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Thread: Flash Question

  1. #1

    Talking Flash Question

    Hi, when u look at the specs of a flash, they give a gn ( guide number ? )
    or something that is indicative of its power... can someone explain how all this gn stuff works out with the aperture and iso, as i see it often mentioned but cannot make head or tail
    Thnx

  2. #2
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    checkout the explanation from shortcourses:
    http://www.shortcourses.com/how/guid...idenumbers.htm

  3. #3

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    In short, the Guide Number (GN) is the distance it can cover. It's usually judged against ISO 100 film...e.g. a GN of 50m means a subject can be illuminated correctly by the flash at a distance of 50 metres using ISO 100 film. Flash units with zoom heads will have different GNs at different zoom settings.

    Nevertheless, a GN is only a rough guide and is only used as a relative comparision of the different flashes' output strength.

  4. #4

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    ah! i get it, thnx alot guys!

  5. #5
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    Quote Originally Posted by Ah Pao
    In short, the Guide Number (GN) is the distance it can cover. It's usually judged against ISO 100 film...e.g. a GN of 50m means a subject can be illuminated correctly by the flash at a distance of 50 metres using ISO 100 film. Flash units with zoom heads will have different GNs at different zoom settings.
    There is one more factor: lens aperture. It is only 50m if you use a lens at f1.0. If you use f2.0, the distance is 25m.

  6. #6

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    Quote Originally Posted by Ah Pao
    a GN of 50m means a subject can be illuminated correctly by the flash at a distance of 50 metres using ISO 100 film. Flash units with zoom heads will have different GNs at different zoom settings.
    GN = f/stop x Flash-to-subject distance

    huh?!? from the shortcourses guide... should'nt it be with a GN of 50m, depends on ur iso and aperture...
    say if iso at 100 and aperture is f2 den its....
    the flash shold have correct exposure at 25m
    but if at iso 400 and aperture f2 den....
    flash should work till 50m
    if iso 1600 and f2 den work till 100m
    so at f20 and at iso 100 flash coan work till about 2.5 metres

    Right?

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