along with data from the Matrix metering system, is analyzed to adjust the flash's output for the most balanced background-to-foreground exposure possible. All of this complex processing happens in a fraction of a second, before each exposure, to provide unprecedented levels of flash precision and performance.