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Thread: Guide Numbers

  1. #1

    Default Guide Numbers

    hi ... i know tat flash has guide numbers ... but i dunno wat is it acutally .. only knows it is related to distance is it ?? ~~

  2. #2

    Talking

    u can uze it to calculate the apperture that u muzt set on ur cam when u uze the flazh .........

    guide number divided by subject diztance = f-stop number

    plz correct me if me am wrong .........


  3. #3

    Default

    guide number can also be use to compare the power of the flashes... a higher guide number means more powerful lah... but be careful though some guide numbers are in feet while some are in meters

  4. #4

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    Usually the GN is also specified at a particular angle of coverage. This is mostly done by stating the 35mm focal length the specified GN specified for. For example, GN 36 at 35mm.

    So when comparing GN, take into consideration the focal length specified for. the shorter the focal length the more powerful the flash, given the same GN.

    chgoh

  5. #5
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    Default

    do note that some manufacturers use feet (e.g. Vivitar 285HV has a GN of 120 feet) while some use meters (e.g. Metz 32Z1 has max GN of 37m at 85mm zoom).
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    Default Re: Guide Numbers

    Originally posted by West_ray
    hi ... i know tat flash has guide numbers ... but i dunno wat is it acutally .. only knows it is related to distance is it ??
    A flash's guide number is a number used to describe the flash units maximum operating range when referenced to a given lens and film speed.

    example: Guide Number 56m at 105mm focal length f1.4 at ISO 100.

    What this means is that with a 105mm lens at f1.4 the flash will produce correct illumination at a range of 56m using 100 ISO rated film.

    Once you know the GN and it's parameters you can then caclulate several variables, the most common ones being aperture and effective range for an aperture.

    Aperture to use:

    GN / Subject distance. eg: GN 56m and subject distance of 12m = 56/12 = f4.6

    Aperture to use for a given distance:

    GN / Distance

    eg: GN 56m distance 10m. Aperture = 56/10 = 5.6

    Note: This assumes a lens that is eqivalent to the lens stipulated in the calculation paramaters of the GN.
    The Ang Moh from Hell
    Professional Photography - many are called, few are chosen!

  7. #7

    Default Re: Re: Guide Numbers

    Originally posted by Ian


    A flash's guide number is a number used to describe the flash units maximum operating range when referenced to a given lens and film speed.

    example: Guide Number 56m at 105mm focal length f1.4 at ISO 100.

    What this means is that with a 105mm lens at f1.4 the flash will produce correct illumination at a range of 56m using 100 ISO rated film.

    Once you know the GN and it's parameters you can then caclulate several variables, the most common ones being aperture and effective range for an aperture.

    Aperture to use:

    GN / Subject distance. eg: GN 56m and subject distance of 12m = 56/12 = f4.6

    Aperture to use for a given distance:

    GN / Distance

    eg: GN 56m distance 10m. Aperture = 56/10 = 5.6

    Note: This assumes a lens that is eqivalent to the lens stipulated in the calculation paramaters of the GN.
    wow ~!! sounds easy but confusing to do though ~~

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    Default Re: Re: Re: Guide Numbers

    Originally posted by West_ray


    wow ~!! sounds easy but confusing to do though ~~
    It's not confusing:

    Aperture to use:

    GN / subject distance.

    Distance to subject to be at for a given aperture:

    GN / Aperture.

    There's a lot more you can do with the GN though such as re-rating for different film speeds, multiple flash unit combined GN to calculate correct aperture/ distance when using multiple flash units etc.
    The Ang Moh from Hell
    Professional Photography - many are called, few are chosen!

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    Originally posted by mpenza
    hi jasonpgc, doubling of ISO actually results in 1.4x change in GN. So at ISO 400, the GN will be higher by 2 times instead of 4 times.
    Thanks for the correction,

    The Table should be

    GN(ISO 200) = 1.4 X GN(ISO 100)
    GN(ISO 400) = 2 X GN(ISO 100)
    GN(ISO 800) = 2.8 X GN(ISO 100)
    GN(ISO1600)= 4 X GN(ISO 100)


    Not to forget, most GN are reference to ISO 100 and can be measured either in meters or ft. So if you're using ISO 400 film. Then the True GN is (GN @ ISO 100)X2.

    Take for example,

    Your newbie SLR has a built in flash of GN 12(m) at ISO 100.

    You load a ISO 400 Fuji superior film. Therefore the True GN = 12(m) * 2 =24(m)

    So you plan to use your 28-80mm len @ f5.6 for a portrait in auto mode.

    To find out the area where the flash would be effective, take the GN and divide by the aperture used.

    Effective Range(ISO 400, GN 12)=24/5.6=4.28 meters.

    So as long as your subject stays inside 4.3 meters, the TTL flash will be able to give sufficent exposure.

    Last edited by jasonpgc; 3rd March 2003 at 07:03 PM.

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    hi jasonpgc, doubling of ISO actually results in 1.4x change in GN. So at ISO 400, the GN will be higher by 2 times instead of 4 times.
    Check out my wildlife pics at www.instagram.com/conrad_nature

  11. #11
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    Originally posted by mpenza
    hi jasonpgc, doubling of ISO actually results in 1.4x change in GN. So at ISO 400, the GN will be higher by 2 times instead of 4 times.
    opps, I forget the teleconvertor rule, 1.4X increase in focal length lost 1 stop light, so 2X increase in focal length lost 2 stop light

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