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Archive 2012 · silly inverse square law question.
  
 
BrianO
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p.6 #1 · p.6 #1 · silly inverse square law question.


jeremymeier wrote:
...The subject...will in effect require adjustment of exposure the further the sensor would get from the subject being lit.


I guess you haven't actually tried it before posting.

The exposure doesn't change; just about everybody here agrees on that. The only question, and the subject of what was until a few posts ago a polite discussion, is why the exposure doesn't change.



Nov 11, 2012 at 02:07 AM
quicksilver33
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p.6 #2 · p.6 #2 · silly inverse square law question.


This has been a really interesting discussion to me since I used to consider myself a bit of a physics buff. I'm a little rusty now so although my understanding says reflected light must also fall off over distance - I understand that practically it doesn't appear to be so. Doing a little searching online I believe I found the answer as to why this occurs from an exposure perspective.

You can check the link below for full details but the answer seems to be that as you move the camera away from the subject you get light spreading as per the inverse square law, but at the same time it's concentrated into a proportionally smaller area so brightness per surface area (per pixel) stays the same.

Reference here: http://www.scantips.com/lights/flashbasics1b.html



Dec 05, 2012 at 03:37 AM
RustyBug
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p.6 #3 · p.6 #3 · silly inverse square law question.


http://www.scantips.com/lights/flashbasics1b.html
There are two cases:
1) light that is incident onto the subject, from the light source, illuminating the subject. And there is
2) light reflected from the subject, entering the camera lens (the subject itself is not a light source).

Light reflected from the subject is a different situation than light incident on the subject.




A single ray of light originates from its source with a given direction from the source, continuing on that path until interacting with another object.

The amount and color of the light reflected from the object is the result of the amount and color of light striking the object, minus the amount and color of the light being absorbed by the object. (Conservation of Energy Law)

The light striking that object is then reflected iaw AI=AR (based on the angular surface structure at the point which the object is struck by the light for an opaque object, refraction & reflection combined for a translucent object, refraction for a transparent object)

The reflected light that is captured by the camera is done so relative to angles of inclusion involved, iaw AI=AR (see Light: Science & Magic)


This is how light travels from its originating source to an object and then to either our eyes or our camera. Of course, the process involves a multitude of rays and angles that work in concert with each other.

Consider this ... most of us have used a magnifying glass to "burn leaves". When playing with a magnifying glass, the angle that we hold the light determines whether the light (energy) is being concentrated or diffused as we watch that "ring of light" change size and shape. The light is not traveling iaw with ISL after it is being refracted by the glass, it is traveling iaw AI=AR.

Whether light is reflected or refracted it continues its path iaw with AI=AR (reflection or refraction) based upon the surface's properties (opaque, translucent, transparent, refractive index, etc.)

Much of what people are trying to associate to ISL regarding reflected light is the lack of accounting for the multitude of originating rays that combine to yield an array of angles, then striking an array of surface angles. The fewer variations in the angular structure of a given surface, the more directly the light appears to travel iaw with AI=AR. The more variation in the angular structure of a given surface the more it appears to deviate from AI=AR. This seems to be when people errantly start trying to insert ISL to explain its reflected distribution based on their observance of what appears to them to be the rationale for how light travels ... rather than apply the tenets of AI=AR to the multitude of angles involved (that they cannot individually see). However, in all cases light travels iaw AI=AR.

ISL is the explanation of initial spherical distribution of a multitude of photons from PLS. Light energy is a vector quantity, thus the angles involved are paramount to understanding light as it continues to be reflected & refracted. Besides, if reflected light truly traveled iaw ISL rather than AI=AR, then parabolic reflectors would not have any different effect on light than flat reflectors, nor would changing the angle of a reflector allow you to "feather" your light any differently.




Edited on Dec 05, 2012 at 06:36 PM · View previous versions



Dec 05, 2012 at 05:12 AM
quicksilver33
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p.6 #4 · p.6 #4 · silly inverse square law question.


Hahah okay I've finished work and had a chance to read back through the thread a bit more and I see this has already been brought up in different words by Curious. Now I understand your reluctance as there seems to have been a lot of back and forth with no consensus.

I'm not sure what you mean by the article being poorly written and "telling on itself". Forgive me for being presumptive but it sounds like you don't have a scientific background so I'm not inclined to consider yourself a credible source on the accuracy of scientific explanation. I guess on a practical level it doesn't matter what you believe since we all agree you don't need to adjust exposure as you move away. However it does ruin your credibility to speak with an air of assumed authority on behalf of a subject that you don't seem to know much about. I'm happy to attempt to explain if you're genuinely interested. Or we can just agree it's a neat phenomenon and drop it.


Curious - Don't be too let down, these nerds get it!

http://www.photography-forums.com/why-isnt-reflected-light-subject-inverse-square-law-t221230.html
http://www.apug.org/forums/archive/index.php/t-87212.html



Dec 05, 2012 at 01:27 PM
RustyBug
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p.6 #5 · p.6 #5 · silly inverse square law question.


My bad ... I suppose I really should be swayed by a few threads from photographers on the internet who are grappling to creatively explain phenomenon rather than hold fast to the teachings of a research physicist from the Army's Night Vision Laboratory. He just made me conduct experiments and perform trigonometrical (vector quantity) mathematical calculations about the behavior and properties of light, but that was before the internet existed. They put it on the internet, so they must be correct.

I understand that there will always be those who continue to believe as they will, based on their perceptions and/or the explanations of others. Thus, to each his own as to what he believes. I've obviously subscribed to AI=AR, vector quantities of reflection & refraction for opaque, translucent and transparent surfaces/materials and the Conservation of Energy Law. Other are welcome to subscribe to whatever they choose. I only offer it in good faith.

BTW, the world is still flat, that whole round thing is just a ruse based on an optical illusion.
Serious question though ...

How come I never heard of anyone offering/teaching this ISL / camera - subject distance / reflected light / cancellation / equalization / offsetting explanation "back in the day"?
Did light somehow magically change once we entered the digital / internet era?
Or was everyone "back in the day" wrong about light all along?
When did this "offsetting" / ISL / camera - subject distance explanation come to replace AI=AR and the vector quantities of reflection and refraction and the Conservation of Energy Law ... 'cause I obviously missed it along the way.


Not so serious question ...
Did Al Gore re-invent light when he invented the internet? Maybe ISL is the real reason we're having global warming.



Dec 05, 2012 at 02:19 PM
Zenon Char
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p.6 #6 · p.6 #6 · silly inverse square law question.


Sorry if this was posted already. This video alone cleared it all up for me. I think if you are into flash photography this is just as important as understanding shutter, aperture and ISO.

http://www.youtube.com/watch?v=f5BIvSBjvLg



Dec 05, 2012 at 05:37 PM
BrianO
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p.6 #7 · p.6 #7 · silly inverse square law question.


RustyBug wrote:
...Did Al Gore re-invent light when he invented the internet?


I know you're kidding, but just because it irks me so much when people say stuff like this seriously I'm going to climb up on my soap box:

Al Gore never claimed to have "invented" the Internet. He said in one speach that he was "instrumental in the creation of the Internet."

The Internet was an expansion of the DARPA Net, and Al Gore was one of the few in a position at the time to recognize the potential for an information superhighway and to act on it. He worked on putting together the funding, organization, etc. needed to move it from a military-oriented network to a public one.

The Worldwide Web, of course, was developed by CERN in Switzerland, but owes its broad success to riding on the Internet.



Dec 05, 2012 at 07:33 PM
RustyBug
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p.6 #8 · p.6 #8 · silly inverse square law question.


Brian, no worries. Glad you understood the "jest" that was in play.

But, it does raise an interesting point @ how "twisted" something can become when the complexity of it isn't fully known ... and how many people will believe the twist, even when it isn't accurate.

I'll readily acknowledge my knowledge regarding the history of the internet is mostly anecdotal from that which circulates (i.e. not credible) ... but I was under the impression that is was somehow connected to the education system and specifically a degree of credence to the U/I @ Champaign/Urbana as well. But, here again, that is only anecdotal.



Dec 05, 2012 at 08:06 PM
BrianO
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p.6 #9 · p.6 #9 · silly inverse square law question.


RustyBug wrote:
...I was under the impression that [the Internet] was somehow connected to the education system...


You are correct. The Defense Advanced Research Projects Agency (DARPA) used the network to connect researchers at various universities and other research sites in order to speed up information exchange.

The first node was at UCLA, and the second at the Stanford Research Institute. Other nodes followed in quick succession.

A brief history can be found here:

http://www.internetsociety.org/internet/what-internet/history-internet/brief-history-internet

As for Al Gore's role, you can read about it here:

http://en.wikipedia.org/wiki/Al_Gore_and_information_technology



Dec 05, 2012 at 09:03 PM
uscmatt99
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p.6 #10 · p.6 #10 · silly inverse square law question.


quicksilver33 wrote:
This has been a really interesting discussion to me since I used to consider myself a bit of a physics buff. I'm a little rusty now so although my understanding says reflected light must also fall off over distance - I understand that practically it doesn't appear to be so. Doing a little searching online I believe I found the answer as to why this occurs from an exposure perspective.

You can check the link below for full details but the answer seems to be that as you move the camera away from the subject you get light spreading as per.

Reference here: http://www.scantips.com/lights/flashbasics1b.html
...Show more

Awesome discussion guys! The quoted point is the salient one. Here's my take, in keeping with curious80. The behavior of light in the universe can't be different whether it's reflected or emitted from a point source. The size of an object (as seen by the sensor) decreases by the inverse square law as you move a sensor away from it. The amount of light reflected off a subject and reaching the sensor decreases by the inverse square law as well. The exposure of the subject stays the same. If you think in terms of photons, less total photons are reaching a sensor the farther you move away, but they are representing a smaller subject. The easiest argument for me to wrap my head around was the following: If all of the reflected light photons made it back to the sensor regardless of distance from the subject, then our subjects would get brighter as they got smaller, which simply isn't the case.



Dec 06, 2012 at 12:39 AM
 

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HelenB
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p.6 #11 · p.6 #11 · silly inverse square law question.


Reading this I feel really sorry for curious80 and find it difficult to believe that curious was given such a hard time on such a fundamental issue.


Dec 10, 2012 at 02:58 AM
BrianO
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p.6 #12 · p.6 #12 · silly inverse square law question.


HelenB wrote:
...curious was given such a hard time...


Hard time? This was a very polite debate/discussion, as Internet forums go, and pretty darned informative, too. If you think anyone here was "given a hard time" then I suggest you stay away from DPReview Forums and POTN; you'll feel like people have been murdered over there!



Dec 10, 2012 at 05:41 AM
HelenB
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p.6 #13 · p.6 #13 · silly inverse square law question.


Thank you for your sage advice Brian. It is always nice to discover that others have one's welfare at heart. I am, however, no stranger to discussions on the interwebs, including ones on this very subject (it gets asked fairly frequently by folks trying to understand the inverse square law). That is why I sympathize with curious. Curious gave a good answer, showing good understanding of the subject, then was told that the answer was wrong by people who did not seem to understand the basics.


Dec 10, 2012 at 11:51 AM
HelenB
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p.6 #14 · p.6 #14 · silly inverse square law question.


Because this subject is fundamentally important to photography it is covered very well in the standard textbooks like Born & Wolf and Ray. There is the formal proof, from first energy flow principles, that although the flux entering a lens from an object is inversely proportional to the square of the object distance, the image illuminance is independent of the object distance (assuming no loss by atmospheric absorption etc).


Dec 10, 2012 at 01:03 PM
Guari
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p.6 #15 · p.6 #15 · silly inverse square law question.


HelenB wrote:
Thank you for your sage advice Brian. It is always nice to discover that others have one's welfare at heart. I am, however, no stranger to discussions on the interwebs, including ones on this very subject (it gets asked fairly frequently by folks trying to understand the inverse square law). That is why I sympathize with curious. Curious gave a good answer, showing good understanding of the subject, then was told that the answer was wrong by people who did not seem to understand the basics.


This thread should die it's course. Disseminating flawed knowledge is bad stuff.

I am sorry but I am in your supposed group of people who do not know what they are talking about.

Google my name, it's on my facebook web page. I am a research scientist,, who did a degree in geophysical engineering, then I did a master and now I'm nailing a PhD. Physics is the second name of my career.

What curious said is flawed on so many levels. But it sounds "alright". Plausible. Well, it is not.

I'm sorry but this threadstill being alive is getting silly... I highly recommend reading the books instead of believing everything a person writes on the internet.

I don't claim to be the wisest or anything, because I'm not. But what curious speaks is flawed physics 101



Dec 10, 2012 at 01:10 PM
RustyBug
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p.6 #16 · p.6 #16 · silly inverse square law question.


Guari wrote:
Disseminating flawed knowledge is bad stuff.

But what curious speaks is flawed physics 101


+1
For photographers, get the book Light: Science & Magic. It does NOT violate physics 101.

The book's topic of "angles of inclusion" is one of the most "illuminating" aspects of the book that has significantly more practical value (even for those who are well versed in the physics of light) than ascribing to "plausible explanations" rooted in flawed physics.

I do appreciate curious80's desire to help his fellow FM'ers understand what he feels is the answer to such observations ... but unfortunately, this are no more correct than "the world is flat". Just because it seems plausible to believe through an offsetting explanation that others agree with doesn't make it true.

What curious80 is stating is somewhat akin to saying that the reason we see the photographs of earth from space as round is due to the optical aberrations induced by the refraction of light passing through the earth's atmosphere, but it is really still flat. For some people they would believe such, because they have simply have no way to really disprove it ... i.e. it strikes them as being a plausible way to explain that which they are otherwise unsure. Meanwhile, for those few individuals who have actually circumnavigated the world (which, btw, I have), they know beyond any plausible explanation that the earth is round, no matter how many people try to explain that it is flat, nor how many people believe such an explanation to validate it as flat. After all, look out your window and you'll clearly "see" the earth doesn't "look" round, therefore it must be flat.

Light: Science & Magic ... get the book.

That, or begin preparing for years of study @ vector forces, the Conservation of Energy, refractive indices, Planck's constant, etc., and be prepared to enter the world of Isaac Newton, Phillip von Jolly, Max Planck, Albert Einstein & others.



Dec 10, 2012 at 01:41 PM
HelenB
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p.6 #17 · p.6 #17 · silly inverse square law question.


Well, I have had LS&M for a few years, and I have read it. I also read physics at Oxford, so I do not have to begin years of study. I began that decades ago. I am just as happy to take on PhD-qualified people now as I was when I was at Oxford and encouraged to do so.


Dec 10, 2012 at 02:07 PM
Guari
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p.6 #18 · p.6 #18 · silly inverse square law question.


HelenB wrote:
Well, I have had LS&M for a few years, and I have read it. I also read physics at Oxford, so I do not have to begin years of study. I began that decades ago. I am just as happy to take on PhD-qualified people now as I was when I was at Oxford and encouraged to do so.


If you read physics at Oxford and you passed brilliantly, then you surely would understand what wrong physics is all about. If in doubt, just consult the bibles. It's all there.

Have a great day



Dec 10, 2012 at 02:09 PM
RustyBug
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p.6 #19 · p.6 #19 · silly inverse square law question.


Excellent ... then you should be fully cognizant of how ISL is the spherical dissemination pattern of energy in particle packets (i.e. photons) from a PLS and that a PLS / ISL is an even distribution pattern based upon diffusion / path of least resistance / until acted upon by another object. Photons are particles of energy. Photons have a mass and are matter (really tiny matter, but matter nonetheless). They follow the laws of Conservation of Energy. Those particles continue until they are acted upon by another object. The subsequent path & energy of the photons after interaction with another object continue iaw with AI=AR as predicated by the object it interacts with ... most notably, the angle of incidence, and index of refraction (relative to opaque, transluscent, transparent) and the energy absorption into the body, rendering the remaining amount of energy to be determinant in the color of the light reflected.

Is there anything there that is objectionable to your understanding of physics of light as taught by Oxford?

http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/first2nd_lawsf_motion.html
Newton's First Law of Motion states that a body at rest will remain at rest unless an outside force acts on it, and a body in motion at a constant velocity will remain in motion in a straight line unless acted upon by an outside force.

Light travels at a constant speed and direction (definition of velocity) in a given medium (i.e. air, vs. water, vs. vacuum, etc.) IAW Newtons' First Law of Motion. In other words, it doesn't slow down or change direction, except when acted upon by another object (outside force). When a photon comes into contact with an object, the interaction / exchange of energy properties, (i.e. color, direction, etc.) occurs. Then, when the photon continues on its new path, it will continue to again travel iaw with Newton's First Law of Motion ... which is STRAIGHT LINE iaw with AI=AR.





Edited on Dec 10, 2012 at 02:57 PM · View previous versions



Dec 10, 2012 at 02:23 PM
HelenB
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p.6 #20 · p.6 #20 · silly inverse square law question.


There is nothing there that I disagree with. It does not, however, disprove what curious explained about constant image illuminance.


Dec 10, 2012 at 02:49 PM
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