# Fall of on par64 1000w

5 replies to this topic

### #1 Iga Mikler

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Posted 23 April 2014 - 04:00 AM

Im doing a music video and wondering the fall of on the par64 cans 1000w?
Also the differences between cp61,62 and 63
Many thanks
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### #2 jeff woods

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Posted 23 April 2014 - 05:24 PM

Hopefully others will chime in, but all light has the same fall-off: twice the distance from the source, 1/4 of the light (inverse square law).

What comes in to play is size the source in relation to the subject. A PAR64 is essentially a point-source, so the shadow edge will be harder than a larger source like a Kino or a PAR through a silk in the same position. That said, a Kino from a large distance becomes a (very dim) point source too. To get a nice wrap around light with quick fall off in the background, the light would need to be a large source close to the subject.

I'm struggling to explain it better without images, so hopefully more well spoken people can chime in.

Hope that helps,

-j

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### #3 Chris Millar

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Posted 23 April 2014 - 06:30 PM

To make it even more concrete, the size of the source with respect to the subject *is* the inverse square law

Agree, much easier to describe in person...
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### #4 Gregg MacPherson

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Posted 23 April 2014 - 11:03 PM

To make it even more concrete, the size of the source with respect to the subject *is* the inverse square law

I knew well (as math) that a point source would have fallof with inverse square of the distance. And at a reasonable distance lamps become point sources. But I didn't think this would be true when close to some common sized lights. I grabbed a redhead (5" parabolic) and a six inch fresnel and at roughly 4'/8' it actually looked roughly linear. Very rough, because I just guessed the measurements. I'll have to find time to do it properly.

Still savouring your ridley expression above.
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### #5 Chris Millar

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Posted 24 April 2014 - 01:59 AM

Hrrrm, let me think about this a moment - at the moment I think yes, you're quite correct. (until I change my mind)

But jeez, how the heck does the math work out in any general sense for the reality? Ouch, I guess you're looking at the algorithms used (or more likely still idealised) in 3D lighting ... and all the attendant computational complexity issues.

Ridley what ??  you read into something not intended ? I hope it was funny/clever - it's yours to use !

(an interesting phenomenon, it's like inspiration out of thin air but with an unfortunate subtext of IP theft)

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### #6 Gregg MacPherson

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Posted 24 April 2014 - 02:26 AM

You're not taking about the math for the point source fall off are you? That one is simple and I think unavoidable.

The experiment was "rough as guts". I moved the meter and saw about a stop difference at twice the distance. I guessed the measurements. Could have measured in cubits I suppose. It's coming back into fashion with ancient Indian architecture. I'll check the experiment before I worry about the real world math.

Re your riddle(ish) statement, can you put that as an equation, so I can see it, rather than guess.
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