Use Fill Flash to Improve Photographs: Add Flash to Fill in Shadows Caused by a Bright Light Source
While it may seem counterintuitive, using flash outdoors can actually improve photos made even in bright sunlight. This is because bright sunlight often causes deep shadows, particularly if the people in the images are wearing baseball caps or are standing in the shadows of a tree or building.
Under such circumstances, the light from the flash unit is used not as the primary light source, but instead just "fills" in the shadows, hence the name "fill flash." This kind of lighting is subtle, but very effective.
Fill flash is easier than ever to use these days thanks to dedicated strobes and through the lens metering technology. Today's cameras and dedicated flash units communicate with each other and the camera shuts the flash output off when the image is properly exposed so there's no need for the photographer to worry about calculating guide numbers to figure out proper exposure.
A photographer should plan on using fill flash outdoors anytime deep shadows are prevalent, particularly when those shadows are in an important part of the image. While the human eye may be able to make out detail in such circumstances, the imaging sensor can't, resulting in blocked up shadows in the photograph if supplemental light isn't provided.
Stay at or Below the Camera's Flash Sync Speed
One thing to keep in mind when using fill flash is to make sure the camera is set at or below the camera's shutter sync speed. For most cameras this is 1/250th of a second or slower. Shooting with the shutter speed set to faster than the camera's sync speed will result in part of the image being black since the shutter will still be traveling while the flash is exposing the image.
To use fill flash, set the camera so that it will properly expose for the ambient (existing) light (shutter priority automation with the shutter speed set to the camera's fastest shutter sync speed) and with the ISO set as low as possible.
Use a Low ISO Setting
The ISO is set to a low setting to make it possible to shoot at a larger aperture since the smaller the aperture, the less effective the light from the flash unit will be. It's very easy in outdoor photography to be shooting at such a small aperture, that the light from the flash has no effect.
Keep in mind that adjusting the exposure compensation setting will help adjust the brightness/darkness of the background. This makes it possible to make the subject pop out against the background if desired.
Fill flash is particularly important when high contrast lighting conditions are present since strong shadows will be a problem. Another advantage of using fill flash is if the subject is squinting because of bright sunlight in their eyes.
Turning the subject around so the sun is behind them solves that problem, but because the subject is now backlit, they will be in heavy shadow. Using fill flash will balance the light properly and give a good exposure without blinding the subject.