adjusting lights from shot to shot

I guess sometimes ignorance is bliss.

I don't think anyone is tearing theories apart. I think we are all confused as hell.

Fact of the matter is: we know how it works practically, we don't know how it works theoretically. Which is okay by me. Because I no longer understand any of this. Haha.
 
Ryan Patrick O'Hara said:
I followed you on the gray card issue, but lost you on the AWB. lol.
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AWB is "Average White Band", an Australian rock group from the...oh wait...sorry...

I was just trying to address the fact that I had referred to it as "manual" white balance before, rather than "automatic" as it's usually labeled on a camera...

I seem to have a knack for de-clarifying things... : )
 
Here's one response, so far...

It's true that the emitted power falls off as 1/r² from the source. For reflection, there will be a kind of two-step process: 1/r² from the source to the object and another 1/R² from the object to the detector (cf. radar equation). If the source and detector are the same--say, a camera with a flash that provides most of the illumination--then this is a total of 1/r4. One can talk about these two steps as separate, treating the reflecting object as if it was a source; this is absolutely not a problem as long as one is clear about it is meant.

Part of the confusion in that thread, it appears to me, is an equivocation between what corresponds to luminous intensity and luminance.

Luminous intensity is simply the power of the source weighted according to some luminosity function. The way this works is that because human eyes have different responsiveness to light of different wavelengths/frequencies, lights of the same subjective brightness may have correspond to different physical power depending on the wavelength/frequency of the light. It's actually a bit more complicated than that, because there are four structures in the human eye, each of them having their own contribution: there 'photopic' (when cones are important, bright light) luminosity functions and one 'scotopic' (when monochrome rods dominate, in low light), and further even individually they may even have some variations depending on the conditions.

Luminous intensity is the same, independent of distance, for a given light-source, e.g., a lightbulb. One can think of it as how much brightness it puts out "in total." On the other hand, luminance is luminous intensity per area, and this is what changes--e.g., a light bulb far away won't be as useful in illuminating things compared to if it was nearby. One can think of luminance as how much brightness a source puts "over here."

What complicates things is when you start taking geometry into account. The most obvious is if the light-source is non-isotropic (e.g., a flash puts light in the front) The inverse-square law still applies int he sense that for any fixed direction, luminance in that direction falls off as 1/r², but it is now also a function of direction, which makes a difference both in relative orientations and in how the light reflected (cf. Lambert's law, which is a good approximation). Additionally, the speed of light is so great that you generally won't have just one reflection to consider, and not just reflection from the object (but also, say, source->floor/etc.->object).
 
Sad Max said:
Here's one response, so far...
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Thanks for that, Max.

I've read through it a couple of times, and I'll have to admit there's just enough science-speak in it that it's a bit tough for me to to grasp completely. However, I think this line:

Luminous intensity is the same, independent of distance, for a given light-source, e.g., a lightbulb.
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...might support my contention that a directly (and exclusively) spot-metered light bulb (or candle, in your example) would read the same at any practical distance (as Patrick found out in his experiments).

Other sections of the statement also clearly support the inverse square concept, but again, seem to point to that being primarily a by-product of light dispersion over the increasing 'square footage' of the affected area.

I'll stand corrected if I've misinterpreted any of this. Lord knows I'm not the expert here...
 
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Here's another...

What we perceive as brightness is light flux. Light flux is defined as the surface brightness or "source intensity"* multiplied by the apparent (angular) area. Source intensity is the power per unit area at the source. Apparent area decreases according to the inverse square of the distance: a person twice as far away takes up half the width and half the height in your vision, which is 1/4 the area; a person five times as far away takes up 1/5 the width and 1/5 the height, which is 1/25 the area, and so on.


Source intensity is constant for a primary light source and, obviously, variable for a secondary light source (i.e. if the moon was a trillion miles further from the sun, it would have very low source intensity and you wouldn't be able to see anything even if you were standing on it).

In the case of a person retreating from a camera flash in the same place as the observer (which in this case is the film), the intensity of the light source falls off according to the inverse square of the distance. Think back to the moon example. If we teleported Earth and Moon a trillion miles away from the sun, would you say that the moon would still be as bright? Of course not. Similarly, the intensity of light reflected from the person walking away from the camera flash decreases according to the inverse squares law.

Because the angular size of the individual decreases according to the inverse square of distance also, the light flux, which is intensity multiplied by apparent area, varies according to k/x^4 where x is distance and k is a constant such that if angular size varies according to a/x^2 and intensity varies according to b/x^2, k = a*b.

Experiment if you doubt the truth of this: Place a flashlight beside a mirror in an open field or very large room. Direct another identical flashlight at the mirror and walk away from the mirror. Observe which light - the stationary flashlight or the reflected flashlight - becomes dimmer at the faster rate.


*surface brightness is the term used in astronomy, if I recall correctly. I don't know if anybody but me uses "source intensity", but what we're measuring is the intensity at the surface and "brightness" is an abstract term with no associated units, so I prefer to use a term that makes sense with the rest of the study of physics.

(Note: sometimes people talk about the intensity of light from distant stars or other objects decreasing with their distance. They are using intensity to mean the intensity of light AT THE OBSERVER, not source intensity, which is the intensity of light at the source and is constant for a lightsource.)


EDIT: Obviously, this only applies to cameras if the light on the subject is FROM THE CAMERA. If you are photographing someone illuminated by sunlight or lots of indoor lights creating a high ambient light level, then their source intensity is constant because the amount of light shining on them stays the same. In this case their light flux only decreases according to 1/x^2, not 1/x^4. Therefore the per-unit-area brightness observed by an observer (or, to use different words to describe the same thing, the amount of exposure on the film of the image being captured) does NOT vary with distance if the observed object is lit at an unchanging level as by sunlight or indoor lights.
 
Sad Max said:
Here's another...

What we perceive as brightness is light flux. Light flux is defined as the surface brightness or "source intensity"* multiplied by the apparent (angular) area. Source intensity is the power per unit area at the source. Apparent area decreases according to the inverse square of the distance: a person twice as far away takes up half the width and half the height in your vision, which is 1/4 the area; a person five times as far away takes up 1/5 the width and 1/5 the height, which is 1/25 the area, and so on.

...Because the angular size of the individual decreases according to the inverse square of distance also, the light flux, which is intensity multiplied by apparent area, varies according to k/x^4 where x is distance and k is a constant such that if angular size varies according to a/x^2 and intensity varies according to b/x^2, k = a*b.


...EDIT: Obviously, this only applies to cameras if the light on the subject is FROM THE CAMERA. If you are photographing someone illuminated by sunlight or lots of indoor lights creating a high ambient light level, then their source intensity is constant because the amount of light shining on them stays the same. In this case their light flux only decreases according to 1/x^2, not 1/x^4. Therefore the per-unit-area brightness observed by an observer (or, to use different words to describe the same thing, the amount of exposure on the film of the image being captured) does NOT vary with distance if the observed object is lit at an unchanging level as by sunlight or indoor lights.
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Those paragraphs were very helpful. Since the lighting we are discussing is remaining constant or independent from camera movement... I think I agree with everything he said. His answer seemed to have a lot more with observance of light as a person or camera.

The 'inverse square of distance' seems to be what I was trying to describe as the negating factor over the ISL.
 
This is why I keep a pair of physicists chained to my sculpted granite throne.

You have to have someone to consult about these things.
 
Sad Max said:
This is why I keep a pair of physicists chained to my sculpted granite throne.

You have to have someone to consult about these things.
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I've got concubines chained to mine. Lower enlightenment to entertainment ratio (Enl/^/1.6 * Ent/^99.7), but way higher vice-versa. Life is full of such tough choices...
 
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Sad Max said:
...the per-unit-area brightness observed by an observer (or, to use different words to describe the same thing, the amount of exposure on the film of the image being captured) does NOT vary with distance if the observed object is lit at an unchanging level as by sunlight or indoor lights.[/I]
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This again would seem to support my earlier position - that the "per unit-area brightness" (like a 12x12 piece of foamcore) stays the same regardless of (any practical) distance. Of course it gets apparently smaller in size, as do all things as distance from them increases, so the "luminance footprint" if you will, likewise decreases, but the actual "surface brightness" of that footprint doesn't.

So:

Ryan's spot meter test of a direct source (producing no measured difference with increasing distance) would seem valid.

And:

The 'inverse square' calculation would thus only apply to measurements of light reflected from increasingly distant surfaces that the source illuminates.

Right?
 
Ted Spencer said:
This again would seem to support my earlier position - that the "per unit-area brightness" (like a 12x12 piece of foamcore) stays the same regardless of (any practical) distance. Of course it gets apparently smaller in size, as do all things as distance from them increases, so the "luminance footprint" if you will, likewise decreases, but the actual "surface brightness" of that footprint doesn't.

So:

Ryan's spot meter test of a direct source (producing no measured difference with increasing distance) would seem valid.

And:

The 'inverse square' calculation would thus only apply to measurements of light reflected from increasingly distant surfaces that the source illuminates.

Right?
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Right.

Although the inverse square law does apply, there is an optical reason we can't see it in effect and why we don't take it into calculation. (Nor can our cameras)

So you are right, the inverse square law needs only to be applied (accounted for, rather) in certain situations. When changing the distance of a source of light upon which is being reflected by the subject, it would be obvious to the eye. AKA, what we all knew going into this mess, just not why it didn't apply on the second leg of the trip (subject to eye/cam). haha.

I tried to rephrase it:

As we back away from reflected light, it does become 1/4 as bright every time we 2x the distance. But since we are 2x the distance away, our eyes /camera sensor see's the object reflecting the light as 1/4 the size it was. Therefore you are getting the same energy/photon to surface area (of the sensor/eye) ratio. Thus it appears the same brightness (regardless of distance), but the light has weakened in 'real' space.
 
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