Paul C. Buff, Inc. Technical Forum

1. At what distance do you loose that "ringflash" look? I don't mean the halo on the wall, I couldn't care less. I mean that sculpting effect where you get those highlights on surfaces parallel to the camera, and shadows on surfaces perpendicular to the camera? That darh shadow around the subjects legs.

2. Is it more pronounce with the grid? Working distance?

3. Will it still work with the Moon units? What effect will it have?

What you are seeing is what is referred to sometimes as light depth of field. If you consider the inverse square law, things closer to the light source will be brighter than things further away. The larger the ratio, the more difference there will be. The farther away the light source, the less difference there will be.

A human head is.... +/-10" in depth. If a light is 10" from the front of the face, the the back of the head is 2 stops less exposed. If the light is 14" from the front of the face, the back of the head is 1 stop less, etc. However, it is not only the front vs. the back of the head, but there is also the head/body vs the background.

At a certain distance the ratio of the front of the face vs. the back of the head, or the head/body vs the back ground, is so close, there is no discernible difference in exposure. This results in a flat light.

Combine this with the specular highlight created on convex surfaces. The size of these surfaces will be a result of apparent size and the shape of the surface. The distance combined with actual size will dictate the apparent size of the light source.

Yes, there is a sweet spot for that ring flash look, and that will vary somewhat by subject and environment. A moon unit will change that look a little by making the speculars larger and less apparent. A grid will not change the look on the subject so much, rather it will limit the spill, keeping the light straight forward.

A ring flash will always have a look not easily obtainable by any other means, but the specific look is a function of usage.

No, I have a degree in nuclear physics, I know all about the inverse square law. The effect is seen because the photons are parallel to the lens. Of course, the light will have dramatic fall off the closer it is to the subject, but you could not get the light close enough to see this effect on a subjects full body.

So in other words you don't know the working distance?

Someone just told me that they thought the effect was pretty much diminished by about 10'

Well, in other words, there is not an absolute working distance. You can still tell a ring flash shot from 15'. 10' sounds reasonable for a pronounced effect.

Thanks!