Fill-in flash is used to soften dark areas or shadows in a photo when a strong source of light, such as a bright sky, comes from behind a subject. This type of light is called backlighting. Fill-in flash can also be effective when a subject is in the shade and within the range of the flash.
A camera is not as capable as the human at capturing subtle ranges of dark and light. To help understand how a camera’s automatic exposure settings can be fooled in back-lit situations, think of your own eyes.
When the sun is bright, you squint to reduce the light coming into your eyes. A camera’s exposure meter acts in a similar manner. The camera is sometimes fooled into “squinting” (closing down the aperture) because the camera exposes for the background light. Less light comes through the lens and causes foreground objects to become dark.
In these situations, select the fill-in flash setting on your camera. Fill-in flash provides a short burst of light which softly fills in dark areas. Color is truer-to-life and more detail appears in dark areas.
As a general rule, the person taking the photo should stand with his or her back to the main source of light. If lighting is bad when you’re photographing scenery, try to return at a different time of day when light falls on the front rather than behind a subject.