Process
Flash diffusion works by scattering direct, or columnar, light to create a softer light with fewer sharp edges on shadows. In photography, diffuse light is preferable to direct light because it creates fewer shadows in facial wrinkles and folds and creates softer, more pleasing shadows.
Translucent Diffusers
The most common way of diffusing camera flash is to place one of several devices between the light source and the photographic subject. Although these materials are always translucent, they vary in how they work.
The most common flash diffuser is a light translucent cover with radial lines. As the photons from the flash tube pass through these lines, they are scattered in different directions. This scattering effect is the diffusion. All on-camera flashes and many hot shoe flashes have this type of diffusion by design. Some cameras and hot shoe flashes have an additional radial diffusion panel that can be either placed on the flash or flipped down over the flash.
Translucent flash covers, with a milky appearance, can be placed over the existing flash for more diffusion. These can be either plastic caps that fit over the front of the flash or cloth boxes that mount over the entire flash unit. The milky nature of the material scatters the photons far more than radial flash diffusers. These types of diffusers come at a cost in effective flash distance. The extreme scattering of photons means that less light falls directly on the subject, reducing the distance at which the flash is effective.
In some cases, simple household materials can be used as a flash diffuser. In a pinch, photographers have held or taped napkins, plastic, paper or tissues to the front of the flash to create a homemade diffuser.
Reflective Diffusers
Reflective flash diffusers scatter the light from the flash by using the reflective properties of certain materials. Some on flash cloth diffusers also use reflection to further soften the light. In these devices, the flash is pointed upward at a reflective surface that then transfers the light outward through the milky white cloth.
Another common flash diffuser is a reflective photographic umbrella. The diffusion is a result of the light bouncing off the umbrellas reflective surface and scattering the light in different directions.
Some hot shoe flashes allow the flash to use low ceilings and walls as reflective diffusers. The hot shoe flash can be tilted upward to reflect, or bounce, the light off light colored walls and ceilings. The result is similar to using a very large, diffuse light source.
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