# Inverse Square Law and Diffusion

diffusion inverse square law

3 replies to this topic

### #1 AJ Young

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Posted 15 July 2015 - 08:39 PM

I  am confused with inverse square law and diffusion material. Do you calculate the intensity of light falling off over distance from the head or from the diffusion?

I love key lights with a "creamy" softness to them as well as being the subjects eye light. (Of course, there are numerous situations where I just can't do that style of lighting) Additionally, to sell the creaminess of the key light, I always try to have the eye light fade away from one side to another.

(an example from Dearest Jane, a feature I shot in August 2013)

The above shot was lit with a Tweenie through a 4x4 cut of quiet grid. It was directly off frame right (and I think I had some 251 on the head for additional spread onto the cloth and to cut some intensity). I used to keep my diffusion as close to my subjects as possible and pull my head back so I can get that fading eye light effect as well as to maximize softness. I've learned the hard way that stacking lights and diffusion right outside of frame limits frame mobility and actor mobility severely. A very bad (and rude) thing for a DP to do!

Today, I typically use a strong source (such as a 1.2 HMI) shot through a large frame of diffusion (such as an 8-by or 10-by) at a substantially far distance away.

However, set ups of that size require a large amount of space. I've recently been shooting on cameras with higher native ISO's (such as the F5). I've started to move back to the above lighting set up I did above, but want to maximize my use of the inverse square law.

There are some situations where I can actually bring my diffusion material very close to the subject and still allow them to move freely and only gaining/losing a quarter of a stop. However, do you start calculating the inverse square law from the lamp or the diffusion material?

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### #2 Stuart Brereton

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Posted 15 July 2015 - 08:46 PM

The inverse square law refers to point sources, so it's not really that useful when dealing with diffusion.

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### #3 David Mullen ASC

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Posted 15 July 2015 - 08:57 PM

I think the more general term is "fall-off", which is more rapid when the source is closer to the subject.

As for whether it is better to be larger and farther with a more gentle fall-off, or smaller and closer and a quick fall-off, there is no right or wrong answer.  To some degree, if you are going for a natural look, then I tend to try to make a soft key as if it were from a window to have a gentler fall-off than if the soft key was supposedly coming from a nearby lamp, where sometimes I feel that it looks more realistic to have a fast fall-off, you get the sense that the source is physically close and not across the room.

For example, you'd expect that a candle on a table near a face would not have a gradual fall-off across the whole room, whereas you'd expect a daytime window to create a soft light that falls off gradually.

I don't know if anyone "calculates" it, they would tend to just see the results and compensate if necessary, or use a meter and read the range of brightness that happens as the actor moves closer and farther from the source.

Now sometimes we count on a faster fall-off to avoid flagging the light off of the background.  A larger, softer source is going to spill more around the space so you might be using bigger flags to control it.

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### #4 AJ Young

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Posted 15 July 2015 - 09:05 PM

The inverse square law refers to point sources, so it's not really that useful when dealing with diffusion.

I think the more general term is "fall-off", which is more rapid when the source is closer to the subject.

As for whether it is better to be larger and farther with a more gentle fall-off, or smaller and closer and a quick fall-off, there is no right or wrong answer.  To some degree, if you are going for a natural look, then I tend to try to make a soft key as if it were from a window to have a gentler fall-off than if the soft key was supposedly coming from a nearby lamp, where sometimes I feel that it looks more realistic to have a fast fall-off, you get the sense that the source is physically close and not across the room.

For example, you'd expect that a candle on a table near a face would not have a gradual fall-off across the whole room, whereas you'd expect a daytime window to create a soft light that falls off gradually.

I don't know if anyone "calculates" it, they would tend to just see the results and compensate if necessary, or use a meter and read the range of brightness that happens as the actor moves closer and farther from the source.

Now sometimes we count on a faster fall-off to avoid flagging the light off of the background.  A larger, softer source is going to spill more around the space so you might be using bigger flags to control it.

Ah, that clears it up very well. Thank you, guys!

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