I just read "Lessons Learned on The Flash Bus Tour" and saw this image. In it, an assistant is holding some sort of white shoot-through diffuser (I got a similar one in my 5-in-one reflector kit) right in front of a soft box, which is lighting a portrait from above.
This is Joe McNally and David Hobby teaching at a workshop, so I assume there's a good reason for this. However, I don't know what it could be, or why it works.
- My understanding is that the larger the effective area of the light source, the softer the light.
- The softbox accomplishes this by bouncing the point-source light from the strobe around inside the box. When it exits, it's coming from a much larger "hole" (the front of the softbox), and from multiple angles, so it's nice and diffuse.
- The hand-held diffuser doesn't increase the surface area of the light much at all, since it's being held so close to the soft box.
- It would definitely reduce the amount of light, and I can understand needing that, but I'd think you could also do that by reducing the flash power (and saving your batteries).
So, what is the intent, results, and mechanism behind adding in the additional diffuser?