stacking diffusers to further soften the light?

Question

i want to achieve the softer possible light possible. i was thinking about using multiple diffusers to further soften the light. for instance, using a Speedlight with a plastic cap diffuser inside a portable "sock" soft diffuser, inside a softbox, or something like a small softbox inside a bigger one using both the inner and outer diffusion screens on each...

i understand that i would lose some stops with each diffuser but i often use speedlights set as low as 1/128 power, so i could just use full or half power if adding more difusion.

is my thinking sound? would that be a good solution for a very soft wraparound light? or am i much better buying a big studio strobe to have a bigger light source and put it in the biggest octabox i can carry?

(it needs to be portable as i usually set up a portable studio on location with several pro speedlights like the 580exii)

Answer 1

The reason for stacking diffusers is generally to make the light from a modifier more even. It's a decent strategy if you're getting "hot spots" on your subject because the light from a flash is coming through the middle of a less-than-ideally-designed modifier hotter than it is coming off the edges of a modifier. That's why many of the best soft boxes, particularly for use with portable speedlights, place the flash facing backwards so they bounce off the rear of the inside of the box before their light comes through the front diffusion panel. All of that bouncing around helps to even out the light.

It's not really going to make the light any softer, though. Only a larger size light source, or multiple light sources spread out over a larger area, will do that. Keep in mind that when talking about light sources, 'size' is always in terms of angular size as measured from the subject.

Answer 2

Softness and solid angle

In order to get the softest light, you need to have the largest solid angle (like angle but in 3D space) between the subject and the light source.

You can increase your solid angle or softness by 1) increasing the light source size : C0 to C1 or 2) having the source light closer : C0 to C2. I made C1 and C2 having same solid angle: softness will be the same, only the strategy differs.

Role of diffusion

Diffusion in the light source is part of the softening process. The optimal diffusion makes the apparent size (solid angle) of the source as big as the diffuser. A sub optimal diffusion makes the light source smaller, as if it was a combination of strong small light source (light bulb) + big weak light source (diffuser).

You can check the quality of the diffusion by making photography of your flash setup (in manual), and see how uniform is the light coming from the diffuser.

Comments for advanced photography

I shoot with Profoto RFI octa, and I have 2 diffusers stacked inside each octa. It helps indeed having a better (more uniform) light source. Without any diffuser, only the silver umbrella remains, and the apparent size of the light source is barely changed from bare bulb (in the case of Profoto Octa).

With tunable parabolic-like shaped light sources such as Briese Focus, we tune the apparent size of the light source; there is no diffusion but the apparent size is big: that's the secret in order to have soft shadows transitions and crispy textures, which is wanted in beauty commercials.

The actual job of a good photographer is to craft the light; a light source too big makes the subject look flat, a light source too small makes the subject look hard and knife cut.

To improve your skills, I suggest to look at (or photography) the light sources from the subject point of view and try to get a feeling of the coming result.

A practical tactic is to multiply your light sources, instead of having one big light shaper; it's modular and more useful in complex scenes, but for beauty commercials it's not interesting.

For equal solid angle, a bigger (and hence more distant) light source is affecting less the lightning of the subject when it is in movement or extended in space.

Crafting the outdoor light and solid angles

You can without any flash craft the light source size outdoor, with the clouds in the sky: clouds are like new sources and diffusers. A sky without any cloud is almost like bare light bulb; cumulus (especially after storm) in front of blue sky are making an amazing light source; a fully cloudy sky makes the light source as big as possible, contrast and feeling of 3D is minimized.

Conclusion

Adding diffusion makes the light softer by increasing the solid angle when the diffusion is suboptimal; but textures are softened. Increasing the light source size makes shadows transitions softer, but textures are only a bit changed. Working on both is the actual light crafting, and is required to achieve a certain result.

Good luck.

Answer 3

No. Diffusion is NOT how light is softened. Multiple layers of diffusion cannot help at all. Size is what helps.

Large light size is how light is softened. A diffuser can merely scatter the light. But hanging up a larger panel (as a diffuser) softens because it is large, less so because it scatters (trying to say size makes the diffusion useful). True regardless if the light comes through a larger cloth, or if the light is bounced from a larger panel. Large is soft.

Diffusion scatter causes different light paths, but a diffuser on a small light merely scatters the light outward, most of it missing the subject. A small diffuser is too small to redirect the light inward to the subject. It has no width from which to scatter inward. Small only can scatter outward, missing the subject.

A large diffuser (like an umbrella, or like a hanging cloth sheet (for the light to either illuminate through or bounce from), or a large bounce panel, or like the front panel on a softbox), being large, was width that can scatter the light at the edges back into the subject. Especially a reflected umbrella, curved to do exactly that. Many light paths from large angles. Large is what works.

A light is soft only because it is large (and because it is close, since close makes it appear even larger to the subject). Place a softbox close, so that it will appear as and act as larger. Consider a light panel the same size as its distance to the subject (like a four foot light panel at distance four feet). The math is that it will appear 53 degrees size to the subject. That is light from 26 degrees to the left, and 26 degrees to the right. Top and bottom too. So there are many directions of light paths hitting the subject from many directions, and each path fills the shadows of all the other paths, which is what soft is. Because it is large. Up to a reasonable size. In practice, much larger than this "same size as distance" will have diminishing return.

In contrast, our Sun is 1/2 degree wide, quite small due to the 93 million mile distance, so much less than this 53 degree example. That is a Hard light. No matter how you diffuse it, if it stays 1/2 degree, you might make it dim, but it won't help soft. But if you use a four foot reflector of it placed up close, it can do much. Soft is only about Large.

See http://www.scantips.com/lights/flashbasics3.html about What is Soft.