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Gene Alberts

Dimming Fresnels changes colour temp?

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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.photosnack.com/F8E7D87D75E/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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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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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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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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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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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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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.photosnack.com/F8E7D87D75E/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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.. 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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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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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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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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... 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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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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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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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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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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While these things are all true and worthy - but they don't make the question clever ;)

 

(or dumber I guess)

 

I suspect it's not the question, but the use made of the answer makes it clever. "Why does an apple fall?" could have been seen as a dumb question to many people at the time, with a possible religious answer that would possibly satisfy most people at the time.

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I used the watch Brian Lamb's interviews on CSPAN, and one reoccurring thing he did was ask the author to explain very common terms -- the guy would be suddenly mention communists in the 1950's and Lamb would interrupt and say "what was communism?" It wasn't that Lamb didn't know the answers, it's just that he felt that we use terms sometimes too casually to the point where it is just assumed that we all agree on the definition, so he wanted to know how the author defined the term and wanted the discussion to follow that specific definition.

 

So there is a lot of value in answering basic questions sometimes because it can reinforce / clarify ideas that perhaps were only vaguely defined before or misunderstood before.

 

Of course, there are times when I wish people understood how to use a search engine so that we could further explain or clarify a confusing definition rather than simply providing that definition.

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Quick correction Kenny, it's the Set Lighting Technician's Handbook by Harry C. Box. Generally referred to as the Harry Box Book on set because we low-minded juicers like the dirty pun :)

Speaking of said book, re: Harry Box you lose or gain 10K of color temperature for every one volt up or down. So a 10V adjustment will also result in 100K of color temperature adjustment. As for empirical data, there is none posted in the book but I have no reason to disbelieve him; this is a law of physics that is measurable and will follow the same mathematical formula every time, like gravity. I'm sure empirical data exists somewhere and I'm sure Mr. Box researched said data before publication. But ultimately when you think about the basic principals at play common sense will tell you that CT shift is an inevitability anyway: a tungsten filament is the closest thing that exists in nature to Lord Kelvin's ideal black body radiator. Fewer volts means less energy, therefore less heat resulting in a drop in CT towards red. More volts means more energy, means more heat so it shifts higher towards blue (up to a certain point, then you blow the bulb).

 

A few additional notes on dimmers to further the discussion: If you're using a variac dimmer they usually have the actual voltage level listed on the dial, so you can set it to a precise voltage level, then do the math to see where you are on the Kelvin scale, and make any correction adjustments accordingly. That's assuming that correcting it is important to the shot, I've noticed not all DPs will bother with it. I'm not inside the mind of a DP, so I don't know when they do or don't think CT is critical. Some general things I've noticed: If the scene is very warm i.e. lots of practicals/fire effects; CT is not that important because the DP is going for a very desaturated look; CT is important but the dimmed lights are not keying principal talent...these are some of the situations I've seen DPs be totally OK with CT drop when dimming a light. I'd be very curious to hear from any other DPs on what their thoughts are on this.

 

Also, I think it's worth discussing SRC/electronic dimmers in this thread. They work differently than variac or rheostat dimmers do in that they don't use the voltage to effect intensity, but rather the voltage remains the same and the frequency is adjusted; there is mechanism in these dimmers that shuts itself on and off at different points in the sine wave, so the light is just off during a certain portion of the cycle. This can cause flickering in some scenarios but typically is not an issue. What I just realized I don't know is whether or not these dimmers effect color temperature...I've tried researching that online but haven't found anything that conclusive. Does anybody know the answer to that?

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Considering the MIRED system had to be developed because changes in Kelvin do not create a change in visible color at an equal rate (in other words, the color-correction gel that changes 3200K to 3400K does not convert 5400K to 5600K), I don't see how every volt of dimming always produces 100K of color temperature change. But that's just me, I haven't tested that. It may be that in the tungsten realm, it's close enough for practical purposes since most people are dimming 3200K down into the mid 2000's at the lowest.

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