Fill Flash???

I've thought fill flash is changing my flash exposure compensation on my camera. Is this right? Or do you have to have a flash that has a changing power output? Thanks

Fill flash technically is taking 'two exposures in one shot'...that is, using the ambient light source to made the main exposure, then using a supplemental flash to fill in shadow areas (such as, in the eye sockets or under the chin, for example). Since ambient light varies in brightness (clear sun vs. partly cloudy vs. overcast) you need a flash to cope with that difference. While a skilled photographer can balance even a fixed-output flash to the ambient light, it is far easier for the typical snapshooter to use a variable output flash, with the flash either on Manual or on Auto or on ETTL