Do you know what i-TTL stands for?
Nikon’s i-TTL (intelligent through-the-lens) 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, …