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GN Number calculation is for manual flash, right?

StephenStephen Member
edited September 2016 in flash & lighting
I was reading Neil's "On-Camera Flash" 2nd edition.  In the chapter on the GN Number, an equation is presented to help find the distance for correct flash exposure.

GN Number = f-stop x distance

I presume this is for manual flash only, since that is where distance is actually a controlling factor in the flash exposure, correct?  (Power of Flash, Aperture, ISO, Distance between flash and subject)  The text in that section doesn't state explicitly "manual flash."


  • Hi Stephen,

    You are correct. This is for manual flash. The TTL flash with follow your camera settings and fluctuate from shot to shot and thus the output will vary within those settings. 

  • Neil vNNeil vN Administrator
    The Guide Number of your flash is the indication of how powerful your flash is ... and it is *this* powerful regardless of whether you use TTL or manual flash. 

    However, in using the GN to determine distance / aperture, then it is for manual flash only. 
  • Thank you both for clarifying this.
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