How and When to Use Fill in Flash?

Fill-in used to be an advanced technique that made pro work stand out, particularly in colour. Its main use was for photographing people in bright sunlight. People still occasionally come up to when I'm using it and ask if I know I'm using the flash, despite the sun being out. Modern digital cameras in particular have made this technique straightforward for all of us.

Built in Flash

Most new cameras now come with flash built in. Few of these flashes are powerful enough to be more than a makeshift solution for pictures in the dark, and their closeness to the lens makes 'red-eye' an almost certain result in portraits. I never bother to use the 'red-eye' reduction methods with pre-flashes, as red-eye removal is so easy in your image editing software.
The good news about built in flash systems is that generally they make what used to be the tricky practice of 'fill-in flash' a no-brainer. A few years ago, only those with expensive professional equipment including a dedicated flash unit could be sure of getting this right, at least without a few calculations and a bit of luck.
Unless you had an expensive Nikon or Canon system, fill in flash was too often a matter of hoping and praying. Sometimes it worked for me, other times the flash was too predominant, or failed to have enough effect. Most digital cameras seem to cope well with balancing - it is simply a matter of telling the camera to always use flash and shooting.

What fill-in does

The difference in lighting between the sunlight and shadow areas in a bright sunny scene is too much for film to handle, particularly for colour film. If you expose to keep the highlight detail, the shadows will be black and empty. Using the flash on camera supplies extra light evenly across the subject. This has little or no effect on the brighter areas, but boosts the shadows dramatically to give a much more preferable result. Now as digital camera was easy to determine if you need to use build in flash since you can easily deleted if the imaged was not good.

(fill in flash used (Nikon SB-800)

If you overdo the flash, your subjects will look abnormally bright against the background, often making them seem rather like those life-size advertising cut-outs of people. It is an effect that has been deliberately used, but generally not what you want. Correct fill-in is not noticeable in a picture, or perhaps just makes the subject look a little healthier, but it lightens those deep black shadows, producing full detail in them.
Flash also helps to liven up your subjects on dull days, as in this image of

(build in flash used)

this girl standing against the sunlight. You can see how the flash helps make him stand out from the background.