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Fill-in flash

Discussion in 'Help Team' started by mark_m, Apr 1, 2001.

  1. mark_m

    mark_m Member


    Bit of a basic one, this -- how do you calculate exposure for fill-in flash (assuming non-TTL flash camera, sync speeds not an issue, etc)? How do I calculate exposure if I'm using two flashguns/flash as main source/ambient as main source ... Sorry, relearning techniques I first learned 20 years ago....Is there an article on the Web (or even in a back-issue of AP)?


    Mark M
  2. PJMorley

    PJMorley Well-Known Member

    From what I remember from articles in AP and other photography magazines, the basic formula for correct flash exposure is A=G/D where A is the aperture, G is the Guide Number of the flash and D is the distance in metres between the flash and the subject.

    So a flash with a guide number of 30 with a subject 4 metres away requires an aperture of
    = 7.5 so set the aperture to 8...

    That's for correct exposure.. If you just want fill in flash then reduce the aperture by 1 or 2 stops to f11 or f16 and I believe they would give 1:2 or 1:4 ratio respectively. You'd have to try it to see which you prefer.

    If you want to use a certain aperture size then you'd have to rearrange the equation to determine D. Assuming you have a detachable flash that is which from your question it sounds as though you have.

    The above is for a single flash only..

    I don't know the answer for a 2 flash set up. I don't know whether you should do the calculation for each flash as if it were the only one and then calculate an average aperture but I think you'd need to be able to alter the power output of at least one of the flashes also.

    Anyway.. hope this helps.. and hope I remembered rightly <S>


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