Calculate exposure time

Discussion in 'Beyond the Basics' started by taurus, Sep 3, 2007.

  1. taurus

    taurus TPF Noob!

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    I have got to write a program to output exposure time from ISO and Aperture settings. Is there a certain calculation to calculate the Exposure Time after the user has entered the ISO and Aperarure values??
    If so how?

    Thanks
    phil
     
  2. abraxas

    abraxas No longer a newbie, moving up!

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  3. selmerdave

    selmerdave TPF Noob!

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    You'll need shutter speed as well, unless that was intended to be the output of your program. Really, it's all related so once you can calculate one value you can calculate all the others as one "stop" of difference is the same as doubling ISO, doubling shutter speed or moving aperature one step (with 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32 being the standard values - mulitply each by root 2 to get the next). Just be careful of the direction, more light means more exposure. Can't tell you how to calculate that first value, not sure how you would express the light level.

    Dave
     
  4. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Shutter speed, aperture and ISO are all Logarithmic series.
    In each moving up one or down one is halving or doubling the exposure.
    f5.6 admits twice the amount of light into the lens as f8.
    f11 admits half as much light as f8.
    ISO50 is twice as sensitive to light as ISO25 but only half as sensitive as ISO100.
    And so on.
    So if you have a fixed value (ISO) the aperture and shutter speed are reciprocal - that is, if you increase one you decrease the other by the same amount.
    ISO change will affect the overall (total) exposure thus:
    ISO100: 1/30@f8 = 1/60@f5.6 = 1/15@f11
    And these all give the same exposure as:
    ISO200 1/60@f8 = 1/125@f5.6
    Because of this relationship between aperture and shutter speed EV or Exposure Values are often used. All the above exposures for ISO100 being identical could be expressed by the use of just one EV number.
    EV0 is equivalent to 1s@f1 for any ISO.
     
  5. castrol

    castrol TPF Noob!

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    Aaaand kablewy...there went my head.
     
  6. Garbz

    Garbz No longer a newbie, moving up!

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    Search for Exposure Value on Wikipedia :)
     
  7. taurus

    taurus TPF Noob!

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    hmm yea makes a bit more sense. thanks.
    and wikipedia did the trick as well, give the formula for it:

    EV = log((N^2)/T)

    WHERE N = aperture and T=Exposure time.
    But one question. is ISO the same as shutter speed (exposure time)???

    Does that look like it?'


    haha and if u think ur head went boom, well i have to program this as well :confused:
     
  8. selmerdave

    selmerdave TPF Noob!

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    No, ISO is the sensitivity of the film or sensor. ISO 400 needs one quarter the light that ISO 100 needs for the same exposure. That difference could be made by a shutter speed 1/4 as long or an aperature two stops smaller (for ISO 400).

    Dave
     
  9. RacePhoto

    RacePhoto TPF Noob!

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    Also, check out the web for the inverse square law which helps explain why you need twice as much to get half the gain.

    I look at ISO as somewhat of a constant. Probably back from film days, when what you had in the camera was "IT", or carry two bodies.

    Generally when someone is shooting something, there isn't a need to be changing ISO all the time. If you pick an appropriate ISO, they you would only need to deal with two variables. Speed and aperture.

    When you change the ISO, the whole scale just slides up or down on one portion. The relationships between shutter speed and f-stop, stay the same.
     
  10. taurus

    taurus TPF Noob!

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    so like the formula above, theres nothing like that for calcualtions exposure time?

    If not do u have a general algorithm (procedure) to get the exposure time after ISO and aperture have been presented?

    thanks
     
  11. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Oh, come on people. It's easily worked out from the above information.

    Exposure = Illuminance x Duration

    Or to put it into a proper formula using the standard symbols:

    H = E x t

    The unit of illuminance is the lux.
    Time is in seconds.
    Exposure is therefore given in lux seconds.
    If you look up manufacturers data guides the sensitometric data is always expressed in lux seconds. But if you Google you should find a conversion formula.

    *EDIT*
    I just picked up my Weston light meter and the dial does all the calculations for you. We just need to convert what it does into a program and we are there :lol:
     
  12. Hertz van Rental

    Hertz van Rental TPF Noob!

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    I've just found the manual for my Sekonic meter.
    It has a 'Lumidisc' attachment that turns it into a Lux meter - with a conversion table for Lux to EV.
     

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