Slow Sync Flash mode, also known as Night Scene or Night Portrait mode, is used to capture a dimly lit background at night while keeping the foreground properly lit.
When using this mode, a camera uses a slow shutter speed in combination with the flash.
The flash fires briefly to light the foreground subject and capture ambient light to reveal more detail in the background. If Slow Sync flash in not used, the background may appear black or underexposed.
Slow Sync mode is effective when photographing people at twilight. It is also useful when wanting to capture a subject in sharp focus with the background in motion, such as a moving car at night or a concert artist moving across a stage. When using Slow Sync mode on moving subjects, lights will appear in streams.
You can also get similar effects by using shutter-priority mode and a slow shutter speed under 1/60th of a second.
If you don’t want the main subjects to become blurred, a camera support is recommended when using Slow Sync flash.