With current i-TTL capable Nikon DSLR cameras, the exposure compensation adjustment also affects the i-TTL flash exposure when shooting in an automated exposure mode like aperture priority. In another word, exposure compensation affects both ambient and flash exposure. If you want to reduce the ambient exposure while keeping the flash exposure unchanged, you’d have to make two adjustments: first dial down the exposure compensation, then increase the flash exposure compensation.
With the recently announced Nikon D4, the exposure compensation can be decoupled from flash exposure compensation by making a change in custom settings. This affects i-TTL flash photos taken in Aperture Priority, Shutter Priority or Program modes with a Speedlight (wired via SC-29 or wireless via commander unit).
More information: Exposure Compensation When Using i-TTL Gets Easier with the D4
Keywords: Flash photography, Nikon D4