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Why is double quarter blue not half blue?


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#1 Phil Rhodes

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Posted 06 August 2015 - 02:28 PM

Two bits of Lee 203 have a mired shift of -35 each, for a total of -70.

 

One bit of Lee 202 has a mired shift of -78.

 

Um.

 

P


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#2 Adrian Sierkowski

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Posted 06 August 2015 - 02:47 PM

This is common knowledge that two quarters don't make a half, just as two halves don't make a full. I suspect it has to do with; well  physics (moreso the thickness of the gel; the air-gap between two gels, a slight color compounding in the underlying medium to which the gel pigmentation is applied, and general goblins. ) Though in truth; often, though not perfect, douvbling halves or quarters or eights etc is as they say "good enough for government work"


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#3 David Mullen ASC

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Posted 06 August 2015 - 03:52 PM

I agree that it's not exact, the MIRED shift isn't even consistent across manufacturers for standard color correction gels. The question is how often it's a problem.
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#4 JD Hartman

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Posted 06 August 2015 - 05:53 PM

Speaking of "air gap", is is better to have the two sheets touching or spaced apart?  I've had DPs insist on spacing the two sheets applied to opposite side of an open frame with some wadded gaff.  Any science between one method or the other?


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#5 David Mullen ASC

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Posted 06 August 2015 - 06:03 PM

Not sure why that would matter.


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#6 Robin R Probyn

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Posted 07 August 2015 - 05:27 AM

Building up the overtime.. ? 


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#7 JD Hartman

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Posted 07 August 2015 - 05:41 AM

Building up the overtime.. ? 

That would only make sense on a union shoot. 

 

Anyway....I think I going to shoot an an email to Rosco.  Would be good to be able to get a definitive answer on "air gap", if one exists.  It will probably fall into the Double Jeopardy category: "Misconceptions in the Grip department".


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#8 Robin R Probyn

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Posted 07 August 2015 - 05:51 AM

worried about the heat welding them together.. only thing I can think of.. 


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#9 Phil Rhodes

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Posted 07 August 2015 - 06:25 AM

I was gonna say, cooling.


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#10 aapo lettinen

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Posted 07 August 2015 - 08:44 AM

Speaking of "air gap", is is better to have the two sheets touching or spaced apart?  I've had DPs insist on spacing the two sheets applied to opposite side of an open frame with some wadded gaff.  Any science between one method or the other?

probably for cooling. if you have dense gels they would build up lots of heat and the one closer to the lens may melt at least a bit


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#11 David Mullen ASC

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Posted 07 August 2015 - 11:12 AM

Yes, and maybe it also slows down the fading from heat.


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#12 Sanjay Sami

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Posted 07 August 2015 - 12:05 PM

JD, would love to know what Rosco or Lee have to say.

Ive always stuck them together.

Maybe its time to change :-)


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#13 aapo lettinen

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Posted 07 August 2015 - 05:15 PM

it depends a lot on what gels you use. for example CTB heats a LOT more than a CTO (you can see this by comparing the transmission percentage. if a gel has for example 25% transmission then you can assume that about 75% of the visible light hitting the gel converts to heat (maybe less because the gel surface also reflects small amount of the light back to the fixture) . and that is A LOT of heat if you are using for example 5K tungsten behind a CTB 


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#14 JD Hartman

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Posted 07 August 2015 - 07:14 PM

Rosco said:
 
In general, if you place two filters in a frame together so that they are touching, they will eventually melt together. So, although I don’t specifically know, my guess is that yes what you are describing is an in the field solution to keep the filters from melting together. That will help them remain usable longer.
 
Hope that helps!
 
Wendy
 
Wendy Luedtke
Product Manager for Color and Lighting
Rosco Labs

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#15 Robin R Probyn

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Posted 07 August 2015 - 08:07 PM

 thanks for the up date.. 


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#16 Sanjay Sami

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Posted 07 August 2015 - 10:41 PM

Thanks JD


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#17 JD Hartman

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Posted 08 August 2015 - 09:35 AM

I'm hoping that she will forward the question on at Rosco Labs and I'll be able to share a more scientific answer.

 

The heat dissipation thing does make sense, now we need empty flat frames with heat dissipation channels so we can have convective cooling of the gels.


Edited by JD Hartman, 08 August 2015 - 09:39 AM.

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#18 Sanjay Sami

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Posted 09 August 2015 - 03:57 AM

Ha ha ha :-)

Ive honestly never had gels melt together in a situation that would not have melted a single one.

However, if it helps maintain the colour for run of show I wouldn't mind trying that.


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#19 Brian Drysdale

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Posted 09 August 2015 - 04:52 AM

I suspect the colour will fade or discolour before the gels actually melt together.


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#20 Phil Rhodes

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Posted 09 August 2015 - 05:54 AM

I have these PARs which are extremely efficient but create a very high intensity beam. They melt gels like an industrial cutting laser, and I'm only exaggerating slightly.


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