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Thread: Flash exposure

  1. #1

    Default Flash exposure

    Hi all,
    I'm using a 100% manual camera. I have a flash of GN24, it has fixed flash output too. There is a exposure table telling me what aperture to use when using ISO, say 100, when my subject is, is 2m away.
    Frenz told me that i can actually calculate the aperture by using formula below:
    GN/Distance from subject= aperture

    would anyone like to comment on either methods.
    Can anyone teach me a better way of setting the correct aperture.

  2. #2

    Default

    errr... come on leh, give some comments, so many ppl viewed but no one wanna give me so advice, pls give me some ideas leh

  3. #3

    Default

    Maybe coz the way u described is the best way already?
    at ISO100, GN/dist from subject = aperature.

  4. #4
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    both are the same. also, for every doubling of ISO, multiply the guide number by 1.4x if you are manually calculating.
    Last edited by mpenza; 24th January 2004 at 12:03 PM.

  5. #5

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    you already have the answer with the formula

  6. #6

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    Quote Originally Posted by mpenza
    both are the same. also, for every doubling of ISO, multiply the gain number by 1.4x.
    U sure 1.4x ?
    unless u talking about aperature.

  7. #7
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    Quote Originally Posted by AReality
    U sure 1.4x ?
    unless u talking about aperature.
    yup, I'm sure. not talking about aperture, but the GN.

  8. #8

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    Quote Originally Posted by mpenza
    yup, I'm sure. not talking about aperture, but the GN.
    u double sure?
    Any webby to refer to?
    Coz I always tot is x2.

  9. #9

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    Quote Originally Posted by AReality
    Maybe coz the way u described is the best way already?
    at ISO100, GN/dist from subject = aperature.
    take note, the GN is extended when your iso is increased, i think its doubled with evey stop increase in iso.

    also, check whether the GN is described in feet or meters...yours i think should be in meters. then your dist to subject should be in meters.

    check at what focal length your GN is rated at. a wider focal length than the rated focal length used will imply a smaller GN and coversely a longer focal length (than the rated focal length) used implies a larger GN available.

  10. #10

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    i think mpenza may be rite abt the doubling of iso with GN X 1.4.....go check on the web.......

    go read abt the inverse square law too....i.e. every doubling of subject distance reduces the rated flash output to 1/4 its power....

  11. #11

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    Quote Originally Posted by mpenza
    both are the same. also, for every doubling of ISO, multiply the guide number by 1.4x if you are manually calculating.
    Ok ok... got it liao...
    aperature x1.4, so the LHS of the eqn also x1.4

  12. #12
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    yup, doubly sure. you could also check the maunal of your flash and do the calculations if it's not explicitly stated.

    here's one article:
    http://www.jafaphotography.com/flashex.htm

    GN of the new film= GN of known film x the square root of the known ISO/new ISO.

    (A bit slow cos was searching for articles)
    Last edited by mpenza; 24th January 2004 at 12:31 PM.

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