How light diffusion works

Diffusers are often made of paper, plastic sheets, thin layers of foam etc.. Basically, these materials all share the same basic structure, they are all more or less sheets of meshing and intertwining transparent or near transparent fibers. These fibers serve to refract light. Due to this, every single point of the diffuser on the side of the subject can be taken to be a point light source. The image below illustrates an infinitesimal cross-section of a diffuser and what happens to the light that hits that section:




As we are interested only in light that hits our subject, the light ray diagram can be simplified by ignoring the light rays emerging from the diffuser in the wrong direction that do not hit the subject. This will result in the light ray diagram given below:


When a diffuser is too near to the light source and too far from the subject, light from the light source hitting the edges of the diffuser travel a longer distance and are therefore dimmer than light hitting the middle. The effective light from the diffuser will thus look like the image below, where light from the edges are very dim compared to light from the middle (having a hot spot in the diffuser). The apparent light size is therefore small and lighting from such a setup will therefore be less diffused, no matter how big the diffuser is.



When the diffuser is far away from the light source, all light rays hitting the diffuser are approximately the same length, and therefore there is no hot spot on the diffuser. However, if the diffuser is not curved, the light rays emerging from the diffuser will not be equal in length and there will be a similar result as for diffuser A, but to a lesser extent.

Therefore, to achieve very even and diffused lighting, the light source has to be far enough away from the diffuser, and the diffuser shape has to roughly follow the contour of the subject. Happily most macro subjects can be approximated to be a small sphere and a simple curved diffuser placed as near to the subject as possible will suffice. As an example, an almost perfect diffuser for a salticid is to place it in a ping pong ball.

The nearer the diffuser is to the subject, the smaller it needs to be, as can be seen by the two diffusers above. Diffuser A is much larger than diffuser B, yet the light rays coming from both diffusers are of roughly the same direction as seen in this overlap:



However, light that is too diffused will result in very flat 2-D images which do not look realistic (like most of my images, as I am usually too lazy to angle my flash and diffuser). This is because shadows and highlights are what bring out the texture and 3-D form of subjects. One way to solve this problem is not to have the diffuser too head-on or top-down, perhaps have the diffuser from the side or at an angle. Another way, for those using twin lights, is to adjust the power of one side to be less than the other.

Also of note is that although an omnibounce increases the spread of the light, it does not increase apparent light size. This is why people do not aim an omnibounce directly at the subject, but at a nearby wall or ceiling instead. An omnibounce's main function is to spread the light over as much wall or ceiling as possible, effectively turning a larger section of the wall or ceiling into the light source compared to a bare flash.