Fill in Flash Photography
Great For Evening Out Heavily Back-Lit Subjects
Fill in flash can literally save a picture that is in danger of having very hard shadows on the subjects face, or being overly backlit meaning the face is underexposed.
It can also simply add a pleasing effect to an already good shot, such as as highlights or catchlights in the eyes.
Using Fill in Flash Outdoors
You can see in the image below that the subject is being back-lit by the setting sun. Ordinarily, this would create harsh shadows on the face as the camera meters for the bright sunlight. However, by adding a "pop" of fill in flash, you can see that it has:
To get shots like this, I generally just turn the flashgun on and set it to E-TTL (Auto) mode. In this image above, the fill in flash was diffused and placed off camera to the right of the model. See the light in the eyes?
As the flashgun (Canon 580EX II) is dedicated to the camera, it works out when the subject has received enough light and shuts off accordingly.
As to what method you would use in this situation,
...it is up to you and dependent entirely on the lighting available and what you want to achieve.
Weddings and Fill in Flash
I love back lighting, and fill in flash comes in particularly useful at weddings that I shoot.
You never know what the light is going to be like whether it is from the sun, the reception venue or the DJ.
In the image to the left (or above on desktop), we had a bright sunny day that was creating harsh shadows. For this shot, I used two off-camera speedlights with Pocket Wizards.
One to the right and just behind the couple and the other pretty much directly at them. It has worked well and created a nice "separation light" down the sides where there would normally be shadows.
Without fill in flash, the shot above could have been ruined or at least not as good. There were no clouds in the sky and the air was clear creating a harsh light. The fill in flash pulled out the features and shadows perfectly.
If you are unsure, use the normal settings that the camera's meter chooses. Then put the flashgun onto Auto or E-TTL and you should be ok. Of course, the beauty of digital is that you can see the results instantly. If you are shooting in direct sunlight, always try to turn the subject so that the sun is behind them. Otherwise you will end up with some serious shadows to deal with in Photoshop!
If you ARE shooting a wedding, I would seriously suggest that you go to the venue a week or so beforehand. Take some test shots at the time of the actual event. You don't want to be keeping the couple and guests waiting while you play around with the flash settings!
Lastly, if you have some images already taken that could do with some fill in flash, fear not. One of my favourite tools from the Kubota Image Tools range for Photoshop is an action called (strangely enough), digital fill-in-flash. It mimics actual speedlight flash perfectly and I use it all the time.
You can find out more about this, and all the other actions that come with it, in Kubota's Image Tools Vol 2.