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How does Light Stack?


Gabriel Devereux
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Dear All,

I have been wondering and testing how light stacks and its correlation to light fall off. The term 'stack' is most likely wrong, what I mean by it is when two beams of light overlap does the luminance value say foot candles in this instance add up? It makes sense as for example a maxi brute or other Fays that have several lower wattage lamps such as 1k's or 650's add up to produce a larger output. So for instance if I had several Par64 cans and rigged them in an array in which the distance between fixtures was minimal the total output would be the addition of all the outputs of the par cans combined?

However I question this further (reference diagram bellow) in which then if you bounced several sources into the same area of say a frame of ultrabounce. If the beams overlapped would they add together create a more powerful source with therefore less fall off. Putting terms into words is not my strong suit so I have drawn a crude diagram bellow. The tests I have created had mixed results and while practical is good and something I shall test further for a future rig the theory behind it interests me greatly! 

 

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Please do note nothing on this diagram is accurate from the refraction etc to anything. Just a crude idea.

Edited by Gabriel Devereux
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It stacks additively.

What makes this complicated is the fact that f-stops represent a doubling of light, so if you have one parcan and you're getting an f/2, add another to get f/2.8, add two more to get f/4, add four more to get f/5.6, etc.

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On 9/28/2020 at 8:50 PM, Gabriel Devereux said:

the beams overlapped would they add together create a more powerful source with therefore less fall off.

Additive, yes. Falloff is the inverse square law, and is a constant. By adding more light, you only make it brighter; it's falloff shape will remain the same, only brighter as it steps down. If you require less falloff, then you must move the source further away. Notice now that this source becomes dimmer by a great deal, thus you'll need more light to reach your desired exposure in the falloff bracket you want.

Also, your math on the diagram is incorrect. The inverse square law is always in effect. Your original sources, the 75FC and 150FC, should drop in FC, not increase. You cannot create more light than you begin with. Also, the "double distance" marker is missing it's informing marker, an original point of measurement. It is double what distance from the source? You must meter at a measured distance first, then you will be able to identify the intensity at double that distance from the source.

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Thank you for the replies!

4 hours ago, David Mullen ASC said:

Yes, fall-off rate is determined by distance, not starting intensity. So adding more light only decreases fall-off if it allows you to back up the lights farther from the subject.

Thank you Mr Mullen! Yes sorry I used the wrong terminology. So basically if my shooting stop is a T2.8 at 400 ASA 180 degree so on I need 50fc for a stop over key. So I can move the sources back further if I in a sense combine them. Therefore creating a lesser/smoother fall off.

 

4 hours ago, Stephen Sanchez said:

Additive, yes. Falloff is the inverse square law, and is a constant. By adding more light, you only make it brighter; it's falloff shape will remain the same, only brighter as it steps down. If you require less falloff, then you must move the source further away. Notice now that this source becomes dimmer by a great deal, thus you'll need more light to reach your desired exposure in the falloff bracket you want.

Also, your math on the diagram is incorrect. The inverse square law is always in effect. Your original sources, the 75FC and 150FC, should drop in FC, not increase. You cannot create more light than you begin with. Also, the "double distance" marker is missing it's informing marker, an original point of measurement. It is double what distance from the source? You must meter at a measured distance first, then you will be able to identify the intensity at double that distance from the source.

I really used the incorrect terms. The idea was if I bounced multiple sources into a singular bounce I could potentially move the bounce further back getting a lesser fall-off. Re-diagram read from top to bottom. The top two triangles are the lights and so on. With the distance marker I meant double the distance from the bounce as the bounce becomes the source.

So the overall idea is if I bounced several sources into a singular small frame the light which is additive (thanks everyone!). Would in a sense combine creating a more powerful soft source which I can therefore move further away creating a smoother fall-off.

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Yes, fall-off rate is affected by distance and softness is affected by size relative to subject, so if you move the soft light farther, make it bigger, and use more light to keep the same exposure on the subject, the fall-off will be more gradual but the softness and exposure would be the same.

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12 hours ago, Gabriel Devereux said:

Re-diagram read from top to bottom. The top two triangles are the lights and so on. With the distance marker I meant double the distance from the bounce as the bounce becomes the source.

So the overall idea is if I bounced several sources into a singular small frame the light which is additive (thanks everyone!). Would in a sense combine creating a more powerful soft source which I can therefore move further away creating a smoother fall-off.

Well that makes more sense. I thought the arrows were direction.

I'll note that your diagram is representative of a mirror, the way you have the beams leaving unaffected.

The more matte the surface, the more each beam is reflected in all directions. So a super matte surface like rough concrete or ultrabounce become omnidirectional upon reflection. They glow in all directions.

Keep in mind that bounce is dependent on the reflective quality of the material you are using. Paper for example used in a foamcore bounce card has a slight hard reflection to it, I believe due to the fibers although it could be how it was pressed. This is why if you play with the angle you can get "more punch", but you are actually catching the angle of reflection from the light source. The same is true for Griffolyn material which is white and slightly glossy.

Some may say then that a Griff is brighter than ultrabounce, and that would be true only at that particular angle it reflects a light. Outside of that specific angle, it is darker.

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The rate more or less follows the inverse square law though that technically only refers to a point source.

I think you can imagine what it looks like if a face is one foot from a soft light and then moves three feet away from it, as opposed to being 20 feet from a soft light and then moving three feet away from it.

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