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Using Diffusion

Using Diffusion

Diffusion is invaluable for softening harsh light and creating natural-looking lighting designs.

Light-diffusing material (just plain diffusion in professional jargon) softens directional light by scattering it. In the short version, that's all there is to tell, but selecting, mounting and fine-tuning diffusion is one of the gaffer's subtler arts. The rest of this column is the long version. We'll see how to determine the lighting look you want, match diffusion media to the lights you're using and deploy all this stuff to good effect.

The Long of It


Placed in front of a light source, diffusion discombobulates the directional light rays that pass through it, scattering them widely. The resulting beam appears softer and more general, with shadows lessened or even completely absent. This softer light has several advantages. It looks more natural and more like daylight or uniform overhead office lighting. It flatters subjects by diplomatically suppressing pits, crags, wrinkles and other facial blemishes. It reduces uncomfortable glare in a subject's eyes. By minimizing shadows, it can conceal the fact that a subject is lit movie-style, by two or more instruments. (Nothing gives the game away like a matched pair of key and fill light shadows behind a subject.)

In addition to softening, diffusion widens the pattern of the light beam and reduces light intensity. The broader beam lets you light a larger area per instrument, but the lower light level means you may need more units to achieve a given light level, so that's often a wash. The key point here is that diffusion will always reduce the light intensity on a subject.

Finally, diffusion reduces undesirable patterns, such as lamp filaments, the lamps themselves and the lenses in front of them. These often place noticeable shadows right in the middle of the light beam, where it's most visible to the camera. Diffusion usually solves this problem.

Why not use self-diffusing soft boxes instead and dispense with diffusion entirely? The answer is control. You can never make a soft light any harder, but you can soften a spotlight progressively until you get exactly the diffusion you want. And when you're traveling light (or cheap) a unit that works both hard and soft can do the jobs of two instruments.

Know Your Instruments


As noted, soft boxes are already completely diffused and umbrellas can't usually be softened, but every other common type of lighting instrument can benefit from the soothing influence of diffusion material.

Fluorescent banks are already quite soft and they can be ultra-creamy when covered with spun glass. One of the best materials is the milk plastic panels sold for fluorescent ceiling lights. These can add significant weight, however.

Scoops are large, open, circular floods often used in studios. Because they throw soft but distinct shadows, they're usually fitted with big rings holding diffusion material.

Broads are floodlights, just big enough to hold a long halogen lamp, a half-cylinder reflector behind it and a barn door on each side. Because of their small size, broads throw obvious shadows and their beams can be unpleasantly mottled. The workhorses of location lighting, broads can be much improved by judicious diffusion.

Spotlights are small light sources throwing hard-edged, directional beams. Although you can often move the lamp back in the instrument (closer to the reflector) to soften the light a bit, this approach makes the barn doors less effective. You will probably spend much of your light-diffusing efforts on spotlights.

Diffusion Media


What will you diffuse these instruments with? As always: it depends. Milky plastic filter material creates a very soft effect, but make absolutely sure you get the strongly fire-resistant plastic specifically intended for movie and theatrical lighting. If you'll be supporting the diffusion on separate C-stands you can (carefully) use standard milk plastic 2x4 fluorescent light panels.

Spun glass is a pure-white cousin of pink insulation batting (with the same nasty habit of leaving fiberglass stickers in your hands). Supplied in large, thin, flexible sheets, it is perhaps the most versatile diffusion because you can cut it to any size, double or triple it in thickness and place it wherever you want. Like plastic filters, you can buy spun glass from theatrical supply houses. Or you can get furnace air filters at your local hardware store. One caution: the glass material will not easily burn, but it will melt.

White sheeting is useful for tenting, a special form of ultra-diffusion. Once, I even covered a picture window with a pale yellow sheet, softening the sunshine streaming in and changing the color temperature from daylight to (roughly) tungsten.

Finally, translucent white umbrellas can turn a soft umbrella setup into an ultra-soft box. Simply install the white brolly in place of the silver or gold one, reverse the unit and aim the umbrella at the subject so that the light shoots through it.

Diffusion Setups


The farther from the light source, the greater the diffusion. You can hang diffusers on the light itself, on its barndoors or on a Century stand between the light and the subject. For a really diffuse effect, you can bounce the light off a reflector.

Spotlights (and some scoops and broads) have filter holders that accept a metal sandwich frame holding spun glass or gel cut to size. This setup is good for lights too high or otherwise too difficult to reach easily. Scoops are generally safe, but broads and spots should be checked periodically to make sure the media isn't charring. Focusing spot lamps in the "flood" position can reduce the heat a little.

Spots and broads have side barn doors that are ideal for holding spun glass, but plastic doesn't work quite so well. Spread the diffusion across the already-positioned barn doors and clip it on with wooden clothes pins. The extra distance from the lamp makes the diffusion more effective, and the media will stay safely cool indefinitely.

For a really soft effect, try clamping spun glass or plastic with a C-stand placed several feet in front of the spotlight. Of course, if you go too far out, the cone of the light beam will be wider than the diffusion, but sometimes that's just what you want. For example, you can soften the light on just the face with a circle of spun glass stapled to a thin wire frame.

The larger a light source, the more diffuse it is. Bouncing a spotlight beam off a 4x4-foot white foamcore reflector creates a very soft illumination. A bed sheet is even bigger and softer, but the dramatic light falloff means that this solution isn't very flexible.

For product shots and special portraits, there's nothing like a cloth tent. Simply string a clothes line and hang a sheet to create a side and a roof. Lighting through the sheet creates an essentially shadowless design.

Diffusion Outdoors


Small tents work well in a studio, but large ones do better outside where the sun is your light source. (Of course clouds make fabulous diffusers, but they are notoriously hard to control.) Bed sheets and black gardening shade cloth are good options. Orient the clothes line and sheet so that the sun filters through both the top and the side. The results will amaze you.

Also, in bright, filtered sunlight, the camcorder's aperture will be small enough to deliver good depth of field, which is essential for shooting very small objects especially.

Incidentally, outdoor tents are also useful for copying photos onto videotape. Even glossy prints behave themselves in the tent's shadowless environment.

You can light human subjects through sheets hemmed and mounted on PVC pipe frames. An assistant can hold a framed diffusion sheet between sun and subject, even for setups as wide as a medium shot.

You can often omit diffusion in wider shots, where higher contrast is not as big a problem. Unlike flexible reflectors, diffusion screens work fine on somewhat windy days because the light travels through them, rather than bounces off them. Do make your assistant aware, however, that a 4x4-foot diffusion screen is a very efficient sail in a brisk wind.

Diffusion does have limitations in bright sunlight since it changes the character of the light. This doesn't matter if you're shooting only close shots, but your close shots won't match your long shots. When I need both kinds of setups, I prefer screening, which reduces light intensity without changing its quality too much.

Good shooting!

Contributing Editor Jim Stinson is the author of the book Video Communication and Production.

[Sidebar: Safety First, Last, & Always]


Before we look at diffusion setups, we need to emphasize safety precautions that really ought to be obvious. Excepting fluorescents, all lights grow hot enough to char filter media, burn fingers and start fires. Before deploying any diffusion material, you need to understand:

  • How hot the light gets
  • How fire-resistant the medium is
  • How long the diffusion must remain on the hot light

Even knowing these things, a conscientious gaffer will re-check the diffusion every few minutes to make sure it's not overheating. Unless the light is inaccessible, I prefer to avoid using diffusion in circular light filters. Even spun glass and the best heat-resistant gels will eventually brown and char like forgotten pot roasts.

Tags:  November 2004
Jim
Stinson
Mon, 11/01/2004 - 12:00am