A ring flash is a flash unit that is formed in a ring. The camera lens can be poked through the middle of the ring to take the shot. Alternatively the flash is mounted on the lens and flash is set in an ‘around-the-lens’ position. Ring flash shares many of the common features of an ordinary off-camera flash and can be controlled by ‘through-the-lens’ technology where the camera takes control of the unit and calculates the best flash for the shot based on the normal exposure settings of ISO, aperture and shutter speed.
The ring flash is commonly used for close portraiture and macro work. Both benefit from the ring of light that is generated by the flash. The idea is that the ring provides an image without shadows or Modeling, especially for smaller items in the shot.
Ring flash units can be very expensive, as with other off-camera options. In recent years the advent of the LED (light emitting diode) lights has made the use of ring flash cheaper and more durable. However, the LED type are not as good at longer distances since they lack the power of true high intensity flash. However, this is not necessarily a bad thing since a ring flash is most effective at close quarters.
There are many DIY ring flash units although the recent LED commercial units are actually almost as cheap as making it yourself.
Recently ring flash units have started to be made in larger sizes. They are particularly good for portrait work. The use of a ring flash creates particularly flattering ring catchlights in the eyes. Large rings can also be non-flash too.