Definition: Diffuser; Diffusion Filter

Diffuser; Diffusion Filter

A diffuser is a device we use to scatter light. As light is scattered it creates softer light where the shadows are muted without hard edges.

When you use a diffuser it will reduce the light you shine through it. Some of your light will be reflected off the diffuser before passing through it. Some of you light will also be scattered. So, you can use a diffuser to reduce the brightness of the the light in your shot and to soften the light too.

You can use a diffuser for a wide variety of photographic activities. Creating soft light and fill are probably the most common.

The equipment you would use to create scattered light is called the diffuser. On the other hand, diffusion is the physical process that creates scattered light.

Diffusion is the opposite to reflection where most of the light is bounced straight off your reflection surface. In a reflection you will probably see an image or near image. (See: Specular reflection. On a diffuser you will not see an image, the light is so scattered so that no image is formed in your eye.

Creating diffused light

Diffused light can be created in three main ways.

• The optical method uses a photographic filter or glass element. In this case the lens of your optical diffuser is probably frosted in some way. The translucence prevents you seeing through it, but you will be able to see light passing through. You will see scattered light. Therefore the the eye cannot resolve an image.
• Alternatively, diffused light can be created by using some form of cloth or other translucent material that allows light through. A cloth diffuser is as effective as an optical diffuser if used in the right way. For example you can use them to reduce harsh Sunlight, create partial shadows, or soften deep shadows.
• In the above methods the light shines through the diffuser. However, you can also use a diffusion surface of some sort to create diffused light. This too is a diffuser. You might use a reflection diffuser to create fill around shadows on the face or a table-top study for example. Diffused reflections are relatively weak so the diffusion surface needs to be near to the subject. For that reason, you will need to be work close up.

Diffuser filters and elements

With an optical diffuser you create filter the light entering your lens. That softens the focus of your lens. Your filter or special glass element is screwed onto the end of the lens using the filter threads. Alternatively, you can use a filter frame placed on the front end of the lens. Then you drop the diffuser glass 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 or where you want a romantic atmosphere. One way to make a diffuser is to smear petroleum jelly onto a glass filter. Watch it though – you can make your equipment really messy that way!

The Cloth or translucent diffuser

This type of diffuser is normally a fine mesh, light white cloth, translucent paper or white polythene material. Your light source will will pass through, but it will be reduced in intensity and will be scattered too. The diffuser will allow no direct beam of light through to the shadow side of the diffuser.

You can use a fine cloth or grid, like net curtain, to create a diffuser. It scatters the harsh light in your shot.
View Large

The cloth type of diffuser you might use on a shoot can be almost any size. A small one can be used in still-life or table-top studies. If you have a large one you can hoist it above a large scene to diffuse Sunlight 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 from your flash will create a harsh, washed-out scene. Using one on your on-camera flash, or with a flash or studio light, softens the flash. This reduces the harshness of light in your shot. The scattered light tends to be gentle and create more flattering lines, especially on the face of your subjects. You can create a flash diffusers with a simple tissue paper sheet over a flash unit. Alternatively you can buy fittings that go over you flash unit to scatter the light. Most off-camera flash units you can buy have diffusion screens. You just pull them out out so they fit over the main flash lens.

Diffusion Gels:

These gels are mainly used to diffuse theatre or studio lights. Gels provide either diffusion or colour or both. They are a type of optical diffuser, similar to the filter you might screw on your camera lens.

Diffusion reflectors:
Some diffusers are actually reflectors. You simply direct a beam of light  at the diffusion surface. The light partially reflects off and partially scatters. The diffusion surface has a rough micro-surface which causes your light to scatter. See: Diffusion; diffuse reflection (A Glossary entry).

Softbox

A softbox is one of our most useful diffusers in photography. They routinely have a large opening at the front which spreads the light. They have a bright light in the box and the light also reflects internally, then out to the subject. You can fit a cloth diffuser on the front which further diffuses the light.

Umbrella:

These useful diffusers are great fun. They give you a great diffused light and are excellent tools, especially for your portrait shots. You might use them to diffuse light by reflective diffusion. Alternatively you can create diffused light by shooting through a fine white cloth umbrella. 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 you measure the light after the diffuser is in place to get a true reading.

If you are using on-camera flash with a fitted diffuser, ensure that you do test shots when chimping to check the light intensity.  When doing test shots you should not rely on one test.