Sunday, July 12, 2020

Diffusing a Macro Twin Flash

I've probably spent the better part of a decade trying to wrap my head around flash photography and how to diffuse a harsh point source like a twin flash in as short of a space as possible. For field macro you just don't have a lot of room to work, so any diffusion scheme has to be compact. I also like to use a twin flash as two separate light sources, in a key (one head at the top of the lens) and fill (one head off to the side) configuration because it gives me a lot of control over the highlights and shadows. It also allow me to partially wrap light around the subject so that it doesn't look flat. If you place the flash heads on opposite sides of the subject, or fire them through a single diffuser that's connected to the end of the lens, the light will be too even across the subject and will potentially make the subject look flat. I don't focus stack, but for those of you who do flat light is a composition buzz kill. Get everything in focus and evenly lit and your images will look 2D. So here's what I've learned while loosing my mind trying to diffuse a twin flash:
  • The diffusers that Sto-Fen sells, and the set that Cannon supplies with the MT26EX RT, are better at blocking the light than they are at forcing it to spread out. You'll lose about a stop of light with either of them for a very small gain in diffusion and I think the same can be said for just about every hard diffusion plastic. I'm not going to do a "how to" on this one cause I don't want you to blame me if you ruin the diffuser set that comes with your MT26EX RT. I bought a second set from Canon to test out my theory that they weren't really diffusing the light very well, and as soon as I got them I used a Dremel tool to remove the front diffusion plastic so I could use it as a base for my own design (I just needed the frame and the clips that hold it to the flash heads). Due to some shipping issues I ended up paying over 60 USD for the set, but it was worth it cause I was right. If you follow me down this rabbit hole with the only set of diffusers that you have for your MT26EX RT you do so at your own risk...
  • 1/4 stop white China silk is the only material I've found that can force the hot spot in the MT24EX (and to a greater extent the MT26EX RT because it has a better built in diffuser) to spread out. It's best to use two layers separated by an air gap, preferably with at least a centimeter between them. Putting one layer of silk directly over another will cause the light to drop by at least a stop, so separate them. You can get it at B&H Photo but it's a little pricey and they have to special order it. I've been looking for a better, cheaper source. Note: The MT24EX knock off flash units perform about the same as the MT24EX, so 1/4 stop silk should also work for them but I have no experience with those flashes. At some point I'm going to experiment with other materials because I'd like to find a cheaper material than silk and not all diffusion surfaces are the same. Each of them diffuses the light in a different way.
  • Gary Fong's Puffer Plus makes an excellent last diffusion stage, if your trying to keep the size of your diffusers as small as possible, because the light transmittal is good and the surface is dimpled (it acts like a much larger diffusion surface). I've been looking for a similar photographic grade material that's not curved, but so far no joy. I've experimented with a lot of different diffusion plastics for that last diffusion stage and none of them performed better than the Puffer Plus. Even just using another layer of 1/4 stop white China silk didn't work as well.
Here's where things get "tricky": There's a difference between soft light and diffused light, and they are not the same. Using a diffuser that is large relative to the subject will give you soft light, and you can see it in the quality of the shadows. But diffusing the light means forcing the light to spread out, and a large diffusion surface relative to the subject won't necessarily do that. Diffused light means forcing the light to spread out so that the intensity of the light across the diffusion surface is the same. You can have soft light (soft shadows) and a hard well defined specular area. This video explains the difference between soft/hard light and specular/diffused light better than I can.



Understanding the difference between soft and diffused light was a eureka moment for me, because I initially thought that I had hit a hard limit in my diffusion because using a larger diffuser wasn't practical. But my light was soft enough with my current diffusers, or another way of saying it is that my diffusers were large enough relative to the subject, because I was getting soft shadows. Take a look at the shadow under the Sourgrass petal just below the Sweat Bee's antenna:

Sweat Bee in a Sourgrass Flower VI

The shadow is soft, so my diffusers are large enough. But the light that they were creating could best be described as "soft specular" and not "soft diffused" because the specular area is still well defined and almost harsh. My light wasn't really diffused. But after modifying the internal structure of my original diffuser (the details of which I'm not going into) I'm now getting soft diffused light with almost no hot specular highlights (the intensity of the light across my diffusers is almost the same) while only losing about 1 and 1/4 stops when compared to the bare flash heads. Light that's so well diffused I'm having a tough time determining if I've nailed critical focus on a some subjects, when viewing images on my camera's LCD screen, because there is no noticeable specular reflection in their eyes.

Foraging Cricket II

The Death of the Kaiser

After some experimenting with a cooperative, and very metallic, Chafer Beetle I've determined that using a Kaiser Adjustable Flash Shoe to elevate the key is working against me in most situations. Depending on the angle that I'm shooting from it can cause the fill light to be too harsh and/or pump a lot of light into the flower that a critter is perched on creating a third specular highlight. So I'm going to be shooting with both flash heads on the Canon flash mount and use the Kaiser for special situations where it will work to my advantage.

The quality of your light and the angle between the light, subject, and sensor will determine how much detail you can capture. You can easily lose more detail to poor light quality than to diffraction, and if you focus stack you really should be putting some serious effort into your light. Doesn't make sense to spend all that time to create a stack only to blow out detail with poor specular highlights and a lot of micro contrast (reflective areas throughout a scene where the detail is lost due to harsh light).

Note: All of my diffuser design has been geared toward creating a relatively small, compact, diffuser that would give me the light quality that I wanted but still be practical for field macro. However light is light, and I'm sure that the information I've provided in this post can be applied to any diffusion setup that you might build for yourself.