Diffuser

A diffuser is a sheet of translucent material, such as white fabric, frosted plastic, paper or cloud, placed between a light and your subject so the light passes through it and scatters. It matters because the lit diffuser becomes the new light source, and if that glowing surface looks large from where the subject stands, shadow edges turn soft, highlights spread out and skin and shiny surfaces look calmer.

The field above is lit by the sun, but the sun never reaches it directly. A thick layer of fog and cloud scatters the sunlight first, so the light arrives from the whole sky at once. That is diffusion at its largest scale. A panel held beside a face, the front of a softbox and a sheer curtain over a window all do the same job on a smaller one. Once you see a diffuser as “the thing that becomes the light”, every practical question about where to put it, how big it should be and why it sometimes does nothing has a clear answer.

How a diffuser works: the panel becomes the light

Light from a bare flash, a lamp or the sun comes from a small area and travels in more or less one direction. When it hits a translucent material, each tiny fiber or particle sends it off in a new direction. Some goes forward through the material, spread out in all directions. Some bounces back toward the lamp. A little is absorbed. What your subject sees is a glowing surface, not the small bright source behind it.

The softness of light depends on how big its source looks from the subject, not on how bright it is. So the diffuser’s job is to make the source look bigger. The sun fills about half a degree of sky, which is why its shadows are sharp. A 1 meter panel held 1 meter from a face fills about 53 degrees. Move the same panel back to 4 meters and it fills about 14 degrees, and the shadows harden again. The diffuser did not change. Its apparent size did.

Two things follow, and most of what goes wrong with diffusers comes from forgetting them:

• A diffuser only softens if it is bigger than the light behind it. A plastic cap on a flash turns a flash head about 7 cm wide into a source perhaps 10 cm wide. From 3 meters away that is still tiny.
• Dimming is not softening. Every diffuser costs light, but a weak light that comes from a small point casts shadows exactly as crisp as a strong one.

Want the geometry of why big, close sources make soft shadows, with the shadow-edge formula worked out? Hard light vs soft light covers apparent size in full.

Size, distance and evenness: the three controls

Every diffuser setup has two distances, and they do different jobs.

Panel to subject sets softness. This is the apparent size rule. A useful habit: keep the panel no farther from the subject than one to two times its own width. A 1 meter panel works at 1 to 2 meters; a 30 cm sheet of baking paper works at 30 to 60 cm, which is plenty for a mug, a flower or a watch.

Lamp to panel sets evenness. A diffuser is only as big as the part of it that is lit. Put a lamp or flash right behind a large panel and you get a bright spot in the middle and dim edges, a hotspot. The subject then sees a small, bright source plus a faint glow around it, and you get shadows with a fairly hard core and a soft halo. The quick test: stand where the subject stands and look at the panel. If you can see a bright blob where the lamp is, the panel is not doing its full job.

How far back the lamp needs to be is simple geometry. A flat panel lit by a small lamp gets less light at its edges for two reasons: the edges are farther from the lamp, and the light strikes them at a slant. This table works that out for the middle of the edge of a 1 meter square panel, 0.5 meters from its center, before counting the lamp’s own beam, which usually darkens the edges further:

So, as a rule of thumb, put the lamp at least as far behind the panel as the panel is wide, and aim the lamp’s beam so it covers the whole panel. When you cannot back off, there are two other fixes. Use a lamp with a wide beam, or add a second layer of diffusion between the lamp and the panel. That second layer is what the inner baffle inside a softbox does: it spreads the hotspot out so the front panel glows evenly.

Seeing a hotspot on a face or a shiny product and want to use the softer edge of the light instead of its center? Feathering light shows how to aim past the subject.

What diffusion costs: light, and sometimes a little color

A diffuser always costs light, mostly because some of it scatters back toward the lamp or off to the sides, not because the material soaks it up. How much depends on the material. Very light diffusion, the kind that only slightly blurs a shadow edge, costs well under a stop. Thicker fabric costs more. Because materials vary so much, the honest answer is to meter it: take a reading at the subject with and without the panel and compare. A light meter held at the subject’s face, or two test shots with the same settings and a look at the histogram, both work.

Here is how the share of light that gets through converts into stops. Each stop is a halving, so the loss in stops is how many times you can halve 100% before reaching the figure:

• 70%: Light that passes through: 70%. Loss: About 0.5 stop. To keep the same exposure from f/8: Open to about f/6.7, or raise ISO from 100 to about 140.
• 50%: Light that passes through: 50%. Loss: 1 stop. To keep the same exposure from f/8: Open to f/5.6, or double flash power, or ISO 100 to 200.
• 35%: Light that passes through: 35%. Loss: About 1.5 stops. To keep the same exposure from f/8: Open to about f/4.8.
• 25%: Light that passes through: 25%. Loss: 2 stops. To keep the same exposure from f/8: Open to f/4, or ISO 100 to 400.
• 12.5%: Light that passes through: 12.5%. Loss: 3 stops. To keep the same exposure from f/8: Open to f/2.8, or ISO 100 to 800.

Layers multiply. Two layers that each pass 50% pass 25% together, a 2 stop loss. That matters with flash, where a small speedlight may already be near full power, and especially with high-speed sync, which costs flash power before any diffuser does.

Color is the quieter cost. Many white fabrics and plastics are not perfectly neutral and tint the light slightly warm, cool, green or magenta, and an old panel that has yellowed with age can shift more. The fix is a white balance set with the diffuser in place, for example from a gray card held where the subject is. If you mix diffused flash with daylight, check the color in both parts of the frame.

Falloff starts at the panel, not the lamp

Once light has passed through a diffuser, the panel is the source. Distances for brightness are measured from the panel, not from the lamp hidden behind it. That changes how quickly the light dims as your subject moves away.

A worked example: a round 1 meter panel, evenly lit, with the lamp 1 meter behind it. Move the subject from 1 meter to 2 meters in front of the panel. If you measured from the lamp, the distance would go from 2 to 3 meters and you would expect to lose about 1.2 stops. Measured from the panel, the inverse square law says doubling the distance costs 2 stops. The exact figure for an evenly glowing 1 meter disc works out at about 1.8 stops, close to the panel-based estimate and nowhere near the lamp-based one. A big panel very close falls off a little more gently than the plain inverse square law suggests, but the rule of thumb holds: think from the panel.

This is why a diffuser close to the subject does two things at once. The light gets softer, and the background, which is much farther from the panel than the subject is, goes darker. If you want a light background with a close diffuser, light the background separately or move the subject nearer to it. The flash falloff tool on Flash photography lets you drag a light back and forth and watch how the subject and background exposures pull apart.

Or try it here: treat the light in the tool below as your lit panel, move it toward the subject and back again, and watch the background fall darker as the light comes in close.

Want the numbers behind falloff for any distance, not only this example? The inverse square law works through them.

Types of diffusers and what each one really does

• Diffusion panels and frames. Translucent fabric stretched on a frame, from a small hand-held disc to frames several meters across. Photographers often call these scrims. On film sets, though, a scrim usually means a net or mesh that cuts the amount of light without softening it, so if you rent grip gear, ask for “diffusion” or a “silk”.
• The center of a 5-in-1 reflector. The white translucent disc inside the zip-off covers is a diffuser. It is the cheapest way to own a portable panel.
• Softboxes. A diffuser built into a box: a front panel, usually an inner baffle, and walls that keep light from spilling sideways. See softbox for shapes and sizes.
• Shoot-through umbrellas. The flash fires through white fabric, so the umbrella becomes a large, round source. A lot of light also bounces back off the inside, so in a small white room it lights the walls too, which adds to the softness and to the spill. See umbrella.
• Sheer curtains, frosted glass, shoji screens. A window with direct sun through it is a hard source. Hang a sheer white curtain in it and the whole window becomes a soft one. See window light.
• Improvised materials. Baking paper, a white shower curtain, tracing paper, a thin white bedsheet. They work as well as anything for small subjects. Keep any fabric or paper well away from hot tungsten or halogen lamps; flash and LED panels run cool enough that this is rarely a concern.
• Cloud, haze and fog. The biggest diffuser there is. Thin cloud blurs shadow edges, a thick layer removes them, and fog does it at ground level. See overcast day photography.
• Caps and domes on a flash. These make the flash head a little bigger and throw light in every direction. Indoors, that spill lands on walls and ceiling, which become large sources and soften things. Outdoors, with nothing nearby to bounce from, they change almost nothing about the light on your subject and cost you power.

A bounce card is often listed with diffusers, but it is a different tool. It reflects light rather than passing it through, and a small card is itself a small source. The card helps mainly by sending light onto a ceiling or wall, which becomes the real source. That is bounce flash, and a reflector works the same way outdoors.

Diffusing harsh sun for outdoor portraits

The most useful single trick with a diffuser is putting it over a person in midday sun. Direct overhead sun fills half a degree of sky, carving dark shadows into eye sockets and under the nose. Hold a 1 meter panel between the sun and the face, as close as the frame allows, often just above the head and tilted toward the sun, and the face is lit by a 1 meter source at arm’s length instead. Shadow edges melt and squinting stops.

What catches people out is the background. The panel cuts the light on the face, maybe by a stop or more, but everything behind the subject is still in full sun. Expose for the face and the background comes out that much brighter, possibly white. You have three choices: accept a bright, airy background; choose an angle with a darker or shaded background; or add a little light back to the face with a reflector or fill flash so the face and background sit closer together.

If no panel is to hand, nature offers two alternatives. Open shade, at the edge of a building’s shadow facing open sky, gives soft light with direction. An overcast sky does the diffusing for you, though from overhead, so faces may need a reflector under the chin.

Shooting people outside and want the whole picture, not only the light? Outdoor portrait photography covers backgrounds, settings and posing.

What a diffuser does not do

It does not remove shadows. It softens their edges. The sharp boundary becomes a wide gradient, but the shadow side of a face is still dark unless something adds light to it. How dark the shadows are compared with the lit side is a separate control, the lighting ratio. You change it with a reflector or fill light to open shadows, or with negative fill to deepen them.

It does not make light better. Soft light smooths skin and calms shiny surfaces. Hard light shows texture, draws graphic shadows and gives a sense of strong sun. Both are choices. Brick, bark and weathered faces often look their best in hard light, and shadow photography depends on it.

It changes what shiny things reflect. A glossy surface reflects the source itself, so the diffuser shows up in it. On a bottle or a phone screen you will see the shape and size of your panel as a long, smooth highlight instead of a pinpoint glare; in an eye, it becomes the catchlight. That is why product photography uses big panels, and why their edges and evenness matter as much as their size. Specular highlights explains how to shape them.

Diffuser, reflector, scrim and flag: four different jobs

• Diffuser: What the light does: Passes through and scatters. Changes softness?: Yes, if the lit panel is large and close. Changes brightness?: Lowers it.
• Reflector: What the light does: Bounces off the surface. Changes softness?: A large white one is a soft source; a small or silver one is harder. Changes brightness?: Adds light where you aim it.
• Net (a scrim on film sets): What the light does: Partly blocked by a mesh. Changes softness?: No. Changes brightness?: Lowers it.
• Flag: What the light does: Blocked entirely. Changes softness?: No. Changes brightness?: Removes it from part of the scene.

Common mistakes with diffusers

• Using a small diffuser far away. A flash cap outdoors, or a 40 cm panel 3 meters from a person, makes almost no difference to the shadow edges. Move it in or use something bigger.
• Leaving a hotspot. The lamp is too close to the panel or aimed at only part of it. Back the lamp off or add a second layer between lamp and panel.
• Stacking layers to get softer light. A second layer of the same size only softens if the first one had a hotspot. Otherwise it just costs another stop or so.
• Trusting a stop figure from a label. Real loss depends on the material, its age and the angle. Meter at the subject.
• Forgetting the background. A close diffuser lights the subject much more than anything behind it, and a panel over a subject in sun leaves the background bright. Decide what you want the background to do before you blame the diffuser.
• Calling a bounce card a diffuser. The card itself is small. The softness comes from the wall or ceiling it lights.
• Expecting soft light to fill shadows. Soft edges are not lighter shadows. Add a reflector or fill if the shadow side is too dark.

Try this: the baking paper test

You need ten minutes, a phone or camera, a small lamp or phone flashlight, a mug or an egg on a table, and a sheet of baking paper about 30 cm wide.

1. Darken the room. If your camera or phone lets you lock exposure, set it once and keep it for all four shots. Put the lamp about 1 meter to one side of the mug, a little above it, and take a picture. Look at the shadow on the table: it should have a crisp edge.
2. Hold the baking paper about 10 cm in front of the lamp, between lamp and mug, and take a second picture. The paper is now the source, but only the patch the lamp lights really glows, and it is still almost a meter from the mug. Notice how modest the change is.
3. Now hold the paper about 30 cm from the mug, and move the lamp back until the whole sheet glows evenly. Take a third picture.
4. Bring the lamp right up to the paper, so you can see a bright spot in its middle, and take a fourth.

How to judge it: compare the width of the soft band at the edge of the shadow and the size of the highlight on the mug. Picture three should have the widest, smoothest shadow edge and the biggest, softest highlight. Picture four should sit between two and three, with a darker core to the shadow. Note how much darker picture two is than picture one: the lamp did not move, so that difference is roughly what the paper cost you in light. If you can, repeat step 3 with the paper at 60 cm and see the edge sharpen again.

Want to sketch a setup like this before you build it, with the lights, panels and camera in place? The lighting diagram tool lets you draw one.

Want to check what you know about what makes light hard or soft? Try the natural light quiz, which covers the quality and direction of light.

Frequently asked questions about diffusers

How much light does a diffuser lose?

It depends on the material. Light diffusion costs well under a stop, and thicker fabric costs a stop or more, with stacked layers adding up. Meter at the subject with and without it rather than trusting a figure.

Is a bigger diffuser always softer?

Only at the same distance. What counts is how big the lit panel looks from the subject, so a 60 cm panel at 60 cm gives about the same softness as a 2 meter panel at 2 meters. A big panel pushed far back can be harder than a small one held close.

Does a diffuser cap on my flash help?

Indoors, a little, because it throws light onto the walls and ceiling, which then act as large sources. Outdoors, or in a large dark room, it barely changes the shadows on your subject and costs flash power. Bouncing the flash or using a large modifier does far more.

What is the difference between a diffuser and a softbox?

A softbox is a diffuser with walls. The front panel and inner baffle do the diffusing, and the box keeps light from spilling onto the rest of the room. A loose panel gives the same softness at the same size and distance but lights more of the space around it.

Can I use a bedsheet or shower curtain as a diffuser?

Yes. A thin white bedsheet, a frosted shower curtain or baking paper all work. Check that the material is neutral by setting white balance with it in place, and keep it well away from hot tungsten or halogen lamps.

Does diffusion reduce contrast?

It softens shadow edges and highlights, and in a room with light walls the extra spill can lift shadows a little. But in a dark space a big soft source can still leave deep shadows. To lower contrast reliably, add a reflector or fill light on the shadow side.

Curious how photographers describe light beyond hard and soft, including direction and color? Quality of light brings the parts together.