Posted 14 May 2005 - 08:40 PM
There is too much artsy mumbo-jumbo surrounding different diffusions. The individual types of material (cloth versus plastic versus paper) used have some small affect on the "texture" texture of the soft light (how much specular light mixes into the soft light) and even may affect the color slightly, but the main difference is in how dense the material is and how efficiently it spreads light (which is why I don't use silk much, because it doesn't spread the light very well, creating a star-shaped hot spot. However, it is one of the quietest of materials outdoors in a breeze, so it is still very useful.)
The softness of a light is dependent only on the SIZE of the source (in this case, the diffusion, not the light behind it) relative to the subject -- i.e. a big diffusion far away can be similar in softness to a small diffusion very close. From the subject's perspective, for example, you could place a 20'x20' diffusion frame at a distance and a 4'x4' frame up close just to one side, and to the subject, both frames may look the same size in their field of view, and thus they would be of the same softness. The difference though is in FALL-OFF; the farther, larger diffusion would fall-off in intensity more gradually. With a diffusion up close to a subject, simply leaning towards or away from the source may cause a visible change in brightness.
However, all of this assumes that the light is filling the diffusion completely evenly and from edge to edge. A thinner diffusion may produce more of a hot spot in the center of the diffusion, so the "size" of the source is not quite as large when it's not filling the frame, and thus the quality of the shadow produced will be a little sharper.
There are no hard or fast rules about this; you can make the light very soft or barely soft according to taste and need.
Besides wanting to keep the light a little less soft, another reason for the lighter diffusions may just be that there is too much exposure loss from the heavier ones.