Fill flash on manual

Discussion in 'Beyond the Basics' started by usayit, Aug 18, 2007.

  1. usayit

    usayit No longer a newbie, moving up!

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    I've got a completely manual camera, flash ( Sunpak 422 D), and a flash meter. I am shooting outdoors at high noon in strong light which means areas of highlights as well as shaded. My subject is in partial shade (letsay under a tree) and I want to set my flash to properly fill in the person's face without loosing exposure in the lighted background. I also want to avoid making it obvious that a flash was used (I'm not a big fan of flash). The flash has the ability to adjust from FULL to 1/16th power.

    Indoors relying on flash, I have no problems. Outdoors using no flash (which is what I usually shoot), I have no problems. Using a manual fill flash, I can't seem to get consistent results.. either over or under exposing the subject.

    Normally I'd use a reflector of some sort but this is not a photoshoot and thus I don't have a reflector nor an assistant. Just a candid shot.


    How would you go about setting up for a fill flash properly?
     
  2. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Meter the area lit by daylight and work out the correct exposure.
    Select the aperture appropriate for the flash exposure (based on GN and subject distance).
    This might entail adjusting the shutter speed, but make sure the shutter speed doesn't go above the flash synch speed (usually 1/30th or 1/60th for film. Check the specs for digital).
    At this setting the fill flash will balance the daylight. Best for backlit subjects, for example against a sunset.
    Drop the power on the flash 1 stop and you will lift the shadows and get fill.
    Drop the power 2 stops and you will just lift the shadows.
    Both good for side lit subjects.
    Lower the flash power any more and you won't make much of a difference but it's sometimes usefull, depending on conditions.

    If you have more power available in the flash, stopping down one (say from f5.6 to f8) and setting the flash for f8 will underexpose the b/g whilst making the flash lit subject look right.
    Doing the same but for 2 stops will make the subject look like it has been shot at dusk.

    If you have a flash meter then use that to get the flash exposure rather than calculating it and adjust the power until the flash to daylight ratio is what you want.
     
  3. elsaspet

    elsaspet TPF Noob!

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    Hertz, you are a God!
     
  4. Hertz van Rental

    Hertz van Rental TPF Noob!

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    We all have a little divinity in us. Would you like some more :eyebrows:
     
  5. usayit

    usayit No longer a newbie, moving up!

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    Thank you!! Time to do some experimenting with your advice!

    Question.. Is there anyway to translate a "drop the power # stops" to 1/2, 1/4, 1/8, 1/16 flash power?
     
  6. Hertz van Rental

    Hertz van Rental TPF Noob!

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    Each stop is either a doubling or a halving of exposure.
    For an exposure A dropping down one stop gives an exposure of A/2. Opening up a stop gives an exposure of 2xA.
    The same works for flash.
    If you have a flash of power B to give you correct exposure, then turning the flash power down to 1/2 power = B/2 or 1 stop decrease in power.
    1/4 power = 2 stops decrease and so on.
    So:
    1/2 = 1 stop decrease
    1/4 = 2 stops decrease
    1/8 = 3 stops
    1/16 = 4 stops

    If you set the flash on 1/2 power and put the camera settings to get the correct exposure with that then increasing the flash power to full will give you a 1 stop increase in power (1 stop over-exposure).

    Does that help?
     
  7. Mike_E

    Mike_E No longer a newbie, moving up!

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    Hertz, Nice explanation! Now, if you would, tell him about the relationship between two streps closer and two steps back. ;)

    And then if you are in a charitable mood, explain why if the flash is off camera the only distance that really matters is that between the flash and the subject and not the subject and camera (within reason). :)

    mike
     
  8. Hertz van Rental

    Hertz van Rental TPF Noob!

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    That's the Inverse Square Law.
    Illuminance levels are inversely proportional to the distance from the light source.
    For a given distance D with exposure E:
    D/2 = Ex4
    Dx2 = E/4
    And so on.
    This is because of Geometry.
    If you have a pyramid of base area A and height H, doubling the height will increase the base area by a factor of four. Halving the height will decrease the base area by a factor of four. (It works the same for cones as well, but it is always best to consider illuminance in terms of square area. That's how the ISO units are calculated.*)
    From this you can see that if you move the light source further away from the subject then the same amount of light has to cover a greater area. At twice the distance the light is now illuminating an area four times bigger. As the total light output does not increase the illumination must decrease.
    Light obeys this Law completely.
    You can see the effect with a torch. It's even better if you use an enlarger.

    The level of illumination (illuminance) an object receives is the important factor in Photography because this affects the amount of light reflected (and we are considering objects as being perfect reflectors here - no loss through absorption). That is to say, the illuminance affects the exposure.
    Why does the ISL not appear to work with light travelling from the object to the camera - at least at reasonable distances?
    Because now we are talking in terms of Optics.
    With a light source the light travels away from it covering an ever larger area as it does so reducing the effective illuminance. (See it like a balloon. The bigger you blow it up the more area the same amount of material has to cover and the thinner it gets).
    But with optics you are dealing with individual 'rays' of light. Think of it like this. Each point on the object is reflecting one 'ray' of light towards the camera and they are all parallel.
    If you place your camera in front of an object all the light travelling to the camera from the object is travelling in a straight line - it is not being spread out. That means that the amount of light entering the camera from the object is the same for 5 feet as it is for 50 feet.
    Of course this only holds true for 'local' distances. Various things like dust and water vapour get in the way and absorb or deflect the light at longer distances.
    The above may be hard to understand but do try. It's the cornerstone of Photography. And if it sounds weird - well, it's because you have to look at different situations in different ways. That's Physics for you :mrgreen:

    *Illuminance is measure in Lux. Lux = Lumens per square metre.
    It's important to not confuse illuminance with luminance.
    Illuminance is the light falling on an object.
    Luminance is the light reflected by an object (measured in Candelas per square metre).
    There are two other measures but mentioning them now will only confuse things further ;)
     
  9. Mike_E

    Mike_E No longer a newbie, moving up!

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    :wav:

    10 out of 10!

    mike
     
  10. Hertz van Rental

    Hertz van Rental TPF Noob!

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    :shock: You thought I might not know the answer or get it wrong or something? With my reputation?

    And I wrote all the above from memory and without the aid of Google or books :lmao:
     
  11. Mike_E

    Mike_E No longer a newbie, moving up!

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    LOLOL No, it's just that when I try to describe it people fall asleep. ;)
     
  12. usayit

    usayit No longer a newbie, moving up!

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    Wow.. THANKS! time to soak it up and have some fun.
     

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