In Auto mode, the flash fires when the camera detects low light -ideal for when the subject is in a dark room or backlit. However, it can be an unflattering, hard light if you can’t bounce it, and often the foreground is too bright or the background black. Turning the flash off or switching to Slow Sync may create a more flattering image.
When in the Flash On mode, the flash will always fire regardless of light levels. This is great for when your subject is backlit, such as in our example above, as it will fill in shadows on their face. In Flash Off mode the flash is disabled, which is suited for when there is a lot of natural light and flash would overpower the image.
This mode helps reduce the red-eye effect that occurs when the flash bounces off the retina of the sitter, like in our example here. If we had used red-eye reduction mode, a series of pre-flashes or a bright light would have been fired before the shot was taken, causing the sitter’s pupils to shrink and lessening the red-eye effect.
This mode is useful if you want to use the flash and record the ambient light around and behind your subject. The flash fires and the photo is taken, but as the exposure and shutter speed is longer, natural light is still recorded. It’s great when you want to capture light trails, although you’ll need a tripod for longer exposures.