As we all know the word "Photography" means painting or drawing with light- lighting is the very essence of the craft. With that in mind we should all appreciate how important the discussion of the behavior and physics of light and the resulting aesthetics effects. The word "Diffusion" means breaking up of any given beam of light thus disbursing or scattering the rays in many different ways depending on the methodologies or equipment utilized. Some methods are very subtle and other are are very distinct and easily noticed.
The basic physical rules hold true in most cases but there are some subtleties and nuances that are "between the lines" so to speak. In the case of small diffusion devices that attached to portable and shoe-mount flash units there are many myths and truths. Yes, if we attach one of theses devices to an on-camera flash we do not increase the size of the effective light pattern enough to make a big difference- it is still basically a point light source when compared to a big umbrella or a soft box and it is essentially a flat lighting so it is not gonna magically turn a flat lighted harshly lighted image into a beautiful portrait. Many of theses devices will, however, soften the light just a touch making it a better fill light source in a multiple lighting setup. The fact that the device absorbs some light, a more dramatic ratio can be achieved when the power can not be easily adjusted or the fill light, being attached to the camera, can not be moved further away from the subject.
Some of theses devices will work more effectively in a smaller light colored room,especially the semi-spherical and bounce card models. What happens is the amount of "unseen secondary" bounce light reflects off the ceilings and wall and soften the image just a bit more. In a large room, a vast venue such as a church or our of doors theses devices have little or no effect- the proverbial "flatulence in a windstorm" (please note the polite medical terminology).
It is also true that light sources that are relatively larger and/or closer to the subject ten to "wrap around" better but the degree of that effect is also dependent on reflectivity of the surroundings- thus the use of reflectors and later movement of the lights as well as feathering. Although an umbrella is nothing more that a big parabolic reflector it will wrap around much more effectively that a smaller metallic parabolic reflector. When using say 16" parabolics in a portraiture set up, oftentimes more than one main light is needed to get the wrap around effect.
Sooo..besides the size and configuration of a diffusion device, the surroundings that it is used in, the degree of unseen secondary light, and the position of the lighting equipment also influence the final effect.
Isn't lighting cool stuff? I hope this helps! Ed
