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Help to understand gradually Fall-off regarding to the Diffusion and Distance of light. (2 replies and 9 comments)

leandroimaz
6 days ago
leandroimaz 6 days ago

Hello Roger, 

I would like to ask you a question about this topic I can't fully understand it yet. 

Whats the difference of the  fall-off when we use different diffusion outside the window in a daytime interior, a light diffusion vs a heavy diffusion.  

To get a gradually falloff  What's the best way to achieve it ?

I am a bit confuse about the distance. I know it's best to use a large bounce or diffused source but I am a bit confuse about the distance. 

Best Regards 

Leandro

 

 

gabj3
6 days ago
gabj3 6 days ago

Not Roger,

Before a rather brief answer in comparison to what an answer to those questions could be I do recommend you read up further on this subject (as well as the replies you get here)! These are the building blocks of good lighting so knowing them and understanding them is quite a good idea.

"Whats the difference of the  fall-off when we use different diffusion outside the window in a daytime interior, a light diffusion vs a heavy diffusion.  "

A rather broad subject mixed in with a smaller subject. I'll ignore the daylight interior and focus on the light diffusion vs a heavy diffusion. Diffusion is when 'Light diffuses when it bounces off the many angles of a rough surface, or when it travels through a substance that changes its angles.' A heavier diffusion scatters the light more than a lighter diffusion therefore creating a 'softer' (diffused) look. However a heavier diffusion will also cut more light. I highly recommend you buy a piece of Muslin off amazon and compare it with a piece of tracing paper. Fold it for an even heavier diffusion and I'm sure you'll see a difference!

The fall-off. The inverse square law states that at double the distance you get 1/4 of the light. Now a number of things to take into account. A medium/heavy diffusion will act as a 'new source' in a way (at least from what I've studied and my experience). So you take your inverse square law into account from the diffusion not the original source. On lighter diffusions (much lighter) this doesn't apply as much. For example bounce off a piece of muslin you take inverse square law from the muslin bounce. Bounce off a mirror and you take the inverse square law into account from the original source. 

"To get a gradually falloff  What's the best way to achieve it ?"

Place the source further away from the subject. If you look at the way light falls off it doesn't fall off linearly it falls off in a curve. Inverse square law you get 1/4 of the light at double the distance. So from 1ft to 2ft you loose 3/4 of your light. From 100ft to 200ft you loose 3/4 of your light (5ft-10ft 20ft-40ft so on). Extreme examples but I feel it shows the point. Now of course there are always exceptions some fixtures have more "throw" such as a ellipsoidal reflector spotlight (such as a Leko).

"I am a bit confuse about the distance. I know it's best to use a large bounce or diffused source but I am a bit confuse about the distance."

'Best to use a large bounce' that is entirely up to you! Softness of the light is dictated by the size of the source relatively to the subject. If you want a hard light for example a kicker. A larger bounce would be a bad idea a smaller bounce that's less diffused such as a mirror would be great! 

The size of the source relative to the subject. The sun is a very large source however very very very far away so it casts a harsh shadow and is considered a hard light. A fluorescent tube in comparison is tiny however at 6ft away from your subject placed horizontally it is a softer source. 

I feel like I've only scratched the surface and it is quite late (midnight) so I have probably missed a fair bit. Theres a lot to read into for example a larger bounce may be better for a softer quality of light but if you don't have the firepower to fill the bounce the larger bounce is essentially useless. 

Best thing to do is grab a light even a flashlight and practice!

 

gabj3
6 days ago

Also do note. The inverse square law refers to point sources, so it's not really that useful when dealing with diffusion. I probably shouldn't of used the term 'inverse square law' but rather 'fall-off'. I am afraid I can't edit the post.

Mike
6 days ago

You should be able to edit once you have pressed the ‘wrench’ symbol.
Give it a try.

dmullenasc
6 days ago

Hitting the wrench symbol only causes "report" to pop up for me.

Mike
6 days ago

I have reported it to Robert for when he is feeling better.

dmullenasc
6 days ago
dmullenasc 6 days ago

The general rule is that when you bounce or diffuse the light, that surface becomes the "source" for discussing fall-off even if inverse square law doesn't technically apply anymore.  So if you have the real sun beaming straight into a window, let's say almost horizontally, that has a very gradual fall-off, in fact, the sun hitting someone outside the building is the same intensity as someone's face being hit by the sun inside the room. However, but 216 diffusion over the window and the fall-off inside the room starting at the window is very fast as you move away from the diffusion.
BUT... I've been meaning to test this someday, but I believe in the case of very, very light diffusion (Hampshire, etc.) that what passes through is a mix in terms of fall-off rate, a mix of a far and a near source. I definitely could see that if the diffuser was very porous, like a curtain sheer with a loose weave. However, I've never tested this theory because it would need a stage with a lot of distance so the point source could be very far away so as a base in the experiment (no diffusion) I had a very gradual fall-off rate starting in the center of the room where I would set up the diffusion frame.

dmullenasc
6 days ago

I could be wrong and discover that the rate starting at the diffuser is the same no matter what the material is.

dmullenasc
6 days ago

"but 216 diffusion" I meant "put" not "but" -- I can't edit my posts.

Mike
6 days ago

Oh dear! Will get Robert or James to see what’s going wrong.

Mike
6 days ago

Probably quicker for James and Reese to check it. How frustrating for you, so sorry.

Mike
6 days ago

Have reported problem to the ‘management’ for action.

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