Digital cameras with built-in flash units often provide several selectable flash modes. In Automatic mode, the camera decides when flash is necessary. In Fill Flash mode (or often called Flash ON mode), the flash goes off with every exposure, no matter what lighting conditions exist to eliminate any facial shadows. In Flash Off mode, the flash does not go off, even in dark settings.
Red-Eye Reduction has a short pre-flash, which helps to reduce the subject’s pupil size before the final flash. Some cameras also have a red beam that prevents the red eye effect. Slow Synch (or Slow Synchro) is similar to a special effect, because it triggers the flash as usual, but leaves the lens aperture open a little longer. This is a nice option when photographing people at night when you want the flash to expose your subjects, but also capture the lights in the surrounding scene.