Fill flash is used to put light into areas of a picture where it is lacking, such as deep shadows on days with strong light and backlight subjects. Typically fill flash is used when working with close subjects but can be expanded by using multiple flash units. Fill flash has become particularly effective since the advent of TTL, Through The Lens, metering and pre-flash metering on the Nikon systems. This determines the amount of flash to use. To help balance the effect of flash and ambient light it is possible to exposure adjust the amount of flash as you would the normal exposure. Typically when using fill flash I will work with between -1 and -2 compensation on the flash output to get a more balance feel to the flash/ambient light mix.