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Fresnel shadow/ penumbra


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#1 G McMahon

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Posted 07 July 2006 - 10:32 AM

Why does the shadow thrown from a fresnel lens sharper at the flood end then the spot end? It does not make sense. Or am I make these judgements after too many hours on set with little food and sleep and too many cigarettes?
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Graeme
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#2 Alex Haspel

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Posted 07 July 2006 - 12:00 PM

i never had this verified yet, but this image shows
#1: my unbelievable wacom tablet skills
#2: how i always explained myself the phenomen you described.

Posted Image

i invite anyone to enlighten me, in the quite likely case that i am wrong.


cheers,
Alex "Mommy says i'm totally clever" Haspel

Edited by haspel, 07 July 2006 - 12:01 PM.

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#3 Michael Nash

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Posted 07 July 2006 - 02:43 PM

Yes, the diagram is pretty much it. If you look at the front of a fresnel unit (wear sunglasses or dim it first), when it's in full flood you'll see that the light is a small spot in the center. When it's full spot, it will fill up the whole lens and also be a lot brighter -- but as soon as you move off axis, it goes away almost completely (as you move out of the "spot"). Naturally, the larger surface creates a softer shdaow, and the smaller point-source (full flood) creates a sharper shadow.

This is true of other types of lamps as well, it's just that fresnel lenses help focus the beam more smoothly than what you typically get with just a refelctor (such as an open-faced unit).
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#4 Laurent Andrieux

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Posted 09 July 2006 - 05:59 PM

All that is correct. I could just add one thing that might help you out understading, if ever needed : if you open up the Fresnel lens, you will find out that the flood position corresponds to the lamp very close to the Fresnel lens. It's like the light coming out the lamp would only pass through its center, where the lens is close to a parallel glass, like if there was no lens at all. Therefore, the source is nearly the lamp only, a point source. That means a flood beam (coming out from one point) and hard shadows. When you spot it, the lamp goes further from the lamp. Therefore, it enlights the whole Fresnel lens (this why you see it all enlighten) and is then spotted by the lens, so the beam is narrower and since the whole lens becomes the secondary source, gives softer shadows. There is nothing like experiencing : look at the beam's width, the lens aspect, the sahdows' aspect, the lamp position, in both settings : spot and wide.
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