Am I calculating the Inverse Square Law properly?
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  1. #1
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    Default Am I calculating the Inverse Square Law properly?

    I know the law - that moving the light twice the distance makes the light intensity 1/4 as strong as it was originally - but am I doing the math right here?

    So, if my flash is 5' from my subject, and my subject is properly exposed at ISO 100, f/8 at 1/125, if I move my flash to 10' from the subject, I would need to adjust my exposure by two stops, so it would be come ISO 100, f/4 at 1/125?

    Is that right? And conversely, if I move the light from 5' away to 2.5' away, my exposure would become ISO 100, f/16 at 1/125?

    Or am I totally messing this up? Oh, and by the way... please tell me there aren't 100 "calculate this" questions on the CPP exam! Please???

  2. #2
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    oh gosh, please tell me too because that's math I don't like to figure out!

  3. #3
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    I don't know about the exam question but you are correct about how it works in practice.

    There is an easier way to do it - put your light at distances you already know (2', 2.8', 4', 5.6', 8', 11', 16', etc.) and you will be working in 1 stop increments instead of 2 stop increments.

    If your light is 8 feet from your subject, move it to 5.6 feet from your subject and gain 1 stop (instead of 2 stops by halving the distance.) If you light is 4 feet from your subject move it to 5.6 feet from the subject and lose 1 stop of light.
    Yes, Hassel is my real name.
    Hassel Weems Photography
    www.hasselweems.com
    www.hasselweemsweddings.com

  4. #4
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    Ladies - there ARE math questions on the test... not 100 of them, but a handful. Knowing how your settings change when things move around will be important.

  5. #5
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    Right on the money, Anne.
    Mark Levesque, CPP, M. Photog., Cr. Photog, A.C. Ph., CPP Liaison, PPCC Judge

  6. #6
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    You need to be intensely intimate and comfortable with the relationships between ISOs, apertures, and shutter speeds. If they give you questions like "which of the following exposures is not equivalent to the others?" it should not worry you even if they throw an f/45 option at you.
    --Elephants can swim...
    ...and very gracefully.
    Knowing that,
    I do believe
    Anything is possible for me.

    Kirk Darling, CPP

  7. #7
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    Love the new avatar, Kirk. Love it even better bigger. link?
    Mark Levesque, CPP, M. Photog., Cr. Photog, A.C. Ph., CPP Liaison, PPCC Judge

  8. #8
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    Feb 2006
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    Default

    Thanks guys! I am glad I'm calculating that right. I am expecting "math" questions about the exposure triangle, about filter factors and how they affect exposure, about lighting ratios and adjusting them with fill light, and about the inverse square law. Is there more "math" beyond that, and did anyone's test deal with any of that in half or third stops, or was it all in whole stops? If it's going to be all in whole stops, I think I'm going to be fine.

    My bigger problem is that I still have four chapters to go in the book - The cyanotype/platinum printing chapter (which I fell asleep reading last night), the view camera chapter, the composition chapter and the history chapter.
    So, if you see me online (anywhere, not just here ), feel free to tell me to get off the computer and get back to the books!

  9. #9
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    Aug 2005
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    Default

    Anne,

    Get back to the books!
    Regards,
    Howard Kier, CPP Weddings, ABI, MBA and DAD
    Magical Moments Photography

  10. #10
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    Feb 2006
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    Thanks Howard! I am off right now to do just that!

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