Do incident light meters assume camera dist.?

Discussion in 'Beyond the Basics' started by geo in lb, Mar 18, 2006.

  1. geo in lb

    geo in lb TPF Noob!

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    I understand the practical benefits of using an incident light meter. (have a Sekonic L-308 use it and like it. FYI - Sekonic has striking illustrative/comparative shots at http://www.sekonic.com/BenefitsOfIncident.html.)

    What I do not get is how any incident meter can theoretically work unless there is some unspoken assumption about how far away the camera is from the subject. By the square law of light intensity fall-off moving a light twice as far away from a subject reduces the light reaching the subject by a factor of 4 (2 stops). Similarly, moving the camera away from the subject has to reduce the amount of reflected light from the subject which reaches the camera. A reflective meter in or at the camera location can know how much light is getting to the camera, an incident meter doesnÂ’t know if the camera is inches, feet or yards away! What am I missing?


    thanks

    ---george
     
  2. Azuth

    Azuth TPF Noob!

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  3. geo in lb

    geo in lb TPF Noob!

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    thanks - that site was responding to essentially the same question but to me it did so by analogy and by stating it really works so don't worry about it. So I'm still in the dark.
     
  4. zedin

    zedin TPF Supporters Supporting Member

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    If the entire area is in the same light (ie what you measured with the incident light meter) it shouldn't matter how far you get. Remember that as you move back while the light from a given area might be decreasing your field of view is increasing thus bring back your light level.
     
  5. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Your confusion comes from a confusion as to how light works.
    There are four measures of light levels and although they are connected they work in different ways.
    1. The first is Luminous Intensity
    This measures the total all-round light output of a source (it's power, if you like) and is measured in Candelas.
    2. The second is Luminous Flux.
    This is the amount of light flow (brightness or intensity) from the light source. It is measured in Lumens.
    3. The third is Illuminance.
    This is the amount of incident light - that is, the amount of light falling on a surface - and is measured in Lux (Lumens per square meter).
    4. The fourth is Luminance.
    This is the amount of light emitted from a source or reflected from a surface and is measured in Candelas per square metre.

    The Inverse Square Law affects (2) the Luminous flux.
    The amount of light flow from a source remains constant so at a given distance the amount of light remains constant. If you move away from the light by twice the distance then the volume that the Luminous flux has to fill is 4 times the original volume, and so the Luminous flux at the second distance is 1/4 of the original (the same amount of light is having to fill 4 times the original cubic area).
    That means that moving the light source twice the distance from the subject reduces the amount of light reaching it by 1/4.
    Thus the illuminance is reduced by 1/4.
    With me so far?
    Whatever the distance you move from the subject with your camera, the amount of light reaching the subject from the light source remains the same (providing you don't move the subject or the light).
    Incident light meters measure the illuminance (3). If the light/subject distance remains the same then the amount of illuminance remains the same whether you stand 5 inches, 5 feet or 5 miles from the subject.
    Of course, the Inverse Square Law will affect the Luminance, that is the light reflected from the subject, but not as much as you would think. In practical terms you won't be able to get far enough away from the subject under normal circumstances for this to become a problem.

    I appreciate that getting your head around all this is quite difficult - even I have a job with it sometimes. So the easiest way - and the one I always advise students to follow - is to look upon light and it's behaviour as if it is magic. The more you find out about light and the way it behaves the more you start to think this is true :lol:


    (I won't even begin to try and explain why, when talking about light generation, you treat the individual light rays as diverging but when you talk about optics you treat them as travelling parallel....)
     
  6. DestinDave

    DestinDave Master of Non Sequitur

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    This is commonly referred to as the FM principle...

    F...ing Magic!
    :lmao::lmao::lmao:
     
  7. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    The inverse square law for distibution of light is compensated by another square law:

    The farther away the camera is from the object, the smaller the area of the image forming on film/sensor/retina.

    both of them are square relationships and they cancel each other out.
     
  8. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Or to put it another way, in light generation the 'rays' are treated as being divergent. In optics the 'rays' are treated as parallel ;)
     
  9. DocFrankenstein

    DocFrankenstein Clinically Insane?

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    I dont' get it. Optics vs light generation?
     
  10. selmerdave

    selmerdave TPF Noob!

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    With one you're talking about emitting light, with the other you're talking about receiving light.

    Dave
     
  11. Hertz van Rental

    Hertz van Rental TPF Noob!

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    The way light is deemed to behave when you are talking Quantum Physics (light generation) differs from the way light is deemed to behave when you are talking Optical Theory.
    It's a bit like Einstein's Theory of General Relativity and Newtonian Mechanics.
    The latter in both cases explains what happens adequately for local conditions but fall down when you look at the whole picture. That's when the former both come in to explain why it happens.
    Quantum Physics treats light as particles (Photons).
    Optics treats it like 'rays'.
    Clearer?

    *edit* Nicely put SD.
     

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