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Dimming Fresnels changes colour temp?

fresnel color temperature as arri kit

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#1 Gene Alberts

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Posted 10 September 2014 - 07:01 AM

Hey Guys,

 

Am I a total newbie or is everyone else aware that when you dim lights the colour temperature changes? I Just bought an As Arri fresnel kit 150w, 300w, 650w & 1000w and when dimming some lights and keeping others on full they no longer match. Seems if you want to dim you'll be needing some 1/4, 1/8, 1/16 CTBs.. but i've really never heard of anyone doing that? Do cinematographers basically never use dimmers and just ND lights?

 

This picture http://www.photosnac...87D75E/pdzne9m3 is of the 650w dimmed about 50%  (left) and the 150w undimmed (right) and the difference is huge.

 

Love to hear your thoughts, work-arounds or what you guys do with dimming situations.

 

Cheers, 

Gene

 


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#2 Mark Dunn

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Posted 10 September 2014 - 08:10 AM

I'm afraid you are a total newbie.

Incandescent lights are rarely dimmed for exactly this reason, unless an effect is wanted. The change in CT is not predictable. The bulbs also drop in CT somewhat with age, hence the use of a meter.

Intensity control is done with the various modifiers, of which ND is one.


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#3 Lance Soltys

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Posted 10 September 2014 - 10:51 AM

You may want to look into getting some scrims (wire mesh that goes in front of the light).  They come in various configurations (I especially like the half scrims) and are not expensive.  Arri makes them, so if those knock-offs has the same lens diameter, I imagine they would work.


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#4 Albion Hockney

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Posted 10 September 2014 - 10:52 AM

Mark, your attitude is not necessary.

 

Gene, Yes as Mark said dimming tungsten lights warms them up.... people do use dimmers all the time but with knowning the source will get warmer.

 

 

Fresenl lights come with scrims usually a set includes two singles, a double, and a half.  Which are little wire nets that cut down the light and slip right infront of the lens of the light. A red double scrim cuts a full stop of light effectively turning a 650 into a 325w light or a 1k into a 500w light while the green single scrims cut 1/2 stop of light. you can stack them infront of the light as well so if you have a 1k light you can turn it into anything between 1k-250W just with a few scrims


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#5 Mark Dunn

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Posted 10 September 2014 - 11:46 AM

Well the OP say it himself. The British are direct, Australians even more so. At least I didn't call him a galah.


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#6 Phil Rhodes

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Posted 10 September 2014 - 12:39 PM

I don't want to put beginners off from asking questions, but I think Mark's approach is more than justified by the existence of this.


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#7 Albion Hockney

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Posted 10 September 2014 - 01:01 PM

Haha I'll give you that this is a total google question.

 

but I don't think you need to shame a self admitted beginner either


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#8 Gene Alberts

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Posted 10 September 2014 - 07:31 PM

Haha ok seems I was the only one who didn't know that. I know about scrims but yeah was always wondering why cine's rarely used dimmers. Thanks for the quick reply guys.. loved the Lmgtfy by the way I'll be using that with my parents from now on.
thanks again!
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#9 Brian Drysdale

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Posted 11 September 2014 - 01:25 AM

I shouldn't worry, I did a teaching in life long learning course last year and one of the first things taught is that there is no such thing as a silly or stupid question. Often it can be the one that most of the students want to ask.

 

They use dimmers in multi camera TV studios, or at least they did when i worked in them at the BBC. However, as you've discovered, that has a disadvantage (TV studios have vision control engineers who colour correct all the cameras on the fly), but dimmers do get used for effects, so it's worth experimenting . 


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#10 Bill DiPietra

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Posted 11 September 2014 - 02:49 AM

Hey Guys,
 
Am I a total newbie or is everyone else aware that when you dim lights the colour temperature changes? I Just bought an As Arri fresnel kit 150w, 300w, 650w & 1000w and when dimming some lights and keeping others on full they no longer match. Seems if you want to dim you'll be needing some 1/4, 1/8, 1/16 CTBs.. but i've really never heard of anyone doing that? Do cinematographers basically never use dimmers and just ND lights?
 
This picture http://www.photosnac...87D75E/pdzne9m3 is of the 650w dimmed about 50%  (left) and the 150w undimmed (right) and the difference is huge.
 
Love to hear your thoughts, work-arounds or what you guys do with dimming situations.
 
Cheers, 
Gene


Take a look at Storaro's work on "Apocalypse Now." He dimmed the lights very effectively. It can work as long as you plan it out.

Also, I too have a set of Arri fresnels. With regard to scrims, they work up to a point. But the more you use to cut down the light, the more of the character of the light you will lose. Too many of those will also act like a dimmer (especially if the lights are gelled) because even though you are not changing the voltage (as you would be with a dimmer) you are still changing the intensity of the light.
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#11 Gregg MacPherson

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Posted 11 September 2014 - 10:46 AM

 

.. loved the Lmgtfy by the way.....



What? Now I'm googling about Google. So the humor works both ways. Seemed far too sophisticated for an Australian, but that's part of the joke, for me. Well done.

Re the color shift with dimming. If the image is a collage of different colors vs a rigid and precise or heavily normalized representation then it's not a problem, just something to work with.

Cheers,
Gregg.
(from Nouvelle Zealandia, almost an Australian)
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#12 Adrian Sierkowski

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Posted 11 September 2014 - 02:15 PM

My biggest use of dimmers is on practical lights in the scene; though I often will use them on "bleached," such as silk china balls as I tend to personally like things a little bit warmer on the whole (i even has a whole set of "unbleached" paper china balls)

My one gaffer loves dimming lights, which is great, though truthfully I much prefer scrims, especially if you're just working to bring down just one light-- as the color shift could then become distracting.


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#13 Chad Griepentrog

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Posted 12 September 2014 - 08:03 PM

Also, the Arri 650's tend to make a buzzing noise when dimmed. You can fit a 300w bulb in the 650 fixture to avoid this.
I've been forced to use ND on lights in the past and it's never been good. Resulted in a greenish, muddied light.
You can get practical in-line hand dimmers at most hardware stores that will work for the 300's and 150's. Anything of higher wattage requires a bigger dimmer (you can make these or spend a lot of money for a proper one- YouTube should have videos on how to make them)
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#14 Bill DiPietra

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Posted 12 September 2014 - 08:37 PM

Also, the Arri 650's tend to make a buzzing noise when dimmed.

 

Yup, very true.  Never thought about that since I usually shoot MOS, but for sound shoots that would definitley be a bit of an issue.


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#15 Rob McGreevy

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Posted 13 September 2014 - 07:01 PM

This is what variacs are for, they don't make noise - more expensive and a lot heavier though.


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#16 Derek Leffew

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Posted 28 October 2014 - 11:36 PM

... The change in CT is not predictable. ...

Au contraire, mon cherie. 

 

coltemp/COLTEMP = (volts/VOLTS)^0.42

 

Running a lamp at 90% of rated voltage reduces color temp by ~5%.

Running a lamp at 75% of rated voltage reduces color temp by ~11%.

Running a lamp at 50% of rated voltage reduces color temp by ~25%.

and so on.


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#17 Chris Millar

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Posted 29 October 2014 - 12:46 AM

I did a teaching in life long learning course last year and one of the first things taught is that there is no such thing as a silly or stupid question. 

 

I never fully understood this, it's up there with 'the customer is always right' - but doesn't have the motivation of sales pushing it through.

 

coltemp/COLTEMP = (volts/VOLTS)^0.42

 

and so on.

 

Is that formula a model based on empirical results or does it come from theory?  (and how much has that theoretical basis been simplified since...).

 

Either way, I figure Mark meant there is variability in results (manufacturing etc.), the term 'unpredictable' simply refers to this variability crossing of a bound of acceptability. Something a human decides.

 

...in other words:  whatevs   :)


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#18 David Mullen ASC

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Posted 29 October 2014 - 01:12 AM

Dimming of tungstens is done all the time on film sets -- Storaro has all of his lights going through a dimmer board.  Deakins has often talked about how some of his warm scenes were lit with dimmed tungsten bulbs.


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#19 Brian Drysdale

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Posted 29 October 2014 - 02:21 AM

 

I never fully understood this, it's up there with 'the customer is always right' - but doesn't have the motivation of sales pushing it through.

 

 

My understanding is that the dumb question can reveal something that's fundamental, or can be used to reveal something that's fundamental. Although, in teaching it can used to open a dialogue with students and  encourages a two way exchange between teacher and students, plus it can be the question that a number of students want answered, but didn't dare to ask. Also, it can initiate that something has been lost in the presentation of the lesson. 


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#20 Chris Millar

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Posted 29 October 2014 - 02:33 AM

My understanding is that the dumb question can reveal something that's fundamental, or can be used to reveal something that's fundamental. Although, in teaching it can used to open a dialogue with students and  encourages a two way exchange between teacher and students, plus it can be the question that a number of students want answered, but didn't dare to ask. Also, it can initiate that something has been lost in the presentation of the lesson. 

While these things are all true and worthy - but they don't make the question clever  ;)

 

(or dumber I guess)


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