Fill flash math.

Discussion in '35mm Cameras' started by David Farber, Sep 17, 2008.

  1. David Farber

    David Farber Guest

    I was wondering about how fill flash affects the existing light. Of course
    it will take the shadows and lighten them up a bit, but what about the parts
    of the picture that are correctly exposed by the existing light? I mean the
    light emanating from the flash (attached to the hot shoe) does not just go
    to the shadows. Wouldn't the areas not in shadow then be overexposed by a
    half stop? My fill flash photographs (from print film) have been coming out
    nicely but this conundrum has me scratching my head. On second thought,
    should I be using some ratio method to compensate for the existing light
    exposure or is that not how it's done? I'm using a Minolta Maxxum 7 with a
    3600HS flash unit.
    David Farber, Sep 17, 2008
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  2. David Farber

    Noons Guest

    David Farber wrote,on my timestamp of 17/09/2008 9:01 AM:
    What you forgot is why/how folks use fill flash.
    Yes, it's additive. Have a look at most examples
    in manuals for fill-flash: it's usually a portrait of a
    person in shadow, with a light or shadow background,
    AWAY from the main subject. That means if you shine the
    flash at the subject, it'll light it up ok but the
    background will be too far away to be seriously influenced
    by the amount of flash light used.
    Flash lighting is very dependent on distance from the flash,
    given constant intensity.
    Noons, Sep 19, 2008
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  3. David Farber

    Noons Guest

    Harold Gough wrote,on my timestamp of 21/09/2008 9:07 PM:
    Where below do you see ANY reference to "film", 1/4 stop steps
    or "film users"? Can you even comprehend that using a flash is
    something that has NOTHING to do with film or dslr? And that
    is NOT the point of the OP question in any way, shape or format?
    Or do you just interject off topic to say, essentially, nothing?
    Noons, Sep 21, 2008
  4. David Farber

    David Farber Guest

    Thanks for pointing this out. I use fill flash mostly outdoors where there
    is a bright sun and harsh shadows. With the sun high overhead, the light is
    not even in the same direction as my on camera fill flash. I think just
    taking more pictures, taking good notes, and experimenting more will help me
    understand this subject better. With all the options on my camera, I
    sometimes pay too much attention to the numbers and start missing the good

    Thanks for your reply.
    David Farber, Sep 23, 2008
  5. David Farber

    David Farber Guest

    Initially my logic was this:

    You have a normally exposed outdoor scene. Let's call the amount of light in
    this scene 100 units. Now, you add a fill flash which is set to -.5. That
    would add 50 units of light. If you add them together, you get 150 units of
    light for a scene which was properly exposed with 100 units of light. Of
    course the real world is much more complicated than this especially since it
    was pointed out to me that the fill flash is not pointing in the same
    direction as the main light.

    As far as manual vs. automatic, the "how to" aritcles I've read say to set
    your exposure manually as if you didn't have the flash. Then, dial in the
    flash compensation to whatever you like, usually between. -2 to -.5. Then,
    the automatic circuits in the flash will figure it out.

    Thanks for your reply.
    David Farber, Sep 23, 2008
  6. David Farber

    David Farber Guest

    Hi Alan,

    I've discovered that when you're in "A" mode (and I can only speak for what
    the Maxxum 7 does), the selected exposure seems to ignore the ambient light.
    For example, in a scene where f./8 @ 1/30th of a second would be the proper
    exposure, the camera will recognize that the flash is enabled and then might
    choose an exposure of f/8 @ 1/125 of a second. This pretty much decides for
    you that the flash is going to be the main light instead of the fill light.
    That is why I like to meter the scene in "M" manual mode so I'll know the
    exposure will be correct with the given ambient light.

    Thanks for your reply.
    David Farber, Sep 23, 2008
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