Fill flash is using your flash to fill in darker parts of your image already lit by other light. I like to use it for all my people photos outdoors as it makes for better, stronger images.
Adding fill flash to your images is easy, just turn on your camera’s flash and automatically get the results shown here. You don’t need to fiddle with any settings; just press the Flash (bolt) icon button and your camera does all the rest of the work, no camera pops up the flash by itself, so you have to turn it on by yourself and the camera does the rest.
When shooting outdoors, fill-flash lights up people’s faces so they aren’t drowned in shadows and also puts highlights in their eyes so they look alive. Additionally, fill-flash lightens shadows and adds details to darker areas, while preserving the highlights. When shooting in direct sunlight, fill flash helps to make your images better, especially when the subject is back lit.
Using an external Speedlight is better than using the pop up flash built into your camera, mostly because the add-on flash has more power, which means greater distance and more fill light for your images. I recommend the 430 EXII if you are a Canon user and the SB-400 if you are shooting Nikon as these are two of the best units for fill-flash and these units are less expensive than the SB-500 or 580EX II which generally run around $500-$600.
Fill-flash also helps with making your colors render more vivid and natural. When shooting with the flash alone I often feel that the shots seem to have a blue tone to them, which is why I use either a slight orange gel or an orange flash diffuser to make the light warmer and the colors stand out better. To have more or less fill flash, use the +/- to increase or decrease the power output of the flash and alter the overall light, which in turn changes the way the image looks.
One last thing to remember is when using fill flash, make sure you take your ISO off auto, especially if using the pop up flash as the camera will set the ISO to the lowest setting, which means if you are shooting indoors, it will be too low.
*Images used for this blog post are courtsey of Ken Rockwell with his permission and are copyrighted by @KenRockwell.com