Calculating Guide Numbers

Discussion in 'Beyond the Basics' started by selmerdave, Feb 11, 2005.

  1. selmerdave

    selmerdave TPF Noob!

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    Can anyone tell me how to calculate flash guide numbers for film speeds other than those provided in the table in the manual? My Pentax 360FGZ gives a table for ISO 100 and 400 film, but I use mostly 50, 64 and 160 film. Of course a lot of the time I use auto flash or TTL depending on the camera, but I would like to be able to use it manually as well. Thanks.

    Dave
     
  2. selmerdave

    selmerdave TPF Noob!

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    Okay, I figured it out. Going up by one film speed (ie. 100 to 125) you multiply the guide number by roughly 1.12 (the sixth root of two), going down you divide. Double the film speed (ie. 100 to 200) is by roughly 1.41 (square root of two).

    Dave
     
  3. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Sorry I'm a bit slow. I missed this post.
    The actual calculation for changing film speeds is:

    New Guide No.= Old Guide No. x the sq root of (new film speed/old film speed)

    [that is New Film Speed divided by Old Film Speed]

    For manual use the f-number you use is found by dividing the guide number by the flash to subject distance.
    There are, however, lots of variables that can affect the outcome so it is best to shoot a test roll to determine the actual values. Set the camera and flash up 10 feet from a target image. Shoot the roll off bracketing at 1/2 stop intervals going from widest to smallest aperture. Check the results and select the exposure that gives the best result. Multiply that f-number by 10 to get the guide number.

    But it is easier to use a flash meter.
     

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