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Photographer
Jeanloup De Loupe
Posts: 117
Calgary, Alberta, Canada


I'll likely set-up an experiment to measure these effects in the studio but thought I'd ask anyways since I'm concerned my experiment won't take into account all variables.

I know that the further away you place the light source from the subject the more gradual the falloff will be. Here's some info if I'm not being clear.

http://photography.tutsplus.com/article … photo-3483


So if you position a spotlight 30 ft from your subject the falloff will be gradual. What happens to the falloff if you place diffusion material 1 ft away from the subject? Is it similar to a soft box placed 1 ft away or is it similar to the setup without the diffusion material?

My guess would be that the diffusion material now becomes the light source and act more in line with the softbox.
Nov 03 13 09:58 pm  Link  Quote 
Photographer
Thinking Inside The Box
Posts: 219
Diamond Bar, California, US


Jeanloup De Loupe wrote:
So if you position a spotlight 30 ft from your subject the falloff will be gradual. What happens to the falloff if you place diffusion material 1 ft away from the subject? Is it similar to a soft box placed 1 ft away or is it similar to the setup without the diffusion material?

My guess would be that the diffusion material now becomes the light source and act more in line with the softbox.

Your guess would be correct. The diffusion material now becomes the light source and acts more in line with the softbox.

Nov 03 13 11:17 pm  Link  Quote 
Photographer
Mikey McMichaels
Posts: 1,765
New York, New York, US


Jeanloup De Loupe wrote:
I'll likely set-up an experiment to measure these affects in the studio but thought I'd ask anyways since I'm concerned my experiment won't take into account all variables.

I know that the further away you place the light source from the subject the more gradual the falloff will be. Here's some info if I'm not being clear.

http://photography.tutsplus.com/article … photo-3483


So if you position a spotlight 30 ft from your subject the falloff will be gradual. What happens to the falloff if you place diffusion material 1 ft away from the subject? Is it similar to a soft box placed 1 ft away or is it similar to the setup without the diffusion material?

My guess would be that the diffusion material now becomes the light source and act more in line with the softbox.

Think of it as reducing the momentum.

A bullet shot without interference would travel X feet. If it had to pass through something, that would slow it down and it wouldn't travel as far.

While the diffusion material is spreading the light it's also reducing it.

Nov 04 13 01:51 am  Link  Quote 
Photographer
Sendu
Posts: 3,499
Cambridge, England, United Kingdom


Jeanloup De Loupe wrote:
So if you position a spotlight 30 ft from your subject the falloff will be gradual. What happens to the falloff if you place diffusion material 1 ft away from the subject?

I think you're intermixing the 2 different ways of using the term 'light falloff' in a confusing/incorrect way here.

Falloff due to distance is where the inverse square law of your linked article comes in, and tells you how bright the subject will be (what your exposure value should be). 'Gradual' does not make sense when describing this - the falloff is always inverse square, which by definition is not gradual.

Falloff across the frame depends on your light modifier (and perhaps your lens, if you want to also include vignetting in the category of light falloff). An actual focused spot light would have an incredibly harsh/rapid falloff, while a softbox would have a gradual one.


Is it similar to a soft box placed 1 ft away or is it similar to the setup without the diffusion material?

My guess would be that the diffusion material now becomes the light source and act more in line with the softbox.

Correct, if we ignore any light that hits the subject or background direct from the light without passing through the material.

Nov 04 13 02:53 am  Link  Quote 
Photographer
Wolfy4u
Posts: 1,078
Grand Junction, Colorado, US


This is slightly off topic, but maybe useful.
Years ago, I bought a few 'AC Slave lights that plug directly into AC. I
use them as spot lights, to highlight an object in the background or to
splash color onto something. They, of course have no light level control,
but I learned that one sheet of 'Kleenex' lowers the level by about 1 stop.
I call that my 'Kleenex' control. On the side, it says that these lights have a
GN of 66 at 100asa.
Nov 04 13 03:19 am  Link  Quote 
Photographer
Jakov Markovic
Posts: 1,076
Belgrade, Central Serbia, Serbia


Yes, it now acts as a light source, and even if you bounce the light, the object you bounced it off becomes a light source.

Every level of diffusion also reduces the power of light.

Note, that in order to contain the effect, Softboxes have walls, if you don't close in the "diffusion", you'll get a "fill" because light also bounces back towards the light source, and around the studio.
Nov 04 13 03:51 am  Link  Quote 
Photographer
BareLight
Posts: 508
Kansas City, Kansas, US


What's important to understand here is the underlying physics of the inverse square law as it pertains to light falloff.

Any light source emits photons of light in a spread pattern.  A focused spotlight produces a narrow spread of light, a wider angled reflector produces a wider spread, and a softbox produces an even wider spread pattern.

The individual photons of light do not decrease in intensity over distance.  Light falloff across a subject (or a scene) is the direct result of the number of photons that fall on it.

If you have a subject that is one meter wide and you position a light source so it is one meter away from the left edge and two meters away from the right edge, the left edge of your subject will be 4 times brighter than the right edge because it is receiving 4 times the photons of light (twice the distance equals one-fourth the light).

In your scenario, and assuming your subject is a person, placing the spotlight 30 feet away will result in very gradual falloff across your subject because the left and right side of your subject is receiving nearly the same number of photons.

If you then introduce a diffusion material one foot away from your subject you are altering the path of the photons, causing them to scatter in a much wider pattern.  The falloff across your subject will be much greater because the side closest to the diffusion material will receive many more photons than the far side.

Ignoring any other light reflected back from nearby walls, etc. you can think of your  spotlight and diffusion material as one giant softbox that is 30 feet deep.
Nov 04 13 05:17 am  Link  Quote 
Photographer
Creative Image
Posts: 1,316
Avon, Connecticut, US


Actually, the inverse square law is deadly accurate only when the light source is a point in space.  It still has relevance as the light source is enlarged (bigger light source and/or closer to subject) but isn't always what you would expect.  So take a meter reading, look at your tethered image (if you tether) or whatever.

And, there are two variables using a silk: distance from silk to subject and distance from light source to silk.  Nothing is going to beat experimenting to understand how they work together.
Nov 04 13 09:32 am  Link  Quote 
Photographer
Mikey McMichaels
Posts: 1,765
New York, New York, US


BareLight Photography wrote:
What's important to understand here is the underlying physics of the inverse square law as it pertains to light falloff.

Any light source emits photons of light in a spread pattern.  A focused spotlight produces a narrow spread of light, a wider angled reflector produces a wider spread, and a softbox produces an even wider spread pattern.

The individual photons of light do not decrease in intensity over distance.  Light falloff across a subject (or a scene) is the direct result of the number of photons that fall on it.

If you have a subject that is one meter wide and you position a light source so it is one meter away from the left edge and two meters away from the right edge, the left edge of your subject will be 4 times brighter than the right edge because it is receiving 4 times the photons of light (twice the distance equals one-fourth the light).

In your scenario, and assuming your subject is a person, placing the spotlight 30 feet away will result in very gradual falloff across your subject because the left and right side of your subject is receiving nearly the same number of photons.

If you then introduce a diffusion material one foot away from your subject you are altering the path of the photons, causing them to scatter in a much wider pattern.  The falloff across your subject will be much greater because the side closest to the diffusion material will receive many more photons than the far side.

Ignoring any other light reflected back from nearby walls, etc. you can think of your  spotlight and diffusion material as one giant softbox that is 30 feet deep.

Would the following be true if it were possible?

If you placed an opaque material between the light and the subject that did not have any effect on the directionality of the light and then increased the intensity of the light to match the exposure without it, there would be no change in falloff.

Nov 04 13 12:35 pm  Link  Quote 
Photographer
Kevin Stenhouse
Posts: 2,658
Calgary, Alberta, Canada


.
Nov 04 13 06:00 pm  Link  Quote 
Photographer
Jeanloup De Loupe
Posts: 117
Calgary, Alberta, Canada


Thanks everyone, this all helps quite a bit.
Nov 04 13 06:01 pm  Link  Quote 
Photographer
BareLight
Posts: 508
Kansas City, Kansas, US


Mikey McMichaels wrote:

Would the following be true if it were possible?

If you placed an opaque material between the light and the subject that did not have any effect on the directionality of the light and then increased the intensity of the light to match the exposure without it, there would be no change in falloff.

As long as the material had no effect whatsoever on the directionality of the photons then, yes, I imagine that would be true.

Nov 04 13 07:55 pm  Link  Quote 
Photographer
Jeanloup De Loupe
Posts: 117
Calgary, Alberta, Canada


BareLight Photography wrote:

As long as the material had no effect whatsoever on the directionality of the photons then, yes, I imagine that would be true.

A grid spot would be close?

Nov 04 13 07:59 pm  Link  Quote 
Photographer
Capitol City Boudoir
Posts: 738
Sacramento, California, US


Jeanloup De Loupe wrote:
I'll likely set-up an experiment to measure these affects in the studio but thought I'd ask anyways since I'm concerned my experiment won't take into account all variables.

I know that the further away you place the light source from the subject the more gradual the falloff will be. Here's some info if I'm not being clear.

http://photography.tutsplus.com/article … photo-3483


So if you position a spotlight 30 ft from your subject the falloff will be gradual. What happens to the falloff if you place diffusion material 1 ft away from the subject? Is it similar to a soft box placed 1 ft away or is it similar to the setup without the diffusion material?

My guess would be that the diffusion material now becomes the light source and act more in line with the softbox.

What happens when you have the sun that's 93,000,000 miles away and you have a cloudy, overcast day with diffused sky at 5 miles?

Nov 04 13 08:15 pm  Link  Quote 
Photographer
BareLight
Posts: 508
Kansas City, Kansas, US


Jeanloup De Loupe wrote:

A grid spot would be close?

A grid is used to control the spread of light by (mostly) blocking the photons that are escaping the light source at angles. 

A theoretically perfect grid material (one in which the grid material has no thickness and is 100% flat black, resulting in no light being reflected light off the material itself) would have no effect on the number of photons illuminating the subject and, thus, no effect on your exposure.

A snoot is, in effect, a single celled grid.

Nov 04 13 08:38 pm  Link  Quote 
Photographer
Jeanloup De Loupe
Posts: 117
Calgary, Alberta, Canada


BareLight Photography wrote:

A grid is used to control the spread of light by (mostly) blocking the photons that are escaping the light source at angles. 

A theoretically perfect grid material (one in which the grid material has no thickness and is 100% flat black, resulting in no light being reflected light off the material itself) would have no effect on the number of photons illuminating the subject and, thus, no effect on your exposure.

A snoot is, in effect, a single celled grid.

Cool.

Nov 04 13 09:16 pm  Link  Quote 
Photographer
Jeanloup De Loupe
Posts: 117
Calgary, Alberta, Canada


Capitol City Boudoir wrote:
What happens when you have the sun that's 93,000,000 miles away and you have a cloudy, overcast day with diffused sky at 5 miles?

I'd say whether it's 5 or 93,000,000 miles falloff is not an issue.

Nov 04 13 09:17 pm  Link  Quote 
Photographer
Robert Randall
Posts: 13,842
Chicago, Illinois, US


Creative Image wrote:
Actually, the inverse square law is deadly accurate only when the light source is a point in space.  It still has relevance as the light source is enlarged (bigger light source and/or closer to subject) but isn't always what you would expect.  So take a meter reading, look at your tethered image (if you tether) or whatever.

I'm unsure of your meaning... doesn't all light come from a source that is a point in space? Can you elaborate a bit on the emboldened line. I'm not being snarky, I just want to understand what you're saying.

Nov 04 13 09:29 pm  Link  Quote 
Photographer
Jeanloup De Loupe
Posts: 117
Calgary, Alberta, Canada


Sendu wrote:
I think you're intermixing the 2 different ways of using the term 'light falloff' in a confusing/incorrect way here.

I don't think I am, I'm trying only to refer to falloff due to distance.

Sendu wrote:
Falloff due to distance is where the inverse square law of your linked article comes in, and tells you how bright the subject will be (what your exposure value should be). 'Gradual' does not make sense when describing this - the falloff is always inverse square, which by definition is not gradual.

Let me try and clear-up my meanings. If you put a flashlight to your face the falloff is fast since the light source is close. If it's the Sun it's slow since the light source is far away. If it's 5-10 feet away it's gradual. This is what I mean by gradual.. hope that clarifies my intent.

Sendu wrote:
Falloff across the frame depends on your light modifier (and perhaps your lens, if you want to also include vignetting in the category of light falloff). An actual focused spot light would have an incredibly harsh/rapid falloff, while a softbox would have a gradual one.

Let's say we are lighting a matte piece of white paper with a harsh spot light 30 feet away.. angle is 170°.. just skimming across the paper. The bottom and the top would have similar values so the falloff is slow. If we lit the same piece of paper with a spot an inch away the falloff is fast. A soft box would light the subject more evenly but I don't consider that falloff and not what I'm after.

Nov 04 13 10:05 pm  Link  Quote 
Photographer
Jeanloup De Loupe
Posts: 117
Calgary, Alberta, Canada


What I'm after in the long run is super smooth umbra/penumbra under the chin and down to the collarbones and I think I'm starting to figure it out but I'm not sure why it works. Not sure it matters.

If I could ask one more question of the group. Is there a difference to these two set-ups with the assumption that all spill light is controlled?


1. 6 foot square soft box positioned 5 feet away from model.
2. 6 foot square diffusion material (same as in soft box) positioned 5 feet away from model but the actual light source is 30 ft. away.

thank you
Nov 04 13 10:05 pm  Link  Quote 
Photographer
BareLight
Posts: 508
Kansas City, Kansas, US


Jeanloup De Loupe wrote:
What I'm after in the long run is super smooth umbra/penumbra under the chin and down to the collarbones and I think I'm starting to figure it out but I'm not sure why it works. Not sure it matters.

If I could ask one more question of the group. Is there a difference to these two set-ups with the assumption that all spill light is controlled?


1. 6 foot square soft box positioned 5 feet away from model.
2. 6 foot square diffusion material (same as in soft box) positioned 5 feet away from model but the actual light source is 30 ft. away.

thank you

The softbox would exhibit a stronger hotspot in the center due to the closer proximity of the light source to the diffusion material but, I don't think there will be a large real-world difference between these two setups.  It really depends mostly upon the effectiveness of the diffusion.

Nov 04 13 10:46 pm  Link  Quote 
Photographer
Creative Image
Posts: 1,316
Avon, Connecticut, US


Robert Randall wrote:

I'm unsure of your meaning... doesn't all light come from a source that is a point in space? Can you elaborate a bit on the emboldened line. I'm not being snarky, I just want to understand what you're saying.

A point source of light is very, very small. A lightbulb isn't a point source, nor, for sure, a soft box. Nor, especially a 4x8 silk.  I probably overstated my first point, which was to meter or whatever and not count on the math to give the exposure or light ratios.

My second point was that there are two factors at play when using silks.

No, I didn't think you were being snarky.  As a matter of fact, I enjoy both your work and your forum comments very much.

Nov 05 13 07:53 am  Link  Quote 
Photographer
NothingIsRealButTheGirl
Posts: 32,026
Los Angeles, California, US


Robert Randall wrote:
I'm unsure of your meaning... doesn't all light come from a source that is a point in space? Can you elaborate a bit on the emboldened line. I'm not being snarky, I just want to understand what you're saying.

Any given photon comes from a point but they have to all come from the same point and have a random trajectory angles in order to strictly conform to ISL.

Nov 05 13 08:47 am  Link  Quote 
Photographer
NothingIsRealButTheGirl
Posts: 32,026
Los Angeles, California, US


Creative Image wrote:
A point source of light is very, very small. A lightbulb isn't a point source, nor, for sure, a soft box. Nor, especially a 4x8 silk.  I probably overstated my first point, which was to meter or whatever and not count on the math to give the exposure or light ratios.

A lightbulb is pretty close to a point source for most working conditions, don't you think?

Even a softbox conforms fairly closely to point source behavior after a moderate distance.

Nov 05 13 08:54 am  Link  Quote 
Photographer
Herman Surkis
Posts: 8,262
Victoria, British Columbia, Canada


Lots of good points.
I'll toss in my 2c worth.
As the OP is talking about fall off, there are 2 things getting confused.
Fall off meaning amount of light hitting the subject is simply a matter of the inverse square law. The type of light source does not matter that much (not that much).

The quality of light is dependent on the type of light source and its distance from the subject. The quality of light is basically the rapidity of change from hi-light to shadow values.

The sun is a little bigger than you are, but it acts as a point source because it is so bloody far away. If the sun was only a mile away, aside from frying you to a crisp, it would be acting as a jeezus huge softbox (huge source of light).

Regardless of the source of light, the amount of light hitting the subject is effected by the inverse ratio law. The link given is one of the best to illustrate this.
Now if the source is a point source and close enough to the subject, then perhaps there will be a noticeable difference in the amount of light hitting the tip of the nose and that which hits the base of the nose. All things being equal, which they never are.

There is more, but I forget.
Nov 05 13 07:13 pm  Link  Quote 
Photographer
Herman Surkis
Posts: 8,262
Victoria, British Columbia, Canada


Jeanloup De Loupe wrote:
What I'm after in the long run is super smooth umbra/penumbra under the chin and down to the collarbones and I think I'm starting to figure it out but I'm not sure why it works. Not sure it matters.

If I could ask one more question of the group. Is there a difference to these two set-ups with the assumption that all spill light is controlled?


1. 6 foot square soft box positioned 5 feet away from model.
2. 6 foot square diffusion material (same as in soft box) positioned 5 feet away from model but the actual light source is 30 ft. away.

thank you

As mentioned by another in example 2. the effective light source is the 6 foot diffusion.
However what you might see is a lose of intensity because the source of light that is hitting the silk is now far away. ISL come into play again.
Fall off on the subject and quality of light will be the same. The intensity will be different.

Nov 05 13 07:17 pm  Link  Quote 
Photographer
Michael Bots
Posts: 5,526
Kingston, Ontario, Canada


Watch this "behind the scenes" of the making of a high budget Christmas commercial a couple of times to pick up the lighting details.
--note the paper wolf cutout at 0:55

https://www.youtube.com/watch?v=Xv-D3u4PTE8
Nov 06 13 11:11 am  Link  Quote 
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