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Questions about fall-off


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#1 Wilder Smith

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Posted 01 March 2017 - 10:16 AM

I've been trying to understand how different lights create different fall-off lately. I'm going to lay out what I understand here and hopefully some of you will read it and hammer me straight. 

 

With soft light sources, I've always held the belief that the closer a subject is to a light source then the quicker the light cast on the subject will fall-off. This understanding aligns with the inverse-square law and usually seems to be well received by other technicians I talk to. My troubles start to arise when I start thinking about focused light sources and fall off. 

 

For instance, deep, rounded bank-lights like the Briese seem to operate under a different set of laws than flattened light sources. Is this because they focus the light?  I always thought it was silly when people put the diffusion over the front of these banks because then doesn't it just make it a big evenly lit light source that you could achieve at a much lower price? (ie: book lighting)  Could someone explain how the quality of light and fall off differ in these modifiers?

 

also, Is fall-off a concept that is primarily discussed when talking about soft light? I don't see a lot of people talking about fall-off in relation to hard light.  Is there something I'm not understanding?

 

Thanks for reading :) Hope you can help!


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#2 David Mullen ASC

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Posted 01 March 2017 - 11:14 AM

People use "fall-off" when discussing soft light just because the more precise concept of inverse-square law technically only applies to a single hard source, but the reality is that soft light fall-off more or less in a similar manner and rate, it's just that it's not precise since you have rays coming at so many angles onto the subject with a large source.

 

Once you put a diffusion frame in front of a light, even a Briese, then the diffusion frame becomes the light source on the subject and you'd talk about fall-off relative to the distance between the subject and that source, the diffusion frame, not the Briese.

 

Fall-off doesn't describe the softness of a light, just the change in intensity over distance.  I doubt a Briese light does anything unusual in terms of how fast or slowly the light falls off in intensity over distance, but then, I've never used one.

 

Most cinematographers just keep it simple, if you want a faster fall-off, put the lights closer to the subject, and if you want a slower fall-off, work with the lights farther from the subject, which if you want them to also be soft, means work with larger soft lights.

 

Fall off is discussed when using hard lights, it's just that a lot of people don't light close with hard lights.  But if you wanted a point source from behind a candle to create a candlelight effect on a room, you'd get a hard effect whether you put a bare quartz bulb behind the candle or use a 10K fresnel from far away (let's say, a sidelight) but obviously the first technique will have a more realistic fall-off in terms of a candle flame lighting a room.  Same goes when you try to light a scene as if a person is carrying a lantern and you either hide a bare bulb behind the lantern or use a lamp from off camera moving with the subject.


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#3 John E Clark

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Posted 01 March 2017 - 01:00 PM

I've been trying to understand how different lights create different fall-off lately. I'm going to lay out what I understand here and hopefully some of you will read it and hammer me straight. 

 

With soft light sources, I've always held the belief that the closer a subject is to a light source then the quicker the light cast on the subject will fall-off. This understanding aligns with the inverse-square law and usually seems to be well received by other technicians I talk to. My troubles start to arise when I start thinking about focused light sources and fall off. 

 

For instance, deep, rounded bank-lights like the Briese seem to operate under a different set of laws than flattened light sources. Is this because they focus the light?  I always thought it was silly when people put the diffusion over the front of these banks because then doesn't it just make it a big evenly lit light source that you could achieve at a much lower price? (ie: book lighting)  Could someone explain how the quality of light and fall off differ in these modifiers?

 

also, Is fall-off a concept that is primarily discussed when talking about soft light? I don't see a lot of people talking about fall-off in relation to hard light.  Is there something I'm not understanding?

 

Thanks for reading :) Hope you can help!

 

 

Fall off, and using the Inverse Square Law is in regard to 'point source' lights... The single light bulb, candle etc.

 

In the case of a 'soft light', I'm presuming that one is using some sort of 'screen/softbox' or the like, and as such, is not a 'point source'.

 

However, as one moves away from the screen/softbox, the source becomes a 'point source', and so the Inverse Square Law holds at that point and beyond. I believe the usual rule of thumb is Inverse Square Law holds for distances greater than 2x the diameter/diagonal of the screen/softbox.

 

So for something like a 20"x40" screen/softbox, the diagonal is 44", and so 2x would be about 88", or about 7'.

 

Calculating these sorts of things may give a general idea, and good for getting a rough idea of the lighting required for a shot, but in reality, measuring the actual setup is better...


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