adjusting lights from shot to shot

Perhaps I misunderstood what you meant.

The inverse-square law applies to light reflected from your subject, as well as any other light from any other source.
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The first person mentioned that he thought the light halved from light to subject and then halved again from subject to camera, because light has to do the 'round trip'. I interpreted your comment was in regards (and in support) of that.

In this case, (light falling off from subject to camera) the inverse-square law does not apply to light reflected from the subject to camera. The subject becomes the 'source' of the light, and we see it, not the surface area where the light passing us (and camera) is hitting.

I think my approach to wording is the issue. I'll re-attempt.

The initial source makes light energy. The intensity of this light is subject to the inverse-square law... which is all about the beam spreading over a surface area. The larger the surface area of the beam, the more the light reduces in intensity. When light reflects off of the subject, it becomes the 'source'. Since we are looking at the 'source', our eyes are picking up the light as it passes us (to put it simply). This light always is the same 'brightness' no matter the subject to camera distance.

The inverse square law is in effect for the light coming off the subject, but we don't see that effect because we are looking at the source. That is where you are right.

So if I was disproving or proving you, depends on what you meant. Since the comment was juxtaposed when the conversation was regarding the light diminishing from subject to camera, I made the assumption.

All I know is that I've got a huge headache!
 
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fight it out guys.... just fight it out.


I thought I had the inverse square law figured out, but I am so lost now.
 
How would surface "perceived" brightness remain the same regardless of distance relationships? I don't see where that makes any sense.
 
Does anyone else think we are being "had' by the question? - read the 3rd post (the one after I take the bait and give an answer to the original question) and tell me what you think. Especially considering that he has asked the same question a couple of different ways over the last few weeks.
 
Mattykins said:
How would surface "perceived" brightness remain the same regardless of distance relationships? I don't see where that makes any sense.
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It doesn't. Offhand example: stars can be catalogued by apparent brightness or absolute brightness. The former being how-bright-they-look from Earth, the latter being their brightness from a uniform standard closer distance.

Better example: a flashlight pressed against your eyeball appears brighter - and really is delivering substantially more photons to the sensor - than an identical flashlight facing you from twenty-five miles away.
 
Sad Max said:
It doesn't. Offhand example: stars can be catalogued by apparent brightness or absolute brightness. The former being how-bright-they-look from Earth, the latter being their brightness from a uniform standard closer distance.

Better example: a flashlight pressed against your eyeball appears brighter - and really is delivering substantially more photons to the sensor - than an identical flashlight facing you from twenty-five miles away.
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Well I feel like the ass who won't stop beating a dead horse, but I still disagree. I don't claim to know the answers, as all of which I've been talking about was theories I came up with based on my knowledge and experiences. I'm hoping these 'disagreeances' will help find the definitive truth. With that said,

If there is a law of diminishing light when perceiving a light source's surface brightness (right term?) it is much much less then the Inverse Square Law!

I've been roaming around my house with my spot meter (reflectance readings), and have yet to successfully have ANY source of light (including simple reflective properties) read different from being 2 inches away to 20 feet away... which is a considerably far the distance of 120x. (My spot meter can't go much farther away considering it has a fixed angle of view.) Anyhow, the f-stop on reflectance still reads to the exact tenth of a stop.


My thoughts.
 
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My other theory is that it does have the inverse square law, but changing the distance and the size on the image negates any change.

Moving closer to a source should make things brighter... but since it now fills more of our vision or the film negative, it spreads that light over a greater surface area... the opposite of the inverse square law.

Only at the extremes (your flashlight and starlight examples) would this counteractive process reveal itself, as it would overpower (or underpower) our abilities to do so.

So what does that mean? It does have inverse square law... but it is negated? Therefore looking at reflected or source light doesn't appear to change over distance at all (to our eyes and cameras), although it does in the physical real world, despite us not being able to tell.

So perceived brightness does not change despite changes in distance relationships.

That doesn't explain my light meter work... but i'm not quite sure what the reflective spot meter formula is. Also, I don't know how focal lengths and zoom lenses would work, considering the distance doesn't change but the size on the negative/image does. Perhaps it does the adjustments inside the lens housing?

Another crazy theory!
 
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I think the discussion has gotten kind of lost in physics that don't have much practical consideration on set. When you use an incident meter for the light source, the inverse square law is very much in effect.

What Ryan says about the spot metering is valid. If you spot meter a white wall ten feet away or twenty feet away the meter reading will be the same.

On the other hand if I take an incident reading off that wall which is reflecting light, the bounced light reading will decrease with distance inverse squarely.

I can't explain it more scientificaly though I know you're all trying. What I'm trying to say is that for lighting, just worry about the inverse square law from light source (whatever it may be) to subject.
 
Ryan Patrick O'Hara said:
Moving closer to a source should make things brighter... but since it now fills more of our vision or the film negative, it spreads that light over a greater surface area... the opposite of the inverse square law.
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A one-meter square closer to a light source will receive more photons than a one-meter square placed further away. Light 'spreads' and falls over a greater surface area, with decreasing intensity the further you move from the source. Meaning that the closer you are, the more photons-per-unit-of-area your surface-of-choice will receive. I believe you have this precisely backwards...

Ryan Patrick O'Hara said:
Only at the extremes (your flashlight and starlight examples) would this counteractive process reveal itself, as it would overpower (or underpower) our abilities to do so.
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Except that photons moving through the same medium behave in the same way, regardless of distance, regardless of source. Physical laws are called that because they apply universally (although some do break down around singularities, which fortunately is not the kind of environment in which most of us have to light).

Ryan Patrick O'Hara said:
So what does that mean? It does have inverse square law... but it is negated?
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I don't believe you can negate physical laws; inverse-square any more than, say, the law of gravitation.

Ryan Patrick O'Hara said:
therefore looking at reflected or source light doesn't appear to change over distance at all (to our eyes and cameras), although it does in the physical real world, despite us not being able to tell.
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I can tell the difference between a car headlight an inch from my eye, and one a mile away. I think that you can, too. This is not an 'extreme example;' the physics are the same from one light-source source to the next.

Ryan Patrick O'Hara said:
So perceived brightness does not change despite changes in distance relationships.
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I'm requesting that tonight you light a candle and stare at the flame from a foot or two away for a while, then take it outside, place it a half-mile or so away, and look at it for a while. Then let me know what you find.

Ryan Patrick O'Hara said:
Also, I don't know how focal lengths and zoom lenses would work, considering the distance doesn't change but the size on the negative/image does. Perhaps it does the adjustments inside the lens housing?
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I figure you're probably kidding, but, yes, it has to do with the relative positioning of optical elements within the lens housing. And - counterintuitively - a telephoto lens and a wide-angle lens gather the same amount of 'information' - but one is bending light from a relatively small volume of space, and the other is bending light from a larger one.

This is fun. And probably a good belly-laugh at all our expense, for the first physicist who comes across the thread.
 
Here's my non-physicist's, recession-discounted $.002:

I think the inverse law applies identically to both source light (sun, lamp) as it does to reflected (subject, surface) where its *dispersion over distance* is concerned. The finite amount of energy produced or reflected will be increasingly dispersed over a larger and larger area as the source and subject are separated, and thus 'spread thinner'. Its intensity will decrease in a predictable manner *when measured as reflected from a fixed-size target area*. So a square foot of foamcore will *definitely* measure brighter a meter from a lamp than at 10 meters because at 10 meters, it's now spread out over a much larger area. The foamcore's surface represents a tiny fraction of that area, thus it gets correspondingly smaller piece of the total light energy 'pie'. At one meter, the light is largely blocked by the foamcore, so a much higher percentage of the energy is reflected by it, and much less is 'shared' elsewhere.

But I think Ryan's spot meter test does prove that on a direct line of sight to the source, it doesn't decrease. So the eye, looking directly at a lamp or lit subject, will not see a loss in intensity over increasing distance, because its *dispersed* light is not what's being seen. A direct line of sight to a light source by definition does not include reflections, thus it is not dispersed. So the light would travel directly, undiminished, instantly (aka light speed), to the receptor.

Regarding the example of a candle at one foot vs a half mile away, I think factors of scale come into play that would skew the results subjectively. To the naked eye, the distant candle is arguably just too 'small' to reliably quantify. But given a clear atmosphere and a telescopic spot meter, I suspect you might very well find the same reading in both locations.

Stars, still bright as all hell even to the naked eye despite being billions of light years away and viewed through miles of atmospheric diffusion, would seem (to me anyway) to support this theory.
 
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Emphasis on telescopic. A meter that reads through magnifying optics is effectively 'cheating' away the effects of the inverse-square law by gathering light the way a telescope would. This is a mechanical solution to measuring light intensity at a distance, but it doesn't mean anything where the I-S law is concerned.

In other words...

...yeah.
 
Well, it's my armchair-scientists' contention that magnifiers magnify *scale*, not light intensity when scale is matched, after all 'mechanical issues' are factored out. So two images of a candle, one shot at 100 yards with a telephoto and one shot at 1 foot with a wide angle, framed to match each other exactly, assuming identical light sensitivities for the lenses and no atmospheric impediments (haze, fog etc.), the light intensity of the flame should match.

Now we need a *real* scientist to settle this : )
 
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What I don't understand is how I'm trying to justify an obvious real life situation, while Sad Max refuses to try. I appreciate his input thus far, but simply tearing my theories apart and not offering a solution is strange.

I don't think I have it backwards, because I'm referring to collecting the light visually. Moving a camera closer to the reflected light source increases the light value, but that light now has to fall upon a greater surface area on OUR eyes or OUR cameras sensor... thus redistributing the extra light.

If the ISL did apply without being negated by how we see things, or how we photograph things, then if we were to move the camera 10 feet from a subject to 5 feet, we'd need to stop the lens down 2 stops. Reflected (& looking at direct sources), if anything, only changes in relative brightness over extreme distances (stars @ 190 trillion miles or a flashlight against your eye)

I still contend that a car's headlights 20 feet ahead of you or one block away still 'appears' to have the same brightness despite actual loss of photons. I think the further you go, the negating will cease to be even as our ability to resolute the smaller source increases. It may seem less bright as you are registering all that negative space around it. I think distances greater then that might appear darker... as out inability to resolute becomes a new factor. I still think this is because when it is further away the light energy only registers on a smaller part of our visual range/ image receptor. If the car headlights had the same intensity but where 5 feet in diameter, I think it would be easier to see that the distance relationship cancels out the ISL.

All I know is if reflected light obeys the ISL, it has to cancel it out somehow, because I don't change my exposure when I move the camera in a set lighting scenario.

And I know how a zoom works, thank you. I was not kidding. What I was saying is that I don't completely understand how it works within my theory. I'm not quite sure what it is that you believe. You think i'm incredibly wrong, so let me ask you: Do you stop down your lens when you cut the distance in half between the camera and the reflected light from the subject? I'd like to know why you think this light obeys the ISL but you don't witness it in everyday life.

Plus zoom lenses are not 'cheating' when collecting light. They collect the light reaching the lens at the physical space they occupy. I can't throw on a 300mm lens and GAIN light collecting ability. My ability to resolute the brightness increases. It's a different factor which at extremes can make something appear less bright (stars for example). If zooming through space collected more light, then we could rightly see in the dark with a long enough lens (extreme example i know).

I wish we had a physicist on board.

Again, just giving the best I got. I'm gonna wait it out for someone who knows a bit more about light. Perhaps I'll farm this question out to a science board or lens manufacturer.
 
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Ted Spencer said:
Well, it's my armchair-scientists' contention that magnifiers magnify *scale*, not light intensity when scale is matched, after all 'mechanical issues' are factored out.
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I love quoting myself : )

To further this thought a little, if, as SadMax asserted, a telephoto (or telescope) *can* increase light intensity *without changing scale* (maintaining a matched image size and framing) compared to an *identically light-sensitive* non-telephoto, then:

*Where does the extra light come from?*
 
If my theory is correct, then perhaps that explains why zoom lenses can lose light when zooming in. Such as the HVX.

Since moving closer would gain light, but the image would be larger on the negative,thus being negated, zooming is cheating.... in a way.

We zoom in on the reflected light source, so it's larger in the frame, yet we didn't move closer to it so there is no gain in light intensity... therefore since they (the photons) are spread out over a greater number of photosites, the same amount of energy is over a greater surface area making it 'darker'.

That is what was confusing me about lenses. I don't know how they manufacture zooms which keep their f/stop through the zoom range however.

A theory for my theory.

EDIT: Oh! And I just realized that 2x teleconverters always lose two stops! Double the magnification of reflected light w/o moving the camera closer and you lose two stops... the same as ISL. If you had moved the camera instead, it ( perceived brightness) would have had stayed the same regardless of distance.
 
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An 18% gray card doesn't come with a distance spec does it? If it actually mattered, wouldn't it?

The only way manual white balancing could work is if there's no connection with distance and light intensity when measured *directly and exclusively from the source* (in this case, the card), which is exactly how it *does* work. If the 'inverse law' applied to it it would render MWB completely non-functional. And it most certainly isn't...
 
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Just to be clear, maybe I should have used the phrase "auto white balancing" as it's called on most cameras. In any case, I meant when you aim the cam at something white and manually press a WB button to custom-calibrate it.
 
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