Shadow Length

Discussion in 'Lighting' started by Bruce Osgood, Jan 30, 2004.

  1. Bruce Osgood

    Bruce Osgood Membership Council

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    I use the sun as my primary lighting source for 99.9% of my photography. Todays news on CNN reported that Monday, Feb. 2 is Ground Hog Day here in the States and the length of the shadow cast by a certain groundhog in the state of Pennsylvania will determine the length of winter. The longer the shadow, the longer the winter.

    Now that seemed logical to me but my wife jumped all over that idea, arguing that the length of shadow is constant (at the same hour of the same day every year) regardless of luminous, a bright, clear day will produce the same length of shadow as a gray overcast day.

    Now I'm confused. What do you think? Is shadow length proportionate to light intensity?
     
  2. Tom Duffy

    Tom Duffy Member

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    ideally the shadow length is proportional to the angle of the sun, ie, same on cloudy or sunny day.
    practically, on a cloudy day the light is weaker and more diffuse. the apparent shadow (what you can see on the ground) will be shorter than if there are distinct shadows.
     
  3. e_joyner

    e_joyner Member

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    The premise of Groundhog Day is if Phil (the groundhog) SEES his shadow, there will be 6 more weeks of winter, if he doesn't, winter is on the outs. I don't know about the length of shadows being related to light intensity, but I do know it has nothing to do with Groundhog Day.:smile:
     
  4. Donald Miller

    Donald Miller Member

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    Interesting question. The intensity of the light has nothing to do with the length of the shadow. The height of the groundhog and the angle of the sun to the groundhog both play a part on the length of the shadow. The intensity of light would have an effect on the contrast between a shadow and a sunlit surface.
     
  5. blansky

    blansky Subscriber

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    In my opinion there are different definitions available here.

    There is quantity of light vs quality of light.

    Quality of light is like the different between a soft box and a parabolic. The soft box has a softer quality of light than the parabolic. The sun is like the parabolic, harsh and direct. The softbox is like the overcast day. Large diffused light source.

    The quantity of light or the intensity of light is how much light there is. 400ws is more than 200ws. The bright sun is more intense than an overcast day.

    These two factors do not affect the length of a shadow just the deepness( if you will) or darkness of the shadow. That is why on an overcast day there are almost no shadows visible. They are still there but the softness of the lightsource has wrapped around the subject and almost obliterated them.

    The only thing that affects the length of the shadow is the angle of the light source in relation to the subject.

    As for the rodent, my take on it was not on how long the shadow was, but whether the was a shadow at all, that determined the length of time until spring.


    Michael McBlane
     
  6. Aggie

    Aggie Member

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  7. blansky

    blansky Subscriber

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    Aggie,

    I think that for an early spring he is supposed to see his shadow. So abstinance on Groundhog Day is a perhaps a good thing. The saying is I believe, that if he sees his shadow there is only 6 weeks left of winter.

    Therefore he should sober up for groundhog day and then start drinking again right after lunch.

    Michael MCBlane
     
  8. KenM

    KenM Member

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    Amen to that! It's only -25C in Calgary today; it's practically a heat wave.

    Time for shorts!

    Aside: I was at a workshop in Yosemite in October 2001, and I wore shorts *every* day in the field - light spring jacket, no gloves or hat. Everyone else was cold, and was almost wearing winter gear, but not the dumb-*ss Canadian :-D

    Then again, I also slept in a tent at a workshop near Seattle a year ago January. Brain damage? A distinct possibility!
     
  9. bjorke

    bjorke Member

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    The length of Phil's shadow is not the issue, it's the shadow's visibility.

    That said, the length of the shadow umbra is dependant on the distance from the Sun (doesn't vary much) and the amount of atmospheric diffusion (varies a lot). Just think of clouds as really big softboxes or diffusion panels -- available in a variety of sizes and thicknesses.
     
  10. Gim

    Gim Subscriber

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    Yes, If the groundhog can see his shadow. But does it make a difference if the shadow is zone 5 or zone 3. I'm ready for spring.
     
  11. dr bob

    dr bob Member

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    My mom always said that the local woodchuck would come out of his (her?) burrow on Feb. 2. If the light was sufficient to see his shadow (below Zone V, I'm sure), he (she?) was frightened and ran back inside for six more weeks. As a former research scientist, I can assure all that the beast always sees his shadow and spring comes at the usual time (excepting global warming - global warming???).

    Truly dr bob. :smile:
     
  12. blansky

    blansky Subscriber

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    It looks as if my mother had lied to me all these years. I'm adopted? No that's not it.

    She gave me the wrong information on the Groundhogs Day Event.

    Here is a link to the facts;

    http://urbanlegends.about.com/cs/groundhogday/

    Now we know.


    Michael McBlane