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Manual Flash and Soft Boxes

jcgoodsonjcgoodson Member
edited August 2013 in flash & lighting
I'm getting my first soft box soon and am wondering how to best set distance/power for manual flash. For example, if I've decided on aperture, ISO, and distance, what's a quick way to determine power for proper exposure? Or, if aperture, ISO, and power are fixed, then how do I determine distance? I don't have a light meter and I'm guessing the flash's distance meter isn't valid when using a soft box. Is there a better option than trial and error? Thanks in advance for any comments.



  • TrevTrev Moderator
    edited August 2013

    No meter, then trial and error it is until you become accustomed to what to initially set it to for an 'in the ball park figure'.

    However, here is a good starting point.

    Speedlights (75-80 watts/second power rated): @ 3m f11 [10ft] around full power maybe a bit less; f8 around 1/2+ power

    Quantums (150 W/s) @ 3m f11 around 1/2+ power; f8 around 1/4+

    Cheetah (360 W/s) @ 3m f11 around 1/4+ power; f8 around 1/8+

    Einsteins/Profotos (600 W/s) @ 3m f11 around 1/8+ and so on.

    With each doubling of power you generally gain a stop extra light, but that's a ball park estimate, don't hold me to it.

    Those are just off the top of my head, I would take the time to do some 'trial and error' shots, noting the power ratio to f-stop; ISO you need @ a set distance and that way you will get to it a lot quicker.

    Or, buy a meter.

    Obviously you need to check histogram/LCD for correct exposure.

    Experience will get you in the ball park after using it a while.
  • Great work on the numbers Trev. I really enjoy this topic. Flash is technically trial and error because there is no way to predict the amount of flash power required for a perfect exposure. Your camera meters only for ambient and since the flash event has not taken place until you press the shutter there is no way to meter it. Even with a flash meter you enter the ISO, shutter and set the flash power and fire the flash. The meter tells you which aperture to use. If I want f8 and it tells me f11, I power the flash down. The only advantage of a meter is it makes it faster to achieve exposure but really shines when you have more than one flash.

    Determining distance after you set if ISO, aperture and shutter is set is not the way I'd approach it. However to your question about using the flash distance meter which I assume is the Guide Number that you are talking about will not work with a softbox. However you could use it as a starting point, adjust power as required (which will be more) then document it for future use. I would set the distance of my light source first based on the look I want, etc. Soft light is all about the size of the light source and the distance to the subject. You then work on the camera settings.

    Also you can really change the look of your image by changing the distance of your light source and that is why I determine that first.

    So basically you set everything up but you need to do a test shot or two to fine tune.

    For exposure you can use these methods of you don't own a meter. Just be careful not to have something brighter in the background that my throw your histogram off. Set your ISO, shutter and aperture and and fine tune the flash output. White is our friend.


    Scroll down and look at the gent holding the white towel.

  • TrevTrev Moderator
    Zenon said: The only advantage of a meter is it makes it faster to achieve exposure but really shines when you have more than one flash.

  • Neil vNNeil vN Administrator
    Lightmeter, or use the camera's histogram for exposure metering
    (as Zenon linked there.)
  • Learning this method from Neil was a life saver for my first wedding.
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