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manual flash settings 1/2, 1/4, 1/8, and such vs. f/4, f/5.6, f/8, and such.....

Discussion in 'General flash photography, lighting, and technique' started by stayathomedad, Jul 29, 2008.

  1. stayathomedad


    Mar 11, 2008
    I'm reading some books and it refers to flash settings in the f-stops... "set flash to f/4 to blah blah blah"...

    How does both systems of measurement compare with one another?

    If I set my flash to 1/2 power... how would that work out into fstops that the book is referring to?
  2. It doesn't work like that. You'd need to know the guide number of the flash and the subject distance to work out the appropriate aperture for a given manual power setting.

  3. The apppropriate apperture depends on a lot of things, flash power, distance from subject, any light modifiers etc.

    To find out what aperture you need, you need to use a flash meter, the Sekonic L-308 is a good affordable flash meter to get you started.
  4. stayathomedad


    Mar 11, 2008
    Ok I'm totally lost now...

    How is the two different number systems related?

    what am I missing?
  5. the 2 number systems aren't really related.

    the 1/2 1/4 1/8 etc is how much power in relation to the flashes maximum output is being used.

    So 1/2 is half of your flashes maximum power.

    If you use half power, about 6 foot from your subject you may get f/5.6 but if you use the same power at 12 foot, you may get f/4 put a diffuser infront of it and you may get f/2.8

    The f numbers are just a measurement of how much light is actually there, and correlates to what f stop you need for a correct exposure (assuming you're shooting at or just below flash synch speed). The other numbers relate just to your flash and how much power you want to use.

    So if you wanted f/4 at the subject, you'd set your flashes distance and power so that you can measure f/4 (with a flash meter) at the subject.
  6. Who knows? But that's why the book is written that way. You see the author has no idea what kind of flash gear you have, so he has to have a way to reference light intensity that will apply to everyone.

    Rather than give specific numbers for a particular brand and model of camera and flash, he can just talk about f stops.

    For example, he might talk about a set up where the proper exposure for the key light works out to F 4. Now with your flash at a particular distance using the ISO you have selected you might select 1/2 power to get a proper exposure at f-4. However, with MY gear, I might have to use full power to get the same exposure because my flash is less powerful or because I used a lower ISO. But the important thing is that we're both doing what's necessary to get to an exposure which would work out well using an aperture of f-4.

    So now when the author suggests setting up a second flash to illuminate the shadow side of the subject with half that amount of light; i.e. reduce the power by one stop. You will set 1/4 power and I will set 1/2 power if our second flash is identical to our first, but we might set something else if our second flash is more/less powerful. But in any case, now we all have the same flash exposure and the author can just say "Set the second flash to produce f 2.8."

    In other words, using f stops is just a way to talk about light intensity using a common value that will apply to a wide variety of equipment and situations.
    Last edited by a moderator: Jul 29, 2008
  7. Once upon a time, I had an old strobe (sunpak? Vivitar?) that had a little slide rule on the back- you'd set distance on the outer ring, and the inner ring would give you a setting for aperture (f/number). when you turned the whole dial, it powered down the flash on a log scale, and the f/number stepped down as well. Perhaps this is the unit the author was working with.

    Simply put, if you are set up correctly, halving the flash output will call for one full stop more aperture (you are probably set up for 1/3 stop adjustments on the camera).

  8. Nope. f/2.8. It's the old inverse square law. Double the distance, quarter the power so two stops lost.

  9. fks


    Apr 30, 2005
    sf bay area
    guide number (GN) = distance (d) x aperture (f)

    for the SB-600, GN=138ft at ISO200 and 35mm setting on the flash head. using the above formula, you can play with either distance or aperture. for example, if your subject is 10ft away, you need to set your aperture to f/13.8 (or f/14). set the flash to half power, GN drops to 69ft, and you need your subject at 5ft to get proper exposure at f/14, or you open the aperture to f/6.9 (or f/7.1).

    your question can't really be answered as you need two variables to get the third, and half power is only one variable.

    just curious why you're controlling your flash this way instead of setting it to auto? this was the torturous method of calculating flash exposure before they invented auto-exposing flashes and ttl :wink:


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