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.