Diffusion Confusion (take 4!)

Diffusion will be my epitaph

So I lied last time. It is more than once a year that I find myself writing about diffusion. But i still see so much misunderstanding about this topic. This time I will try to be short and sweet.

Diffusion scatters light. Diffusion can come from passing light through a translucent material (such as a white shower curtain, or a scrim, or diffusion material from the camera/cinema store) or by bouncing the light off of a mat surface (a painted wall, a sheet of foamcore, a sheet of art board, etc.). Here I will be concentrating on shining the light through translucent material placed right on the light source (or the metal dish reflector on the light source).

As I said, diffusion scatters the light. But it doesn’t soften the light. Diffusion possibly lowers the contrast as it scatters the light that will then bounce of of nearby surfaces like the floor, walls, and ceiling. This will cause the shadows to be filled in or lighter in density. But the shadow edge transition won’t change if the size of the light source doesn’t change. Don’t confuse contrast with hardness (quality). In a small studio space this is more apparent than in a large space (in the large space the light has to travel farther to a surface to bounce off of to come back and fill in the shadows).

How many layers of diffusion did you use???

It pains me when I read someone saying the put 3 or 4 or 5 (or more!) layers of diffusion over their light to soften it. All they actually did was lower the output of the light, causing them to either have to use a more powerful light, go to a higher ISO, or work at a wider aperture than they might have wanted to. But the one thing they did not do is change the shadow transition to make the light softer. To make the light softer would require using a diffuser that is much larger than the light source. If the diffuser and the light are the same size the light quality remains the same. The time to use double diffusion is if your diffusion material is rather thin and you can see the hot center spot of a light pointed through the material. (NOTE: This is separate from the concept of a softbox or octabox that has an inner and outer diffuser. In that case the inner diffuser’s job is to spread out the light from the strobe to completely illuminate the larger front panel diffuser and is not the same thing as double diffusing the front of the softbox or octa.)

7-inch reflector without and with diffusion

The opposite of diffusion is to narrow the light with some sort of baffle. This could be a snoot, a set of barn doors, or a honeycomb grid. I want to talk about grids here for one big reason. I often see photographers mixing a grid with a diffuser by putting the diffuser between the grid and the subject. I am not sure what they are trying to accomplish with that other than again losing a lot of light (2 stops or more in my testing). And they also lose the effect of the grid. If you feel you need a diffuser with a grid, you need to place the diffusing material between the flash tube and the grid, not in front of the grid. See the Deep Zoom Reflector examples photos later in the article.

First, let’s go to the standard 7-inch reflector that is common to most strobe systems. In the accompanying photos (click on the photo to see a larger version) I have used the 7-inch on its own, then with one layer of diffusion clipped on, and then with two layers of diffusion clipped on. There are a few things to notice. First is the large shadow on the background. Without diffusion there are actually multiple shadows with distinct edges. This comes from the combination of the flash tube itself, the reflector behind the flash tube, and the dish reflector around the flash tube each putting out a little bit different amount of light from their different sizes. When we add diffusion the light is homogenized or evened out so the shadow on the wall is more even. Then look at the shadow of the nose and the highlight on the tip of the nose. By adding the diffusion (one layer, two layers, or a dozen layers) we do not see any changes to these from one photo to the next. The edge of the shadow is still the same hardness and the highlight is still the same brightness in relation to the overall exposure. As noted above the density of the shadow might be a little brighter from light bouncing around the room, but that doesn’t affect the edge quality of that subject. If we want to lessen the brightness of the highlight on the nose one solution is to bring the light in closer, which will make the light bigger and cause it to fall off quicker (See! Everything in photography is interdependent and full of trade-offs). As a specular reflection the brightness will actually stay the same while getting larger, but the exposure on the rest of the face will also get brighter and we will power down the strobe to compensate for that which will then also bring down the brightness of the hot spot.

7-inch reflector bare, with a grid, and with a grid + diffusion

Now let’s look at combining a grid with diffusion. Three examples again this time. The first is with the same one we saw with 7-inch dish in the above examples. Then the 10-degree grid is added which limits the spread of the light for a more dramatic look and deeper shadows because there is less light bouncing around the rest of the room. This is the look I suspect that everyone is looking for with the grids. But then some people put a diffuser over grid. And look what happens when the diffusion material is placed over the grid—the diffusion material does its job (scattering the light into a wider pattern) and completely obliterates the effect from the use of the grid. Take a look at the diagram at the bottom of this post. And that is at the cost of two or more stops of light (translating into using more battery power if you are using a portable battery powered system). If you do feel the need to diffuse the light when using a grid you need to place the diffusion between the flash tube and the grid, not in front of the grid (yes, I am repeating myself for emphasis). We will take a look at how that works in the next set of examples below.

Diffusion scatters light

Baffles restrict light

Don’t confuse light contrast with light quality

Deep Zoom Reflector examples

One more example, this time with the Deep Zoom 11-inch reflector kit from Interfit. The Deep Zoom kit comes with a set of three grids (10-, 20-, and 30-degree) and a diffusion sock. Six examples this time. First we see the Deep Zoom by itself providing a nice crisp look. Adding the diffusion sock lowers the contrast, but retains the same shadow edge quality. Next I put on the 10-degree grid providing a more dramatic look with falloff of light across the background and a bit more contrast. Then I tried the grid with the diffusion sock over the grid and POOF there goes the grid effect that you spent good money on to purchase the grids. The next thing to try was with the sock behind the grid, between the flash tube and the grid. We get the drama back, but with a bit less contrast. So, while we’re at it, what happens wit with the sock behind the grid and a sheet of diffusion material in front of the grid. Again we can see that by adding diffusion in front of the grid we are negating the effect of the grid.

In all six examples we need to take a look at and compare the transition edge of the nose shadow. By now I hope that you don’t really need to look at it to know that it is going to be the same because the size of the light didn’t change between photographs.

So, to recap…

Diffusion scatters light in all directions and makes it cover a wider area. As the light is scattered and some is absorbed by the diffusion material you will lose some power.

Diffusion material can be specific products like those from Lee and Rosco. Or it can be a sheet of tracing paper or a bedsheet or a frosted shower curtain. Here I am assuming that the diffusion is dense enough that you won’t see a hot spot from a light shining through it. This is usually called a full-stop diffuser. Your diffusion material should be neutral in color, but often isn’t. That white shower curtain might contain brighteners that make it cause the light to be more blue, or it might have yellowed with age. Be prepared to have to do some color compensation when processing your raw files.

The opposite of a diffuser is a baffle (though I am befuddled as to why some softbox manufacturers and photographers call their scrims and the inner diffuser on their softboxes baffles). Baffles restrict the flow of light. Common examples are snoots, barn doors, and grids. Grids maintain the shadow edge while restricting the coverage of the light. Hard grids are available for use with metal reflectors like the 7-inch, the Deep Zoom, and beauty dishes. They are rigid with a honeycomb pattern of openings and snap onto the front of the reflector. They usually come in densities from 5-degrees to 40-degrees. The lower the number the narrower the light coming out of them. Soft fabric grids are available for most softboxes and octa boxes. They are usually around 40-degrees and allow you to have a large directional soft light.

Don’t confuse contrast (the difference between the light and dark areas of your image) with quality of light (how quickly the shadows transition from dark to light). While thinking about that, also do not confuse brightness with harshness. Moving a light in closer makes it brighter, but also softer as it becomes bigger in relation to the subject (or as seen by the subject). The exception to this is when using hard grids. Because the light rays are restricted by the honeycomb only the center rays from the light going through the grid will reach the subject, making the light appear smaller as it is brought in closer.

 Expanding on this illustration from a  previous blog post

Expanding on this illustration from a previous blog post

Well, thanks again for indulging me and reading through my thoughts on studio lighting. Now go out and make some photographs! Until next time…

Cheers!
John Cornicello