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Bouncing light-Difference in moving the source or formcore


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#1 Mathew Collins

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Posted 19 May 2017 - 08:34 PM

Hi, 

 

I was assisting a Cinematographer.

 

In a scene he bounced an Arri4K to a foamcore. In this scenario,

 

What would be the difference in,

 

1. Move the 4k away from the formcore

2. Move the foamcore away from the subject

 

Please share your views.


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#2 Satsuki Murashige

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Posted 19 May 2017 - 08:51 PM

The foamcore in this case sounds like it is the key light source.

Assuming the 4k is already filling the foamcore evenly from edge to edge, moving the light further away will simply reduce the light's intensity and thus the intensity of the key light. You could achieve the same effect by using scrims in the 4k. The quality of the light remains the same because the size and specularity of the source remains the same relative to the subject.

Moving the foamcore away from the subject will reduce the key light's intensity and also make the quality of the light harder because the size of the foamcore relative to the subject is smaller.
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#3 Mathew Collins

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Posted 20 May 2017 - 06:14 AM

Thank you Satsuki.

 

Instead of filling the foamcore evenly from edge to edge

if I off-center the foamcore with PAR light,

what would be the changes in light on subject?


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#4 George Ebersole

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Posted 20 May 2017 - 12:08 PM

Isn't bouncing light is also like opening the barn doors and adding a subtle diffusion at the same time?  Or am I just imagining that.


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#5 David Mullen ASC

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Posted 20 May 2017 - 01:40 PM

How evenly you fill the foamcore determines the size of the soft light which determines the softness. Whether the light hits the card at an angle or more flat on matters less the more matte the surface of the bounce card is -- since foamcore often has a matte and a shiny side to it, you may get a harder kick off of the shiny side that will make the bounce less soft.

When bouncing or diffusing light through a frame, the size of the area of soft light determines the softness -- putting diffusion on barndoors opened up might create a soft light shape that is 1'x1' or 2'x2' depending on the size of the light and barndoors, which is not a very large soft light compared to bouncing into a 4'x4' white card or going through a 4'x4' frame of diffusion.
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#6 Mathew Collins

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Posted 21 May 2017 - 03:08 AM

Thank you David.

 

I try to follow all your posts about diffusion. I understand that size of the diffusion frame determines the softness of the light.

But did the type of the material of the diffusion have some effect on the softness or quality? Like light goes thru 216 or 400 or 430?


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#7 Satsuki Murashige

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Posted 21 May 2017 - 02:17 PM

Yes, the type of diffusion material has an effect on the quality of light. 'Quality' in this case refers to how hard/soft and even/specular the light is.

Light quality exists on a spectrum between a perfect point source (like a single distant star in the darkness of empty space) and a perfect global soft source (like an overcast day on a snowy landscape). Most light sources exists somewhere between these two extremes.

The sun is basically a point source, even though it's about the size of a small coin relative to humans on the Earth's surface and not a point. The sunny side of a white building is a relatively large soft source when you stand next to it. Any surface that is not perfectly matte black will reflect some light and become a new light source.

You can observe this in nature every day. Hard reflections from the sun off of windows, water, and metal surfaces are the brightest and most specular sources. Then soft reflections from large bright surfaces like buildings, pavement, and panel trucks. Then even softer reflections from darker color saturated surfaces like foliage, painted signs, and colorful clothing. You can observe how the reflections light the environment, in additional to the primary light sources of direct sunlight and ambient sky light.

For filmmaking purposes, we tend to work at high enough light levels so that most of these secondary sources are relatively weak compared to our key light and don't show up on camera, or they can be removed with a simple negative fill. If you happen to be working at very low light-levels, then secondary sources are a much greater concern.

Similarly with direct diffusion on a light source, the diffusion blocks the original source from the subject and becomes the new larger and less specular source. The thicker the diffusion material is, the more spread out and even the light will be.

Think of it like different thicknesses of clouds passing in front of the sun. With just a thin wispy layer of high clouds, the diffusion effect is minimal. Most of the sun's rays are still passing through and striking the subject directly. Thicker cumulus clouds create softer light as more of the sun's rays are intercepted and diffused. And a thick even blanket of clouds from horizon to horizon creates the ultimate soft overhead source.

If you can look up and still see the shape of the sun through the clouds, then you are still being struck by some of the direct rays. So you know that the quality of the light is still partially specular. Same thing with diffusion in front of a light source. The light's quality is all about how the light appears from the subject's point of view.
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#8 Satsuki Murashige

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Posted 21 May 2017 - 02:21 PM

Isn't bouncing light is also like opening the barn doors and adding a subtle diffusion at the same time?  Or am I just imagining that.


Bouncing light is literally just aiming the light away from the subject and toward a reflective material. The reflective material then becomes the new light source.

If you put diffusion on the barn doors and aim the light at the subject, or put a frame of diffusion between the light and the subject, then you can't really call that bounce lighting.
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#9 David Mullen ASC

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Posted 21 May 2017 - 03:52 PM

If you fill a diffusion frame evenly with light from corner to corner, you've achieved maximum softness because the size of the diffusion is determining the softness. However in practical reality you don't always fill the diffusion frame evenly and sometimes you want to modulate the degree of softness by having there be more of a hot spot in the center like happens with lighter diffusion. And some materials don't spread light evenly from the center of the beam -- silks, for example, smear the light in more of a star-shape. Some thinner cloth materials, thinner silks or muslin, will have some specular hard light leak through so your subject is lit both with a soft light and a faint hard light. This all adds to the "texture" of the light.
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#10 Mathew Collins

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Posted 22 May 2017 - 07:36 PM

Thank you Satsuki, David.

 
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#11 Adam Frisch FSF

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Posted 24 May 2017 - 01:25 AM

Doored in lights cutting and shadowing on a frame is one of my pet peeves. Drives me crazy. Doors serve no purpose once you start diffusing away from the source. And related to that: another thing that drives me crazy is frames that are too close to the source - if there's a hotspot, we're no using the diffusion we agreed on to its full potential. If you wanted a harder source, then use less diffusion, don't go heavy and create a hot spot.  :angry:


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#12 David Mullen ASC

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Posted 24 May 2017 - 07:39 AM

I'd add to that forgetting to put the light at full flood.

However if you just want a mild softening like by using a frame of Opal, you're probably going to see a hot spot if you look at the frame. With the really light diffusions, it's hard to get them far enough from the light to minimize that, but at that point, you really aren't trying to be efficient nor maximize softness potential, you just want to blur the shadow patterns slightly.
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#13 Mathew Collins

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Posted 24 May 2017 - 11:36 AM

Doored in lights cutting and shadowing on a frame is one of my pet peeves. Drives me crazy. Doors serve no purpose once you start diffusing away from the source. And related to that: another thing that drives me crazy is frames that are too close to the source - if there's a hotspot, we're no using the diffusion we agreed on to its full potential. If you wanted a harder source, then use less diffusion, don't go heavy and create a hot spot.  :angry:

 

 

Is 'Doored' means Bandoor?


Edited by Mathew Collins, 24 May 2017 - 11:37 AM.

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#14 Mathew Collins

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Posted 24 May 2017 - 11:40 AM

I'd add to that forgetting to put the light at full flood.

However if you just want a mild softening like by using a frame of Opal, you're probably going to see a hot spot if you look at the frame. With the really light diffusions, it's hard to get them far enough from the light to minimize that, but at that point, you really aren't trying to be efficient nor maximize softness potential, you just want to blur the shadow patterns slightly.

 

 

>With the really light diffusions, it's hard to get them far enough from the light to minimize that,

 

Why can't we increase the distance between the light source and diffusion frame? Is there any issue?


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#15 Satsuki Murashige

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Posted 24 May 2017 - 11:41 AM

Is 'Doored' means Bandoor?


Yes, correct.
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#16 Satsuki Murashige

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Posted 24 May 2017 - 11:50 AM

Actually, AJ Young posted a lighting setup recently where I thought it made total sense to have a hotspot behind the diffusion frame: http://www.cinematog...=70937&p=477755
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#17 Satsuki Murashige

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Posted 24 May 2017 - 12:03 PM

 
>With the really light diffusions, it's hard to get them far enough from the light to minimize that,
 
Why can't we increase the distance between the light source and diffusion frame? Is there any issue?


Usually, if you're using a really light diffusion, it's because you don't have enough light output to use something heavier. So in that case, moving the light further away won't work because you'll lose intensity too. Or it might be an issue of available space.

Mainly though, because you're seeing more of the hard light source through the frame directly, you would have to back the light up really far away to minimize hotspot, at which point it's not worth it because you've lost so much output.

Remember, light diffusion material is like a thin wisp of clouds under the sun. You can still see the sun through it, which means the cloud is not diffusing the light very much. Imagine how far away and small the sun would have to be in order to barely be visible through that thin cloud. If you were trying to get soft even light, wouldn't it be simpler to just have thicker clouds? That's how diffusion works.
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#18 David Mullen ASC

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Posted 24 May 2017 - 01:27 PM

There can be artistic reasons for only mildly softening a light.
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#19 Mathew Collins

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Posted 24 May 2017 - 08:57 PM

Usually, if you're using a really light diffusion, it's because you don't have enough light output to use something heavier. So in that case, moving the light further away won't work because you'll lose intensity too. Or it might be an issue of available space.

Mainly though, because you're seeing more of the hard light source through the frame directly, you would have to back the light up really far away to minimize hotspot, at which point it's not worth it because you've lost so much output.

Remember, light diffusion material is like a thin wisp of clouds under the sun. You can still see the sun through it, which means the cloud is not diffusing the light very much. Imagine how far away and small the sun would have to be in order to barely be visible through that thin cloud. If you were trying to get soft even light, wouldn't it be simpler to just have thicker clouds? That's how diffusion works.

 

>Usually, if you're using a really light diffusion, it's because you don't have enough light output to use something heavier.

 

Where would be the use of light diffusion ? upto 1KW lights? Above that heavy diffusion? 

 

Is there any approximation?  

 

Any light diffusion (say 251, 252 or 452) is required to attach to Kinos(say 4 bank) to provide

light diffusion and make 4 tubes into a single source?


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#20 David Mullen ASC

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Posted 24 May 2017 - 09:15 PM

I think we've reached the point where you just have to start lighting with these gels and discover the effects for yourself.

 

The choice between using a lighter or heavier diffusion is just as much a creative one as it is a technical one.  You don't use the heaviest diffusion you can get away with in all circumstances.  I've put Opal in front of 10K's and I've put Full Grid Cloth in front of 650w Tweenies, for various reasons.


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