Grid Cloth from li lan's blog

Looking at the light source itself through the diffusion we can clearly see how a combination of direct light and diffuse light creates a harder source (as in the Spun and Opal options). As the definition of the original light source becomes increasingly obfuscated by the diffusion material, the overall size of the source increases and the quality of light is softened substantially.

The Tough Spun has very interesting properties. As it is a fine pattern of thin threads, there are clear spots along the material where direct light can shine through. This means that it produces a combination of both hard and diffuse light. You can see this illustrated in the images of the geometric shapes from our test, as both the umbra and penumbra of the shadows are well-defined.

“Tough Spun doesn’t really change the shadow transfer value [from that of a hard-light source],” notes Williams. “The shadows are still sharp, with just a reduction in contrast. All three of the Tough Spun strengths have this same behavior. This effect is clearest in the heaviest of the three, Full Tough Spun, where the penumbra has a clean, hard line, and the umbra of the shadow, although diminished, is still clearly defined. The Light Opal, on the other hand — as minimal an effect as it might have — starts to produce a breakdown in the definition of the penumbra, denoting more softness to the light.

“It was very interesting to me that the Tough Spun lent a texture to the light, a lot like a cookie or a celo cookie,” Williams continues. “It’s subtle, but it does project a pattern onto the subject.” As with any projected pattern, the farther away the pattern-producing material is from the light source, the better-defined the pattern will be on the subject.

The amount of diffusion increases with 1⁄4 216, which, we can see, produces very similar effects in terms of shadow quality to 1⁄2 216 and 1⁄4 Grid Cloth. Full Grid Cloth and 216 also produce very similar effects to one another, although Full Grid Cloth offers more diffusion of the light, as can be seen in the more diffuse penumbra; the definition of the shape is almost completely gone.

To further understand how diffusion works, we need to take a look at the material itself and see our light source behind it. The quality of light is created by the size of the light source relative to the subject, but if our diffusion material isn’t wholly opaque and we can still see some of the original source behind that material, then the actual fixture — in addition to the “new” light source created by the diffusion — has an effect on the overall quality of light. This is how lighter diffusions, even at the same size and placed at the same distance to the subject, create light that is less soft than what results from heavier diffusions — because they effectively combine the scattered light of the new apparent light source with the direct light of the original source, which is farther away and smaller.