Today’s article is about flash diffusers and diffusion in general. I’ll talk about what I mean with the term, with some diagrams and sample photos to explain it. I’ll also provide instructions for two simple diffusers that you can build at home.
Since Facebook’s posts only allow a single link and image, I’m going to publish the full content of this series on my personal site, with photos and diagrams inline there. If I do this right, the article will still mostly make sense without the illustrations, but since we’re talking about visual effects it’s likely that you’ll want to swing by and see them. I’m also planning to convert the content to Epub format, so the articles can be downloaded and viewed on ebook devices.
Let’s get on with it.
What is Flash Diffusion?
Here are a couple definitions, from a search with “define:diffusion” at Google. An excerpt of the results is:
- the spreading of something more widely.
“the diffusion of Duchamp’s thought and art”
- the action of spreading the light from a light source evenly so as to reduce glare and harsh shadows.
The numbered definition gives us the general usage, and the bulleted definition gives us the definition I’m talking about in this article. The thing to realize is that there are still a couple of ways to interpret that specific definition, which I’ll illustrate below.
In the image on the left, we have a narrow cone of light being emitted from a light source. When it hits the diffusion surface, the angle of the cone is dramatically increased, causing the light to spread over a wider area than it would have done without the diffuser. (This is the behavior of the flip-down diffusion panel on external flashes. It’s also what happens if you use a ping-pong ball as a diffuser. You’re casting light in all directions from a relatively small source.) The important thing to notice about this image is that the diffuser is not significantly larger than the light source, and it’s not very far away from the light source. Keep that in mind; I’ll come back to it.
In the image on the right, we again have a narrow cone of light being emitted from a light source. However, this time the cone travels farther, getting larger in area, before it hits the diffusion surface. Also, when the light hits the diffusion surface, the angle in which it’s traveling is not significantly affected. The light illuminates the diffusion surface and continues on its way. Note, however, that the diffusion surface is both larger and closer to the subject in this case.
The Effect on Surfaces and Shadows
Let’s consider the two diffusion surfaces from the point of view of the subject. The left surface is smaller and farther away, while the right surface is both larger and closer. From the point of view of the subject, the fact that the original light sources are the same size is completely unimportant! All that the subject “sees” is the light being transmitted by the diffusion surfaces, one small and one large. Here’s a comparison image to illustrate. I’m going to omit the light sources, since from the point of view of the subjects, the diffusion surfaces are the light sources. I’m also going to omit all the rays of light that don’t actually hit the subject.
The fact the the left surface is spreading the original cone of light over a much larger area doesn’t affect the subject at all. The light that doesn’t hit it is completely irrelevant. All the rest is just wasted photons. Now look at the right surface, and bear in mind that the arrows are a simplification of what the light is actually doing. It’s not just traveling in a straight line from each point of the surface, in only one direction. Instead, it’s radiating out from the surface, from all points of the surface. That means that much more of the light is hitting the subject from a much wider range of angles.
When light strikes an object, the angle at which it strikes the object affects the brightness of the resulting illumination. If the light hits the object square, you get maximum illumination. If the light hits the object at an angle, you get less illumination.
What this means is that a small light source (or a small diffuser, since as far as the subject is concerned the diffuser is the light source) can only give maximum illumination to those surfaces of the subject that are facing directly at the light source. With a larger light source or diffuser (or one placed closer to the subject, making it apparently larger), the light is coming from more angles relative to the subject, and so more surfaces are facing directly toward the light source, which results in more even illumination.
The same type of effect is at work with shadows. Again, below is an illustration to aid my explanation.
On the left there is a tiny light source, for example a flash without a diffuser, positioned pretty far away from the subject. All the light comes from one place. It either hits the subject and provides illumination based upon its angle of incidence (the angle at which the light hits the surface), or it doesn’t hit the subject. Where it hits the subject (assuming the subject is completely opaque), it casts a shadow. The shadow has a hard edge.
On the right we have a larger, area light source. Light is radiated out from all parts of the surface. Some rays of the light are being intercepted by the subject, but other rays of the light are still managing to get past it. This results in a shadow with a soft edge. The area that is completely in shadow is called the “umbra”, and the area partly in shadow is called the “penumbra.” The larger the apparent size of the light source relative to the subject, the larger the penumbra and the smaller the umbra.
The diffusers we’ll be building are of the type on the right: a large diffusion surface, intended to be placed near to the subject.
Here are some photos showing the difference in the character of light from an undiffused source and from a diffused source. In these photos, I’ve attempted to place the light in approximately the same position relative to the subjects. The undiffused shot is on the left, and the diffused shot is on the right. Things to look for:
- The reflection of the light source itself. This is called the “specular” highlight. It’s the brightest part of the illumination. Notice how much more contrast there is between the brightness of the specular highlight in the undiffused examples.
- The hardness and softness of shadows.
Ok, here are plans for two simple diffusers. One is handheld, and the other is intended to be mounted to an external flash. The pictures are of my (well broken-in) diffusers. I don’t use the handheld one so much now, but it still gets occasional use. The flash-mounted one gets used every time I go out to take pictures.
This one is simple enough that looking at it probably tells you everything you need to know about it, but just to make sure I’m communicating, let’s step through it.
What you need:
- a wire coat-hanger
- a white garbage bag, or some other translucent white material
- some transparent tape
- Bend the coat hanger into the shape you want. I chose a square (sort of) for mine. You could use a rectangle, a circle—whatever.
- Cut a piece of your translucent material big enough to fit on your frame.
- Tape it to the frame. I cut mine big enough to wrap around the frame a little, so that I could tape it to itself. I figured that would stick better than trying to adhere tape to the coat-hanger.
Ta-daa! Your first light diffuser! Congratulations! Just hold this between your flash and your subject. The closer it is to the subject, the larger its apparent size relative to the subject, and the more even the light and softer the shadows will be.
(Note: When using a diffuser, some of the light that would normally be illuminating your subject will be absorbed by the diffusion material. This means that you’ll have to use a brighter flash. If you’re using a built-in flash and you have the option to adjust your exposure value, bump up the exposure a little. If you’re using an external flash, try setting it to Manual mode. Then you’ll be able to dial up or dial down the power output. Just bump it up or down as needed.)
This one is more involved, but I find that since I’m usually holding a flash in my hand anyway, it’s nice to be able to just attach this and forget about it.
Read through these instructions before you start building. In particular, compare the dimensions in the diagram to your flash, to make sure it will approximately fit. A little too big is fine, but if it’s too small then you’ll be out of luck. Of course, you could fudge it by adding some space to the joins when you tape it all together.
What you need:
- some kind of sturdy white material. I used white, 1/4-inch foam-core presentation board. If you have white cardboard, that would work, too, but it won’t last as long. It’s good if it has some flexibility to it, since these instructions are designed for something that bends. If you’re using a rigid material, you’ll have to tape it together in a few more places.
- a white garbage bag, or some other translucent white material (I currently use vellum, a high-quality tracing paper, although initially I used a garbage bag)
- duct tape & transparent tape
- (optional) a short (1 inch or so) piece of self-adhesive Velcro
- (optional) some black spray paint (I used flat black)
(Important Note: This doesn’t have to be exactly right. We’re going to be holding it together with duct tape. There’s wiggle room.)
Here’s the plan for each of the two types of pieces. We’re going to need two of each of these.
- Using the provided diagram, lay out the shapes that will be used for the main body of the diffuser onto your foam-core (or whatever you’re using instead).
- Only cut the solid lines. There is a heavy dashed line, which indicates a fold line. There is a thinner dashed line which you should measure if you want to have an second layer of diffusion material inside. I use it, but it’s not essential.
- Cut out the four pieces. There should be two narrow pieces and two wide pieces.
- Using some kind of straight edge to help ensure an even crease, fold the pieces along where the heavy lines are indicated. This will be the sleeve portion that slips around your flash.
- Using the duct tape, tape the pieces together. I put bands of tape along each of the joined edges, as well as around the sleeve portion and around the wide, front opening.
- Place the wide, 6″ x 9″ opening face down on your diffusion material and trace the boundary.
- If you’re going to include an internal diffusion surface, measure the length of the distance along the thin dotted lines, and use those measurements to cut a smaller pieces of diffusion material that will fit on the inside of the diffuser.
- Use transparent tape to attach the smaller diffusion surface to the inside of the diffuser, if you want to.
- If you want to prevent light from spilling out of the diffuser, which can create flare on the lens or otherwise illuminate things that you don’t want lit, put several coats of black paint on the outside of the diffuser. Lighting is as much about keeping light from where you don’t want it as it is about sending light where you want it.
- When the paint is dry (if you painted) attach the diffusion material to the large opening of the diffuser.
- This wasn’t perfectly snug on my flash, so I used a small piece of Velcro to secure it. I put one piece on the flash and one piece on the diffuser, inside the sleeve that slips over the flash.
That should be everything! Keep in mind that this fits my flashes. You may want to measure your flash to verify that it’ll work for you. I’m guessing it will, because most flashes I’ve seen are pretty similar in size, and have been for 30 years at least. But better safe than sorry!
I hope this was helpful! Please leave feedback and questions on the Facebook Group page where I posted the link to this article. Thanks for taking the time to read through it.