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Where did the flash go?

Discussion in 'General Discussion' started by Cliffords, Jul 24, 2008.

  1. Cliffords

    Cliffords Guest

  2. McQ

    McQ Just your average, everyday moderator. Moderator


    I read it through. One thing though...it seems as if the article just stops after explaining the inverse square law (which you did a nice job explaining, btw).

    It's as if I was expecting more of a "how to" section and the only thing I saw was, "increase your ISO.".

    Perhaps there could be a little more of how to adjust your flash with your camera to get the picture using manual? Just suggesting based on what I thought the article would end up being, not a critical slam, please understand. :smile:

    However it does explain nicely "where the flash goes". And too often I forget and mine goes fading out....

    Also, welcome to the forum!
  3. Cliffords

    Cliffords Guest

    All suggestions welcomed!
  4. the inverse square law works well for approximating a bare bulb flash. most camera flashes have reflectors and other optics to help direct the light towards the subject. because of this, the inverse square law will be a poor model for what is happening.

    flash pictures often look awful because no brain power is put to the positioning of the flash.
  5. scooptdoo

    scooptdoo Guest

    wrong the inverse square law is a law of nature period!!! holds up regardles of lens focus system.a freznel lens(spelling may be wrong) used in light house optics will decrease 4 times for every doubling of distance and visa versa!
    id hardy call this an artical covering flash photograghy.maby the first couple of sentances of an artical?

  6. a collimated beam of light does not decay 4 times for every doubling of distance. neglecting diffraction and atmosphere, it doesn't decay at all. the inverse square law is based on a point source approximation with and even 4pi illumination pattern.

    the point source is one basic scenario used for approximating a broad range of situations. there are scenarios frequently used in approximations including infinite line source and infinite area sources.

    if you where right, there would be no point in zoom flash heads since the intensity at the flash head is always the same. How do explain the behavior of laser pointers with the inverse square law anyways?
  7. Lurker


    Jul 21, 2007
    Lasers are the exception - a laser beam has a constant width and doesn't expand. Any other light that behaves like a point source (hint: if there is a reflector, it's a point source) and therefore expands over distance will fall off with the square of the distance.

    A light beam is always a cone (with cut-off sides, maybe). Double the distance and the area that the end of the cone is covering will quadruple since width and height of the projected light circle have doubled. Since the amount of light is the same, the intensity of the light has dropped to 1/4.

    That doesn't mean that zoom heads are useless. What's the point of using a cone that projects a light circle of 10ft when you only need 3ft? By tightening the cone the area that is covered by the beam decreases, and as a result the intensity increases. And that allows you to use the same strength flash over a longer distance.
  8. what makes a laser beam special is that it is highly coherent, not that it is highly collimated. a laser beam can be shaped into a cone and still be a laser beam.

    if you focus the sun with a magnifying glass, does the light decay as you move away from the lens? no, it gets more intense until you pass the focus point.

    a reflector says nothing about what kind of source it is. lasers have reflectors in them.

    you guys have no idea what your talking about.
  9. Just a thought Joe, no one is saying that a reflector or Fresnel lens will not condense the beam and project it further. What they are saying is that what ever the brightness of the light is at say 20 ft. it will be 1/4 that at 40 ft according to the inverse square law. I will admit that I do not know which one of you is right but this makes logical sense to me. At least this is something I have always been taught and do believe.
  10. McQ

    McQ Just your average, everyday moderator. Moderator

    So does anyone actually have anything to say about the OP's article? :wink:
  11. the distances involved in your scenario mush be very large in comparison to the size of the source and the beam must be highly diverging relative to the width of the beam in order to apply the inverse square law. these assumption do not apply to many flash photo applications.
  12. jcovert

    jcovert Guest

    Was that an article? I thought it was affiliate marketing disguised as something useful.
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