1/2 CTO + 1/2 CTO = Full CTO?

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#1 Chris Keth

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Posted 08 November 2007 - 01:12 AM

So an academic question came up on my last job when I was talking to the gaffer. It's something I had never thought about before:

Does 1/2 CTO correct 5500K light to 4350K (halfway between 5500 and 3200) or does it have half the mired value of full CTO?

The gaffer claimed that two layers of 1/2 CTO does not equal one layer of full CTO. That would support it correcting to the halfway point in color temperature, which is 4350K. Full CTO must have a mired value of +131 because it corrects 5500K to 3200K. If 1/2 CTO corrects to 4350K, it would have a mired value of +48 and two layers of it (having a mired value of +96) would correct 5500K light to 3600K, undercorrecting a little bit.

This just seems illogical to me, since the visual interpretation of color temperature (which the mired scale is very useful for) is more important than exactly what color temperature it corrects to.
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Posted 08 November 2007 - 01:36 AM

While I don't want to think of the math, I can tell you that I have always heard that two 1/2s don't make a full in terms of CTO/CTB. Wives tale; maybe, but it's gotten me this far (not that it's a great distance at all but still)
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#3 Matt Workman

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Posted 08 November 2007 - 01:45 AM

I feel like a color temp meter could solve this problem, too bad I don't own one
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Posted 08 November 2007 - 02:13 AM

Ironically enough this post sent me out on a hunt for one. Ebay has a few "vintage" ones around and some minoltas. Dunno if i'd trust them; though calibration-wise
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#5 Chris Keth

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Posted 08 November 2007 - 02:48 AM

Ironically enough this post sent me out on a hunt for one. Ebay has a few "vintage" ones around and some minoltas. Dunno if i'd trust them; though calibration-wise

It's take a call to make sure but I bet quality light metric does calibration on them.
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#6 Michael Nash

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Posted 08 November 2007 - 04:15 AM

And to think I've been doing it wrong all these years, and I'm just know finding out all my footage is ruined...

Hold up a piece of Full CTO. Hold up two pieces of 1/2 CTO (from the same manufacturer) layered together. Tell me what you think.

Besides, you have be certain which numbers you're measuring -- 3200 or 3400; 4350 or 4400; 5500 or 5600. This is one of those things where you can let the theory and math bog you down, or you can just do what you know works and keep shooting. Your gaffer may have a point, "but what does Zeno, anyway?..."
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#7 Brian Drysdale

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Posted 08 November 2007 - 05:14 AM

Although, just to confuse, the mired shift on a 1/2 CTO (81) is greater than the mired shft on a 1/2 CTB (-68).

Rosco's Full CTO has a mired shift of 160.

You do get manufacturing variations and the last time (2 weeks ago) I checked a double 1/2 CTB against a Full CTB, these didn't quite match by eye.
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#8 Kevin Zanit

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Posted 08 November 2007 - 06:59 AM

In my experience, its 'good enough for government work', I end up doing it all the time (doubling up). I'm sure its as accurate as the odds of any of our lighting units delivering an exact 3200 or 5600.
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#9 timHealy

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Posted 08 November 2007 - 10:07 AM

In my experience, its 'good enough for government work', I end up doing it all the time (doubling up). I'm sure its as accurate as the odds of any of our lighting units delivering an exact 3200 or 5600.

I agree with Kevin's sentiment.

best

Tim
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#10 Dan Salzmann

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Posted 08 November 2007 - 11:10 AM

+1 on Kevin's statement.
The length of your extension leads or "stingers" as you call them can make lights shift magenta/red as well as genny performance, power surges, diffusion and bouncing, reflectors that are yellowed from age, etc.
"Good enough for government work" made me laugh. Wonder how that saying got started. Probably with C-rations or something. Anyone?
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#11 Brian Drysdale

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Posted 08 November 2007 - 12:22 PM

Well, I doubled these particular !/2 CTB gels for an interview with a government minster, so it was definitely good enough for government work
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#12 Chris Keth

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Posted 08 November 2007 - 04:43 PM

In my experience, its 'good enough for government work', I end up doing it all the time (doubling up). I'm sure its as accurate as the odds of any of our lighting units delivering an exact 3200 or 5600.

Oh, I'm not arguing with that or trying to start a mass movement to change the usage habits of gaffers far and wide.

That's why I called it an academic question. I'm just curious.
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#13 David Auner aac

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Posted 10 November 2007 - 03:41 AM

That's why I called it an academic question. I'm just curious.

Just checked Box's Set Lighting Technician 3rd Ed.. Here's what he has on the topic:

Lee: 1/2 CTO Mired Shift: +109; 1/1 CTO Mired Shift: +159
Rosco: 1/2 CTO Mired Shift: +81; 1/1 CTO Mired Shift: +167
GAM: 1/2 CTO Mired Shift: +81; 1/1 CTO Mired Shift: +183

Interestiung huh?

Cheers, Dave
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#14 Chris Keth

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Posted 12 November 2007 - 03:26 AM

Just checked Box's Set Lighting Technician 3rd Ed.. Here's what he has on the topic:

Lee: 1/2 CTO Mired Shift: +109; 1/1 CTO Mired Shift: +159
Rosco: 1/2 CTO Mired Shift: +81; 1/1 CTO Mired Shift: +167
GAM: 1/2 CTO Mired Shift: +81; 1/1 CTO Mired Shift: +183

Interestiung huh?

Cheers, Dave

That is interesting. Rosco seems to be the closest one in terms of a half strength actually being half in terms of color to your eye. I'm sure different batches of gel are slightly different, too.

For sake of comparison, lets say we're starting with a perfect 5600K source. Here are the color temperatures that you will get with the various gels applied, based on the mired shifts you supplied:

Lee: 1/2 CTO: 3472K, full CTO: 2959K
Rosco: 1/2 CTO: 3846K, full CTO: 2890K
GAM: 1/2 CTO: 3846K, full CTO: 2762K

I'm surprised that none of them theoretically, based on those mired shift values, correct to 3200K at all but quite a bit warmer.
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#15 Brian Drysdale

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Posted 12 November 2007 - 05:27 AM

That is interesting. Rosco seems to be the closest one in terms of a half strength actually being half in terms of color to your eye. I'm sure different batches of gel are slightly different, too.

For sake of comparison, lets say we're starting with a perfect 5600K source. Here are the color temperatures that you will get with the various gels applied, based on the mired shifts you supplied:

Lee: 1/2 CTO: 3472K, full CTO: 2959K
Rosco: 1/2 CTO: 3846K, full CTO: 2890K
GAM: 1/2 CTO: 3846K, full CTO: 2762K

I'm surprised that none of them theoretically, based on those mired shift values, correct to 3200K at all but quite a bit warmer.

Rosco describe their 3/4 CTO as the standard correction from daylight to tungsten.
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