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  1. #1 The blue channel under 3200K vrs 5600K lighting... 
    Hi all... remember this question I had?

    http://www.reduser.net/forum/showpos...&postcount=185

    Well, thanks to Jon Firestone and Chris Bagley and RED #232 we had a chance to test the whole question about noise in the blue channel as it relates to the color temperature of the lighting. Since the sensor is daylight native it makes sense that it would produce the cleanest image in daylight, and that this might be an advantage when doing blue or green screen work.

    We shot both blue and green screen footage, changing the lighting from tungsten to daylight. Obviously the same effect can be had by filtering at the lens, but we didn't have access to the proper filter in 4 by 5 so we did it with gelling the lights and ND...

    We've just made available raw R3D clips of this test, along with some dpx's here...

    http://www.reduser.net/forum/showthread.php?t=7451

    But to cut to the chase, here's a 1:1 sized sample from both a blue and green screen, shot under both lighting conditions. It's very easy to see that if you can do it, light your key stages with daylight sources. Not only for blue screen work, but as you can see, the blue channel in the green screen plate benefits greatly.

    http://ftp.datausa.com/imageshoppe/o...el_compare.png
    Jim Arthurs
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  2. #2  
    Senior Member Nils Ruinet's Avatar
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    Wow, the difference is huge !
    Did you expose both at 320 ISO ?

    Thanks for doing this test. Maybe we'll have to keep using blue filters after all...

    Nils.
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  3. #3  
    Senior Member M.Halsell's Avatar
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    Wow! That was a gold nugget. Thanks for sharing the R&D.
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  4. #4  
    This is the same thing with every video camera. I figured this out the hard way after lighting a greenscreen and a scene with all tungsten fixtures for an HVX shoot. The closer to daylight the better with a video sensor.
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  5. #5  
    Quote Originally Posted by Slinnn View Post
    Wow, the difference is huge !
    Did you expose both at 320 ISO ?

    Thanks for doing this test. Maybe we'll have to keep using blue filters after all...

    Nils.
    Yes, 35mm zeiss superspeed, f 2.8 and 320 rated on all....

    Understand this is more important for keying than "ordinary" footage... I don't mind the natural look under tungsten in the least and wouldn't worry about it in the slightest in non-key situations. But knowledge is good, and if you know a thing, you understand a thing...

    And while this might become one of those "oh yeah, of course" points that every RED Ninjia will tell a client as rote in a bit, until this test by myself and Jon Firestone it was unknown in fact, and if people read back to just a month ago the party line from folks with deep RAW experience in general was that there was little value to be had by properly color filtering to match the sensor.

    Just sayin'.
    Jim Arthurs
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  6. #6  
    What about using actual green kinoflows ?
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  7. #7 Good to know 
    Great post, thanks a lot the pictures do tell the real story
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  8. #8  
    Senior Member Mike Prevette's Avatar
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    Wow thats such a huge difference it makes me think something might be broken with their white balance transform.


    Were these processed in RedCine and RedAlert to compare?


    You gelled the tungsten lights to go daylight right? How did you manage to keep the stop the same? did you move the lights in?
    _mike

    "One for a meal, One for the reel, or One to learn something"
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  9. #9  
    In this case, the screen lighting was the desired color temp + either 1/2 CTB for the bluescreen, or 1/2 Plus Green filtration for the green. Distance was such that there was no contamination on the subject lighting. Green (or blue) kino's would have been nice, but weren't in the budget.

    RedCine on the PC only for processing, latest version.

    We did white balance the camera to a white card and that's the meta data that was in the files (3003K and 5339K to be exact)... however, when I manually moved it back to either 3200 or 5600 there wasn't a noticable difference in the grain amount. Feel free to download the raw files and have a go... the dpx's will be back on line by evening (client upload right now)...
    Jim Arthurs
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  10. #10 much more blue light in 5600K 
    Quote Originally Posted by Slinnn View Post
    Wow, the difference is huge !
    Did you expose both at 320 ISO ?
    I gather he's putting up raw R3D files, so you can check that directly.

    As I understand it, there is roughly 5 times (that is 2.3 stops) more blue in 5600 K light as compared with 3200 K light, given the same overall exposure (that is, "L" is the same, in Lab color space).

    With 2.3 stops less blue light, I would expect to see a noise difference in the blue channel. I expect this would hold true for any camera, by the way, unless you are far enough above the noise floor not to matter.

    I used the color calculator at http://www.brucelindbloom.com/index.html?Equations.html
    and assumed sRGB has a 2.2 gamma, to work back to get linear light.
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