You need to take control of the intensity of your flash
There are various ways you can fire your external flashgun. The easiest (and the one we’d recommend if your D-SLR has the feature) is the Commander mode. The advantage of using this system is you can alter the flash strength from your D-SLR. If your Nikon doesn’t have this feature, there are alternative ways to fire the flashgun: either use a triggering system or the built-in flash, set your external flashgun to the Remote Shutter setting and select the SU-4 mode.