Light Ratios

Discussion in 'Beyond the Basics' started by DeadEye, Sep 11, 2008.

  1. DeadEye

    DeadEye TPF Supporters Supporting Member

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    Understanding how F stops translate into numbers so as to clearly communicate a scenario. ~

    Just not fully understanding how to do this after studying the light ratios chapter of "Master Lighting Guide"

    If each F stop increase doubles the light reaching the film plane then why a change from f11 to f8 only allows twice the light increase.

    I am truly confused Please explain to me as I am feeling idiotic.:wink:
     
  2. Helen B

    Helen B TPF Noob!

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    There is a one-stop difference between f/11 and f/8. Is that the answer?

    These are the whole-stop intervals, starting at the lowest possible f-number:

    f/0.5, f/0.7, f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45 etc.

    Best,
    Helen
     
  3. Moglex

    Moglex TPF Noob!

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    It's not clear what you are actually objecting to here.

    Each one stop increase in aperture allows twice the light to reach the film.

    So the increase in aperture from f11 to f8 allows twice the light to reach the film.

    Why do you say 'only'?

    (And the word 'increase' in "twice the light increase" is at best superfluous and at worst incorrect and confusing.)

    Can you elaborate a little on what you are actually finding confusing?
     
  4. DeadEye

    DeadEye TPF Supporters Supporting Member

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    Thank you Helen. I was confusing f stop number with the full stop interval. All good now.:thumbup:
     
  5. DeadEye

    DeadEye TPF Supporters Supporting Member

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    I was wrong to think a change from f8 to f9 was a stop.
     
  6. Moglex

    Moglex TPF Noob!

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    If you are wondering why the f-number only doubles for a fourfold increase in light this is because the f-stop is related to a linear measure (diameter) whereas the amount of light passed depends on the area of the opening.

    The f-numbers are actually spaced by a factor of the square root of 2 (but the jump from 1 to 1.4 was rounded). Thus the number doubles every two stops.
     
  7. Moglex

    Moglex TPF Noob!

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    OK, I see where the problem lay now. Glad you've got it sorted out.
     
  8. DeadEye

    DeadEye TPF Supporters Supporting Member

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    Thanks lots.. Now back to studying. :mrgreen:
     
  9. Helen B

    Helen B TPF Noob!

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    I guessed that it was something simple like that. The increase-of-one-equals-one-stop relationship does apply to 'aperture value' (Av or AV) however. A change from an Av of 8 to an Av of 9 is one stop, as is a change from Av 2 to Av 3. That is all based on f/1 being Av 0, f/1.4 being Av 1 etc.

    While you are studying lighting ratios, is it worth mentioning that there are two slightly different ways of stating lighting ratio? Main:main+fill and main:fill? Each have their own logic. The existence of the two different methods sometimes causes confusion.

    Best,
    Helen
     
  10. JerryPH

    JerryPH No longer a newbie, moving up!

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    And that ratio can be played with even more based on how many lights you have. It doesn't stop with 1, 2 or 3 lights... it goes as far as many light sources you have... that includes the sun and reflective/refractive sources as well. ;)
     
  11. fightheheathens

    fightheheathens TPF Noob!

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    the actual F stop number is a ratio.
    the f-number is the focal length divided by the effective aperture diameter


    so on a 50mm lens, F11 has a aperture diameter of 4.5mm, (50mm/F11 = 4.5mm) where as F 8 has a aperture diameter of 6.25. if you convert those into area...(insert math here) F11 has an aproximate area of 63mm^2 and F8 has an area of 122mm^2 which is about twice the area of F11.

    so thats where the numbers come from and what they mean
     
  12. William Petruzzo

    William Petruzzo TPF Noob!

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