Diffusion Confusion

What does it mean to add diffusion to your lights?

There are four main considerations we need to make before we set up the photograph in the studio. A: Depth of field; B: Depth of light; C: Quality of light; D: Amount of light. In this article we will mainly consider C, the quality of light, and how diffusion plays into our needs. Let’s start with a definition of “diffusion:”

DIFFUSION
noun
1. The spreading of something more widely.
2. The action of spreading the light from a light source evenly so as to reduce glare and harsh shadows.

How does this apply to photographic lighting. How do you diffuse your lights? There are a number of ways, all with varied results. My first introduction to diffusion was someone telling me to drape a white handkerchief over my speedlight. In their theory, this would make the light somehow “softer,” but this didn’t make sense to me. You make a light softer by making it larger in relation to the subject. Putting the cloth over the speedlight does nothing to change the size of the light. Yes, it diffuses the light, but all that means is that it disperses the light more widely. That would allow you to use the flash with shorter focal length lens, but it won’t make the light any softer on its own. A few years after that, speedlight manufacturers started adding a diffuser to their lights in the form of a plastic panel that pops out and folds down over the flash for the purpose of working with shorter (wide angle) lenses. The side effect of this is less light output, but the coverage is expanded from side to side.

Now, there is a situation where this type of diffuser might provide a slightly softer light (by hard and soft light, I am referring to the shadow edges. Hard light comes from small light sources and produces a dark shadow with a well defined edge while soft light comes from a larger light source, as seen from the subject, producing a lighter (or no) shadow with a gradual fade of the edge of the shadow.) If you are in a very small room with white walls and ceiling, the wider dispersion of the light might bounce off those walls and ceiling to provide a little bit of fill light to make the shadows a little bit lighter, but not really much, if any, softer.

To make the light softer you need to make it larger as seen from the subject.

What does that mean? It means that distance comes into play along with size. If you bounce your speedlight into a 32” umbrella it becomes a much larger and much softer light if you place the light the same distance from your subject. However, I see a number of photographers who are new to lighting set up their speedlight and umbrella and place it 10 or 15 feet away from the subject. At that distance, the light (as seen from the subject) is not really all that large. They end up with a very flat image (no separation of subject from the background) and harder edge shadows than they were expecting. 

Here is a little tip you can try out. Set up your lights and stand in place of your subject. Hold your hand out arm’s length in front of you towards your light. If you hand covers the light you have a small light source that will produce hard shadows. If, on the other hand, your hand is dwarfed by the light source, the light is large enough to fill in some of its own shadows. Some people say that the large light “wraps” around the subject. A nice description, but not quite true. Light doesn’t wrap or bend. But a large light source provides light from more surface area so that it provides some fill light. 

Before we go any further here, I need to stop and talk about distance. From what we’ve discussed so far, you may be tempted to think that moving the light closer or farther away is the way to control the size and quality of the light. It is. But it isn’t. Moving the light closer or farther away does change the relative size of the light on the subject. But it also changes the depth of the light. This is something I sadly don’t see talked about often enough. This is also the reason why light modifiers like umbrellas, soft boxes, octa boxes, etc. come in so many sizes. The depth of light needs its own article . But for now, please don’t think that changing the distance of the light to subject is the way to control the quality of light and shadow.

So, how does one use diffusion to make a small light become a larger/softer light? By making the light larger, of course. Start with a light we’re all familiar with (even those of us in Seattle), the sun. The sun is massively huge. But it is so far away that it appears to us as a very small light source (remember the hand? You can easily block out the sun from your vision by holding your hand out in front of your eyes). What happens on an overcast day? The overcast layer is much closer to us. It gets lit by the sun (the light origin) and now becomes the light source. And it is huge. Hold your hand up to the overcast sky and you see that light is still coming in from many angles because of the size of the sky.

Now take that to your camera gear. Your speedlight is similar to the sun. It is a small light, producing a hard-edged shadow. To soften the shadow you need to put a bank of clouds between the speedlight and your subject. That bank of clouds can come in many shapes and sizes. If just getting started with a limited budget, an umbrella is often the first choice (and a good choice, despite the bad rap they get from some photographers). There are two basic types of umbrellas for photographic use. A white (shoot through) umbrella that you point your light into and then point the umbrella towards the subject and a reflective umbrella with a black outer cover and a shiny interior that you bounce your light into and then have it reflect on your subject. By placing your small light way back on the shaft of the umbrella, the light illuminates the large umbrella surface, which now becomes the clouds or your light source.

The next light softener of choice is usually a soft box. The same principles go to work here. The small light is placed at the back of a box and that light illuminates a translucent fabric held a distance in front of the light to make a larger light source. After that you have octa-boxes that do the same thing, except that the softboxes are rectangular and the octaboxes offer a more rounded shape. We will talk about the effects of the shape of the diffuser in an upcoming article. Spoiler alert—the most noticeable difference will be in the shape of catch lights in the subject’s eyes. My favorite light modifier combines the concept of umbrella and octa-box in one unit called the Photek Softlighter II. Similar modifiers are now available from a variety of companies, but as far as I know, the Photek Softlighter is still the only one that has the option of a removable shaft so you can move the light in very close to your subject.

Going back to the confusion about what diffusion does, let’s look at some of the images contained in the animation playing near the top of this article. We’ll start with a basic studio light in an 11” wide dish reflector. In all of these images that light is 40” from the subject and the subject is 36” from the background. There is nothing special about these distances. I just happened to set things up this way for these demonstrations.

diffusion_1-3

In the first image we see what the light looks like with no diffusion. Take note of the shadow cast by the nose. It is dark and has an abrupt edge to it. In the second image I simply taped a sheet of 1-stop diffusion material over the light and made the same exposure. You can see that adding the diffusion lowered the light output resulting in an underexposed image. But look at the nose shadow. It remains dark with an abrupt edge. In the third image I opened up the aperture a stop to compensate for the loss of light. Comparing the first and third images, we see that the light on the face remains virtually the same. The diffusion has not softened the light. The main difference is on the background, where the shadow on the right side of the image has lightened up a bit from the wider dispersion of the light. The point being illustrated here is that diffusion added directly to the light does not increase the size of the light and doesn’t soften the edges of shadows. It might fill in the shadow a little bit to make it less dense, but no softer. Image #4 has a different brand/manufacturer of diffusion material, which may affect the spread of light, but maintains the same shadow quality.

The rest of the images in the series show various additions to the light, such as grids and a snoot. I have included photos of some of the modifiers at the bottom of this article in case you are not familiar with some of them. In the photos made with a grid (#5, #6, & #7) on the light you can see that adding diffusion to the grid will give a different look depending on if you place the diffusion material behind the grid (between the light bulb and the grid) or in front of the grid (on the subject side of the grid). Image #5 is the grid alone. With the diffusion inserted behind the grid (image # 7) the light maintains the grid pattern, which is mostly noticeable on the background. With the diffusion in front of the grid (image #6) the light is still concentrated by the grid, but gets spread out a bit, too. 

diffusion_5-7

Diffusion material comes in various densities and surface textures. Examples 8 and 9 show Rosco Tough Silk and Lee’s #253 material, respectively. Subtle difference can be seen. 

diffusion_8-9

In images 10-14 the 11” dish reflector has been replaced by a 22” beauty dish. In #10 you can see that the larger diameter light source has softened the shadows a bit. This is most noticeable in the curl of hair on the forehead and on the background where more light coming from the larger source appears to wrap around the head and fill in the background. #11 and #12 have a diffusion “sock” placed over the beauty dish. Again, the sock doesn’t make the dish any larger, but it does even out the light across the front of the dish and here I did not adjust white balance so you can see that different diffusers will have an effect on the color of your light. Here, the Mola brand sock is a bit more warm/yellow than the Speedotron sock. Some may like the added warmth, others may choose to correct it back to neutral via a custom white balance in camera or by adjusting in post-processing.

Image #13 adds a 35-degree grid to the beauty dish to narrow its beam without affecting the shadow quality. #14 adds the Speedotron sock over the gridded beauty dish. This negates some of the narrowing of the beam, but has little effect on the shadows. 

diffusion_10-14

Going in the totally opposite direction, image #15 is just a bare bulb studio strobe pointed straight up towards an 8’ high ceiling, and throwing light out in all directions. This is my “sunlight” look, with a crisp shadow line and some filling of the shadows from light reflected off the walls and ceiling. #16 uses a 7” dish reflector, but instead of a silver interior, this one is black. It is designed to work with a snoot, but I’m using it on its own here. Again it is a small light source and we see that in the nose shadow. #17 adds the snoot, and we see the shadow quality stay the same, while the area of coverage is shrunk down considerably by the snoot.

diffusion_15-17

Image #18 is another 7” dish reflector, but this time the standard silver interior. Again, a similar quality of light on the subject and the nose shadow. But a little wider dispersion of light lessens the shadow on the wall behind the subject.

Image #19 is the same 7” silver dish reflector, but now I’ve taken a 32” translucent white pop-up diffuser disk and held it about 24” in front of the light, making the disk become the light source. As it is much larger than the 7” dish, we see a drastic change in the shadows on the face. The nose shadow is still there, but much less intense and with a soft/smooth transition from its darkest area to the edge. Full disclosure, in addition to being larger, the light is now closer to the subject as I left the lamp head in the same position and inserted the diffuser disk 20 or so inches closer to the subject. Notice that the background has gone a slight bit darker here. This harkens back to the comments above about the depth of light. The closer your light is to your subject, the darker the background will be. Again, more about that in an upcoming article on light depth.

Image #20 was lit with a somewhat specialized modifier, normally used to light up a background. It looks somewhat like the protective cover that you would put on a light for storage or travel, but is open on one side instead of completely enclosing the flash tube. Again, a small light with strong, hard-edged shadows. 

diffusion_18-20

For the rest of the images (21-27) I have used a speedlight as my light origin. #21 is the bare speedlight pointing directly at the subject. For image #22 I configured the built-in diffuser screen and opened up my aperture to compensate for the loss of light. Again, note that the nose shadow is basically the same with or without the diffuser. The diffuser does not soften the light. But it does spread it out to light the scene more evenly, as the light is not all concentrated in the center of the image.

Image #23 adds a sheet of diffusion material (Lee 3026) taped directly onto the speedlight. Again, the same quality of nose shadow. Diffusion is not softening the light.  

diffusion_21-23

#24 adds a Stofen diffuser. What happens to the nose shadow? Nothing. The stofen does not increase the size of the light, just the width the beam of light coming out of the flash.

Image #25 adds a “tupperware” type of flash diffuser. Here the flash is pointing straight up so most of the light goes up and bounces off the ceiling. Very little of the light goes forward, so we end up with dark eyesockets. The light has bounced off the ceiling, making the light source considerably larger, so shadows are softened. But the look of the face has suffered. For #26 I couldn’t let things go on, do I held a white diffuser disk on the camera right side of the face to try to kick in a little bit more fill. This fill card opens up the shadows and gives an overall improvement to the photograph, but the eye sockets are still a bit dark for my taste. 

diffusion_24-26
diffusion-27

Finally, we have image #27. Back to the straight on speedlight on a stand off to the left side of the camera, but now with the 32” translucent diffuser disk held between the light and the subject, turning the speedlight into a larger light, with softer, but not eliminated, nose shadows.

So, to recap, small light sources like a speedlight or a studio flash head are inherently small and therefore hard light sources. Hard light sources create deep/dark shadows with an abrupt transition from dark to light at the shadow’s edge. Adding diffusion directly to a small light source does not soften the light, it just spreads out the light more evenly and the main use is to help avoid vignetting when using shorter/wide angle lenses. To make the light softer with a less dense shadow and a more gradual transition we need to make the light source much larger. Once we have a larger light source, we might think that moving it closer or farther away is the way to control the size, but that changes other aspects of the image (to be covered in a follow-up article on "depth of light"). The way to control the size and quality of the light is to have a number of different size light modifiers available to us. 

Please scroll down to the bottom of this page for examples of the light modifiers mentioned in this article.

Thanks!
JohnC

 


Here are some of the light modifiers used in preparing this article...

bare bulb flash head

bare bulb flash head

snoot

snoot

11" deep dish reflector

11" deep dish reflector

11" dish with grid and diffusion in front of the grid

11" dish with grid and diffusion in front of the grid

background reflector

background reflector

extending the built-in diffuser on a speedlight

extending the built-in diffuser on a speedlight

"Tupperware" type of diffuser on a speedlight

"Tupperware" type of diffuser on a speedlight

7" black dish reflector

7" black dish reflector

standard 7" silver dish reflector

standard 7" silver dish reflector

11" dish reflector with grid

11" dish reflector with grid

11" dish with grid and diffusion behind the grid

11" dish with grid and diffusion behind the grid

22" beauty dish with diffusion sock cover

22" beauty dish with diffusion sock cover

Stofen diffuser on a speedlight

Stofen diffuser on a speedlight

pop-up translucent foldable diffuser disc shown behind a speedlight

pop-up translucent foldable diffuser disc shown behind a speedlight