Reduce the direct light your flash produces
A flash diffusor won't make sense to you if you are unfamiliar with the concepts of built-in and external flash. Once you understand how an external flash works, a flash diffusor is pretty basic.
A flash diffusor just softens the light produced by your flash.
Varieties of Flash Diffusors
There are many different type of diffusors you can choose from. All of them fit over the light-emitting end of your external flash.
|The most basic diffusor is a white box that attaches to the top of your flash. This white box causes the light emitted from the flash to be diffused over a wider area. It spreads the light out and tones down its intensity.
This photograph shows an external flash attached to a camera, with the basic diffusor placed over the flash. The diffusor is the object outlined in blue.
Other diffusors bounce the flash, and here you can get different diffusors depending upon where you want the light to bounce.
All the light from the flash can be reflected forward. 80% can be diffused toward the ceiling and 20% can be reflected forward. You can even reflect all of the light forward and diffuse the flash at the same time.
|This photograph shows a bounce diffusor sitting on top of the flash. In this case, the flash is pointed straight up, and the diffusor reflects the light forward. This diffusor has a light plastic element in front of the flash that diffuses the light even further.
This style of flash diffusor works really well for portrait photography. It directs light forward onto your subject, but screens it enough to create soft light on your subject's face.
What Flash Diffusors Are Used For
Why in the world would you ever want something like this? It's a valid question.
The simple answer is that unless it is your intention to take a lot of portrait photographs indoors where you will have to use a flash, you don't. If you do want to take photos with the flash, it is a nice accessory to have, since direct flash often causes harsh shadows that do not result in pleasing portraits.
The more complicated answer is that there are times when you DO want to use flash outside with natural light, but you don't want a lot of flash. Let's say that the subject of your photo is sitting in the shade, and just needs a touch of flash so they don't appear as a black blob in your photo.
If you use a diffuse flash, you won't get harsh shadows, but you will add just enough light to improve your photograph and make your subject stand out from the background.