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White Balancing: white or "Kodak" grey card


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#1 Matteo Castelli

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Posted 29 December 2008 - 05:06 PM

Ciao,
I'd like to know your opinion about the difference between balancing the white using a white paper, or white card, or a "kodak" grey card. Actually i can't figure out a clear reason for choosing a grey card in place of a white one. More, a friend of mine told me that for digital supports the right grey is 12%, instead of 18% for film support, is it right?

Thanks,
Ciao.
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#2 Jonathan Bowerbank

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Posted 29 December 2008 - 05:51 PM

ummmm, to white balance you need something white...a grey card wouldn't work. I occasionally use a grey card for judging exposure, but never for white balancing, that wouldn't make much sense.

If it's an issue where you plan on shooting straight 3200K or 5600K and not white balancing in camera, then shooting a grey card might be a good idea so your colorist can balance to the grey card in post.
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#3 David Mullen ASC

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Posted 29 December 2008 - 06:06 PM

It doesn't matter whether you use white or grey IF the grey is truly neutral -- because white balancing means balancing the RGB gains until the image is neutral, near white, with no color cast. I think the card has to have a minimum exposure level / brightness in order for the camera to balance the levels. So you just end up overexposing the grey card until it is near white anyway in order to perform the white balance.

And of course you can white balance to non-white or non-grey objects or cards, or how else would you correct out the greenish cast from fluorescents?

Some old video textbooks I have around the house say something like this:

For white to appear neutral on a TV monitor, the image from the video camera when exposed to a white surface must be 59% green, 30% red, and 11% blue. So when using the manual white balance, basically the separate color signals are adjusted until this proportion is created. Now whether that white surface or subject has to be at a certain brightness level, like 100 IRE, I don't think so, as long as there is enough signal strength for the camera to measure the relative signal strengths in each color channel and balance them accordingly.
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#4 Jonathan Bowerbank

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Posted 30 December 2008 - 01:42 AM

That makes sense, overexposing the grey card until it's enough to white balance off of...would most likely work fine for exteriors, getting that overexposure indoors could be tough.

I forgot about that "59% green, 30% red, and 11% blue" ratio, I'm sure I read that in a similar textbook :)
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#5 Matteo Castelli

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Posted 30 December 2008 - 05:05 AM

Thank you for your answers.. expecially you, David.
I work often with professional video cameras so I must to balance the white almost always, and it's nice. I like to play with color temperature, when I have a mixed temperatures I balance several times in different spots and later I choose the one temperature I like most, even without monitor, but only on my experience.
Once I read in a too fast way something about balancing the white on a grey card, but unfortunately I don't remember exactly what that article said. But I tried to balance on a grey card and it work perfectly, even without overexposing. Usually some collegues of mine let the automatic iris working while balancing on a white paper, so the camera goes towards the "middle", the grey!
My question comes from this point: we are going to make some tests with our new Red One during this day. We want to find the better and reliable monitor to let me, or other DPs, control the images. One of my friends, a great technician here in Italy, wants to check if the white that come from the monito match with the Red one, so we have color temperature meter to have a valide reference, but I'd like to go on top of the problem and be sure about balancing the white in the better way possible.
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#6 Chris Burke

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Posted 30 December 2008 - 05:57 AM

Thank you for your answers.. expecially you, David.
I work often with professional video cameras so I must to balance the white almost always, and it's nice. I like to play with color temperature, when I have a mixed temperatures I balance several times in different spots and later I choose the one temperature I like most, even without monitor, but only on my experience.
Once I read in a too fast way something about balancing the white on a grey card, but unfortunately I don't remember exactly what that article said. But I tried to balance on a grey card and it work perfectly, even without overexposing. Usually some collegues of mine let the automatic iris working while balancing on a white paper, so the camera goes towards the "middle", the grey!
My question comes from this point: we are going to make some tests with our new Red One during this day. We want to find the better and reliable monitor to let me, or other DPs, control the images. One of my friends, a great technician here in Italy, wants to check if the white that come from the monito match with the Red one, so we have color temperature meter to have a valide reference, but I'd like to go on top of the problem and be sure about balancing the white in the better way possible.



What is your video village set up going to be? I ask because I just worked on pilot that was shooting with the Genesis and the guys up in the tent were constantly checking to see that they were recording a dense "negative". At the bare minimum, I would say that you get the best monitor possible on set, in an enclosed darkened room/tent. Cinetal or Eizo, calibrated on set. Shoot a full grey scale not just the middle value. White balancing is fine, but I think the full scale is better for a Red. Treat it more like film than video.
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#7 David Mullen ASC

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Posted 30 December 2008 - 12:40 PM

Just remember that manual white balancing on the RED is just metadata and for monitoring purposes, the camera records with a native 5000K balance or so.
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#8 Satsuki Murashige

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Posted 31 December 2008 - 03:07 AM

... I just worked on pilot that was shooting with the Genesis and the guys up in the tent were constantly checking to see that they were recording a dense "negative".

Hi Chris, were they just looking at the waveform? How do you judge the exposure of an image on set when recording in Panalog? Is there a Red 709 LUT output by the Genesis for viewing?

Shoot a full grey scale not just the middle value. White balancing is fine, but I think the full scale is better for a Red. Treat it more like film than video.

That's strange, I usually see full grey scales like this: http://www.filmtools...ompensated.html used for video shoots all the time, but never for film. What's the benefit of shooting of full grey scale for film as opposed to a regular grey card?

BTW, I recently worked on a Red short where I shot a wedge test with a Kodak grey card plus (18%, plus white and black chips), basically exactly what I'd do with a new film stock, shooting normal/over/underexposed uncorrected and then corrected back to normal while rated at 320ASA. I found it to be a useful way to decide how to rate the camera - we ended up rating the camera at 200ASA under tungsten lighting as overexposing the footage by 2/3 stop made the footage less noisy while still keeping about 3 to 3.5 stops of headroom above the clip. I think we were seeing detail about 4.5 to 5 stops into the shadows. So I think with the Red, a full grey scale would have been a very useful tool.

*Edit: we were on Build 17.

Edited by Satsuki Murashige, 31 December 2008 - 03:08 AM.

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#9 Joshua Jackson

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Posted 31 December 2008 - 03:49 AM

More, a friend of mine told me that for digital supports the right grey is 12%, instead of 18% for film support, is it right?


17.6%, to be more precise. If you divide 0% black (In reality, around 3% black) to 100% white into a "zone" system (Ansel Adams), 17.6% will give you the exact medium between the two. 3.12% (-5), 4.4% (-4), 6.25% (-3), 8.84% (-2), 12.5% (-1), 17.67% (0), 25% (+1 stop), 35.35% (+2), 50% (+3), 70.71% (+4), 100% (+5). These values are per an 11 zone scale. This "zone" scale readily applies how we see light to how the medium sees light when measured against "stops." It's a nice middle man. If you notice, 12% is roughly a full stop under 18%. So balance to 12% grey if you want to overexpose your image by a little more than a stop.

People get into heavy debates over %'s of "middle grey," but most of the time the confusion comes from calibration constants in individual meters. It's like this: REGARDLESS of what your meter tells you, you as a cinematographer will have the ultimate say "where" your "medium" tone will fall and what will and will not be picked up with detail. If you run your tests all the way through (setting printer points for black level to finding your ideal rating), you'll know whether or not that 18% grey card falls dead in the "perceptually" medium point...or not. Remember, it's called "calibrating" for a reason. Just another tool in creative control.
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#10 Walter Graff

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Posted 31 December 2008 - 12:06 PM

Hers a good article to explain the meter thing:

http://www.bythom.com/graycards.htm


As for grey, cameras don't care if it's white or any shade of grey, just as long as it has no color. But then again you don't have to white balance on "white" to get accurate color renditions. The advantage of grey over white is you are usually not overdriving exposure when using it. But then again white has an advantage in a darker scenario whre you don't have a lot of light. As you learn more about white balance you learn tricks such as white balancing on grass, the sky, and sometimes just widening out the shot you are shooting and white balancing on a wide shot for great results. People get hung up on white balance. I remember a guy who white balanced every time he panned the camera. The easy thing is that you see what you get on a monitor so can always try including different colors into a white balance to of a grey or white card to get different saturations of red or blue. What happens when you press the white balance button is the camera uses the green channel as it's 'reference' and then compares red and blue to it and adjusts the red and blue gains to be in line with green. Your camera now knows what white is for that shot.

The numbers used in an earlier post 59/30/11 are the perfect numbers for a weighted average for calculating luminance in a histogram of NTSC, but due to settings in a camera they may or may not be exactly accurate related to white balance. Those colors in the RGB world actually create 101° green meaning the picture would be more like green algae. The white in a camera comes from the fact that RGB is a distortion of the hue/value relationship. In other words every hue has a different brightness in a video camera. Actually like the human eye "white" actually has a green hue to it. This will bring up the conversation of why uncorrected on a film scanner the picture always has a green tinge to it before anyone "normalizes" it.
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#11 Walter Graff

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Posted 31 December 2008 - 01:08 PM

Oh yea, and some of you may have noticed that your cameras color temperature indicator does not match a color meter. This is normal as color meters are three color devices and cameras work off two channel minus scenario to show color temp. Plus if you have adjusted internal controls in the camera such as the red gain in the camera slightly elevated when white balance you will get a different number that is correct for your eye but may not seem accurate for white balance. Any adjustment in the camera when you white balance will affect that number and it's another reason why two of the same cameras might white balance together but show different color temp readouts even though the pictures look the same.
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#12 Matteo Castelli

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Posted 01 January 2009 - 05:28 PM

Thank you friends for your precious answers! I've learn a lot from you and I'll take care of your replies..
Walter you are so perfect in all your expositions!
I'll keep you informed the way our tests with Red will go..

Thanks againg and happy new year

ciao,
Matteo
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