Diffuser; Diffusion; Diffusion Filter
A diffuser is used to scatter the light entering the camera or in the region of the shot. As the light is scattered it creates soft light where the shadows are muted without hard edges. Diffused or scattered light is the opposite of hard light with sharp edged shadows. Diffusion is also opposed to reflection. (See: Specular reflection and Diffuse reflection)
Diffused light can be created in two main ways. The optical method uses a filter or glass element. Alternatively, diffused light can be created by using some form of cloth or other translucent material that allows light through. The use of a diffuser is intended to break up a hard beam of light.
Optical Diffuser: This type of diffuser creates light that softens the focus by the use of a filter or special glass element attached to the camera. The filter is screwed onto the end of the lens using the filter threads, or a filter frame is placed on the end of the lens and the diffuser is placed into the frame. The overall effect is to create a hazy appearance, or a sort of dreamy appearance. Diffusion filters are often used at weddings to create a romantic atmosphere.
Cloth or translucent diffuser: This type of diffuser is normally a fine mesh, light white cloth, translucent paper or white polythene material. The light will pass through but will be reduced in intensity and will be scattered allowing no direct beam of light on the shadow side of the diffuser. This type of diffuser can be almost any size. Small ones may be used on a still-life table-top study. Large ones may be stretched across a frame and hoisted above a large scene to diffuse the sun on bright sunny days. Curtain diffusers may be drawn across a bank of lights to provide a high-key background.
Flash diffuser: Flash light in photography creates a very hard light. The strong directed beam of light often creates a harsh, washed-out scene. Using a diffuser on a camera flash, or over a flash or studio light softens the flash, reducing the harshness. The resultant scattered light tends to be gentle and create more flattering lines, especially on the face. It helps the face to be softer and more rounded. Flash diffusers can be as simple as a tissue paper sheet over a flash unit. They can be more complicated like a special fitting that goes over the flash unit scattering light in all directions. Most off-camera flash units have diffusion screens that can be pulled out and placed over the main flash lens so it spreads out the light in many directions.
Diffusion Gels: These are used mainly to diffuse theatre or studio lights. Gels provide either diffusion or colour or both.
Diffusion reflectors: Some diffusers are actually reflectors. The light hits a surface and the light is scattered, because of the surface characteristics, so that it is diffused. See main article: Definition: Diffuse Reflection.
Softbox: The use of a softbox is mainly as a diffuser of light. They routinely have a large opening at the front which spreads the light. The light can be simply reflected from inside the softbox. Often the soft box will have a diffusion screen fitted on the front to create an even more diffused light.
Umbrella: These are used primarily as diffusers in photography. They are used either to diffuse light by reflective diffusion, or shoot-through diffusion. See the article on Photographic Umbrellas.
The impact of diffusers
By its very nature, a diffuser does two things, it will reflect some of the light beam back toward the source. It will also scatter the light on the side away from the source. Both these have the impact of reducing the light intensity. So, in order to have a high light intensity, but still have diffused, scattered light you may need to have either brighter lights or more lights.You should ensure that your light meter is measuring the actual light after the diffuser has been in place to get a true reading.
If using on-camera flash with a fitted diffuser ensure that you do test shots with chimping to check the light intensity. Do not rely on one shot or you may find that through-the-lens metering is fooled by the fitted diffuser.