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F number and light intensity

  1. Jun 6, 2010 #1
    1. The problem statement, all variables and given/known data
    using the same film and level of illumination two pictures are taken, one with double the f number of the other. for both films to recieve the same total light energy, what factor must the required time of exposure change for the picture with double the f number.

    2. Relevant equations

    f number = focal length/diameter

    3. The attempt at a solution
    double the f number couldbe taken as meaning half the diamter. therefore i can guess the answer should be a factor of two. but the f number formula doesn't relate 'total light energy' to anything does it? i thought it was a rating of light intensity.??
  2. jcsd
  3. Jun 6, 2010 #2


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    Hi Lachlan1! :wink:
    If these are with the same camera (it doesn't say), then the focal length is presumably the same, so the diameter is different.

    Bigger lens … more light! :smile:
  4. Jun 6, 2010 #3
    Hi, thanks for that. But the question is asking what is the difference in the time of exposure. do you know what is being referred to as light energy?
    -- well i just found out the answer is four, but im not sure how to get that?
    Last edited: Jun 6, 2010
  5. Jun 7, 2010 #4


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    Staff: Mentor

    Amount of light is directly proportional to the area through which light comes into the camera.
  6. Jun 7, 2010 #5


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    (just got up :zzz: …)
    light energy is just another name for the amount of light :smile:
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