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BLUE AND GREEN SCREEN


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#1 JA Tadena

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Posted 14 May 2006 - 04:01 AM

Hi guys. Would anyone know what IRE rating should I place my blue screen and green screen at?

I had a problem with a shoot with the F-950. I rated the camera at ASA 125 using the PRO 35 adaptor since you lose 1 1/3 a stop with the adaptor(setting my lens opening at F/2.0). I was shooting on blue chroma and getting the same f-stop reading in my incident light meter on my (person)subject and blue chroma. I usually underexpose my chroma by half a stop but in this case I thought that the blue was good enough.
My HD technician was saying that my chroma was somewhere between 20-30 IRE and she thought that it was a bit low for a blue chroma. I checked the waveform and saw it but I didnt know what IRE the blue should be on. My talent(sublject) was pretty ok hitting 60-70IRE on the waveform. So I told her that I think I have to trust my meter since I wouldnt want light the blue screen stronger than my subject.

Anyway Im sorry if i missed some threads on this.

Edited by jatadena, 14 May 2006 - 04:02 AM.

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#2 Michael Nash

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Posted 14 May 2006 - 04:49 AM

Forget the mathematics invloved with the Pro35, and forget what your meter tells you. What you see on a properly set-up waveform tells you what luminances you're recording.

If you're recording a blue screen at 20-30 IRE then that's underexposed by more than your typical 1/2 stop (but it sounds like you knew that).

Keying depends upon color saturation, and there is a "sweet spot" for most video cameras where they deliver the best color saturation for a green- or blue-screen. Usually this means 50 IRE or less; but if you go too low there is too much noise in the color to pull a clean matte.
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#3 Michael Nash

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Posted 14 May 2006 - 04:49 AM

Forget the mathematics invloved with the Pro35, and forget what your meter tells you. What you see on a properly set-up waveform tells you what luminances you're recording.

If you're recording a blue screen at 20-30 IRE then that's underexposed by more than your typical 1/2 stop (but it sounds like you knew that).

Keying depends upon color saturation, and there is a "sweet spot" for most video cameras where they deliver the best color saturation for a green- or blue-screen. Usually this means 50 IRE or less; but if you go too low there is too much noise in the color to pull a clean matte.
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#4 JA Tadena

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Posted 14 May 2006 - 04:59 AM

Thanks a lot. So you think 40-50IRE is what i should work at? How do I find out which is the best IRE setting for which camera i use?
Thanks again.

Edited by jatadena, 14 May 2006 - 04:59 AM.

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#5 Stephen Williams

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Posted 14 May 2006 - 06:00 AM

Thanks a lot. So you think 40-50IRE is what i should work at? How do I find out which is the best IRE setting for which camera i use?
Thanks again.


Hi,

Look at color bars, that will give you a good indication of the IRE value for different colours. Blue will be much lower than green.

Stephen
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#6 JA Tadena

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Posted 14 May 2006 - 10:42 AM

Oh yeah thats a great idea. Thanks a lot.
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