The Inverse Square Law

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Re: The Inverse Square Law

Postby Dexter St. Clair » Tue Sep 15, 2009 10:22 pm

Lucky Poet wrote:I didn't notice you'd posted to be honest - 'twas mainly a reply to hungryjoe :oops:

*Edit after reading the whole thread again* I'm definitely confused now and give up for the evening. Where's Ansel Adams when we need him?


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Re: The Inverse Square Law

Postby hungryjoe » Tue Sep 15, 2009 11:13 pm

That's a wee contradition there - the light reflected from your subject is the light reaching you. Your camera circuitry absolutely measures the light coming your way, not the light hitting your subject, and if you use your handheld meter to measure reflected light you'll certainly see it drop off as you step back from the model towards your camera.

First, it's highly unlikely that the light reflected by your subject, qualifies your subject as a point source of light, so the Law Of Inverse Square doesn't come in to it.
My camera's metering circuitry measures the light reflected by my subject. In a comparison, the subject would have to be constant, which was why I mentioned zooming in on your subject as you move back - to keep the subject filling the frame. Even if you move back a considerable distance and use a spot meter, if your subject is constant, for argument, say 18% reflectance, you are going to get the same reading.
Using a hand held incident meter, you use that as close to your subject to your subject as possible. Using it anywhere away from the subject, unless illumination is constant and without shadow, is a waste of time.



I'm trying desperately to get the idea of the light source and subject being separate things. Not that I'm doubting it, mind. Um... (Then again, I don't shoot in studios :) )

If Edinburgh Castle is your subject, then, at least in daylight hours, the sky is your light source. ::):
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Re: The Inverse Square Law

Postby scottwramsay » Wed Sep 16, 2009 12:33 am

We agree here again:

hungryjoe wrote:My camera's metering circuitry measures the light reflected by my subject. In a comparison, the subject would have to be constant, which was why I mentioned zooming in on your subject as you move back - to keep the subject filling the frame. Even if you move back a considerable distance and use a spot meter, if your subject is constant, for argument, say 18% reflectance, you are going to get the same reading.


That's because, as I've finally realised, when you half-press your shutter button to get a meter reading your physical aperture size will be larger when you've stood back and extended the focal length of the lens :). And yes, your camera's metering circuitry measures light reflected by your subject, all I'm saying is that as far as your camera's concerned there is no other light source since all it can see is your subject, no flash, no sun, no etc. To your camera, the subject is the source of light, and it measures the light emanating from it (replace with whatever term you like - coming, reflecting, etc, it makes no difference). And I know that you use a hand-held meter as close to your subject as possible, I wasn't suggesting otherwise in a practical situation.

And for the third time:

hungryjoe wrote:First, it's highly unlikely that the light reflected by your subject, qualifies your subject as a point source of light, so the Law Of Inverse Square doesn't come in to it.


I've already accepted that over studio distances it doesn't apply! 50 feet man, 50 feet I said! :D
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Re: The Inverse Square Law

Postby Fossil » Wed Sep 16, 2009 9:32 pm

has this got something to do with set squares?
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Re: The Inverse Square Law

Postby bobrobert » Thu Sep 17, 2009 10:31 am

Quote

Had a chat with another physics PhD student who misunderstood how apertures work, but his wording reminded me that aperture size is expressed as a ratio of the hole-size to lens-length, not an absolute size. This means it's changes as you zoom. There IS less light hitting the front of the lens as you move back from your subject,

Unquote

Nope ...... You are correct when you say that the aperture gets smaller as you zoom in, which means that it will get larger as you zoom out but when the aperture size changes the shutter speed changes which means that the amount of light is constant If you halve the aperture from f5.6 to f8 then the shutter speed will double. Basic exposure theory is that the amount of light is constant.

Look at this link

http://photo.net/casual-conversations-forum/00Kp9A
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Re: The Inverse Square Law

Postby scottwramsay » Thu Sep 17, 2009 11:01 am

Nope ......Basic exposure theory is that the amount of light is constant.


Hi Bob,

I'm talking about the inverse square law, which is all about the fact that as you move back from something the light coming off of it gets dimmer. It's a standard physical fact - imagine shining a torch on a wall and drawing around the circle of illumination. As you step backwards the light beam gets a chance to spread out more before it hits the wall, so the circle gets larger and fainter. This is what I mean when I say less light hits the lens - the lens is equivalent to that circle, and the light beam is equivalent to everything reflecting from the subject (or coming from the original light producer, if you're shooting towards a lamp or other source).

If that still doesn't convince you, think about how the sun looks in artists' impressions of how it would look to stand on Pluto. Or how a street light doesn't still light up your path as brightly as you walk away from it, even though you can still look up and see the element glowing inside. And if you automatically say "But that's because it's smaller from that distance!" well, that's the Father-Ted-and-the-cows-phenomenon, and it still applies as you step back from a model, it's just less pronounced.

This part has never been in question - I completely understand the chart you linked to, but that's about keeping your exposure constant after you change either aperture or shutter speed - what I'm discussing is why it's possible to keep the settings the same after you move to a position where there's just less light reaching you. And it all comes down to the way we measure apertures as a ratio of focal length (length divided by aperture size).
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Re: The Inverse Square Law

Postby scottwramsay » Thu Sep 17, 2009 11:07 am

Fossil wrote:has this got something to do with set squares?


They're just as much a mystery to me! Is that the triangle thing from school?
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Re: The Inverse Square Law

Postby Fossil » Thu Sep 17, 2009 7:27 pm

scottwramsay wrote:
Fossil wrote:has this got something to do with set squares?


They're just as much a mystery to me! Is that the triangle thing from school?


aye. tec drawing I think.
cheers anyhoo
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Re: The Inverse Square Law

Postby scottwramsay » Thu Sep 17, 2009 10:29 pm

Slide rules were the first thing that came into my head. An inverse set square rule? *ponders*
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Re: The Inverse Square Law

Postby barry » Sun Nov 01, 2009 1:04 am

What tutor did you have?
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Re: The Inverse Square Law

Postby scottwramsay » Thu Nov 05, 2009 9:43 pm

Jim. Are you on the course?
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Re: The Inverse Square Law

Postby weedrunkglasgowman » Thu Dec 10, 2009 8:40 pm

50mm for me for portraiture......works fine.
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Re: The Inverse Square Law

Postby barry » Tue Dec 15, 2009 5:26 pm

scottwramsay wrote:Jim. Are you on the course?


Aye Jim's a good guy. I'm on a full time course at the moment.
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Re: The Inverse Square Law

Postby scottwramsay » Fri Dec 18, 2009 3:33 pm

barry wrote:Aye Jim's a good guy. I'm on a full time course at the moment.


Yeah, we all liked him.
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