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Author Topic: white balance cards (raw)  (Read 9245 times)
MartinCh
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« on: January 16, 2010, 09:49:29 PM »
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a product on ebay called Color Perfect Card is selling at around $10. overall, do you think this is a worthy card in purchasing to get a more accurate white/color balance from the start? (in addition to shooting with a raw file.)

http://cgi.ebay.com/White-Balance-Card-Gra...n%3D4%26ps%3D63

here is his response in answer to my sent questions:

"Thank you for your interest in Color Perfect Cards. They were designed and are manufactured by CDI Commercial Photography, our company. Yes, they are laminate and water resistant. They resist moisture but they are not designed to be submerged. Color Perfect Cards have been in use for five years are being used by amateaurs and professionals world wide. There are several photography school in the USA and around the world that use Color Perfect Cards. If you follow the White Balance instruction for your camera using the Color Perfect Card you will get good white balance. You can also use it by taking a picture of the card in each different lighting setting. The when preparing to print, if you have a calibrated monitor you can match the CMYK/RGB color on the Color Perfect Card to the image taken in each lighting situation and know that your image color is correct."

"Again thank you for your interest in Color Perfect Cards. All image source data is on the front side of the Color Perfect Card. RGB and CMY is on the right. White is in the center with 18% gray and black (K) is on the left. A gray scale runs across the bottom of the card. The work of many professional photographers is put into print so the CMYK is important for them and their clients. When following your camera's white balance instructions zoom in close on the entire card. When the white balance is correct the color will be very close. Exact color match for catalog work etc. may require additional adjustment. To get absolute color match everytime requires a Kelvin Tempature sensor such as we use at CDI for critical color. The overall quality of white balance is greatly dependant upon the camera you use. We are unable to release our client list. We are not allowed to release any names nor do we ever sell any client lists. Sorry! I would suggest zooming in closely on the entire card for doing white balance. For doing general color matching I would only zoom in moderately, leaving ample image with the colors you are trying to match. Doing prints of exact color matches requires that your computer monitor is accurately caliberated."

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NikoJorj
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« Reply #1 on: January 17, 2010, 03:19:25 PM »
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From the ebay description:
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Unlike most white balance products the Color Perfect Card also provides the standard RGB and CMYK color matching system for exact color matching.
What are standard RGB and CMYK patches?
ECI/FOGRA29? ECI/FOGRA37? AdobeRGB/US Web coated? sRGB/US web uncoated? Etc...

One of two things :
- either you match grays (and let the color fall where they might, it's quite simple in any decent raw converter and already), and the gray part of this card may (or may not) be able to do the task, depending on what is it made of and how will it old.
- or you match colors, and then you need something more documented to rely on, like eg a colorchecker and the DNGprofile editor software.

At the very least, providing the Lab values of the CMYK/RGB color patches of this card would allow some tinkering with DNG profile creation or basic color correction work, but adding unknown colors on the card won't help I'm afraid.
As said, that doesn't preclude using the gray part if it's neutral enough.
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Nicolas from Grenoble
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BartvanderWolf
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« Reply #2 on: January 17, 2010, 05:54:47 PM »
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Quote from: NikoJorj
As said, that doesn't preclude using the gray part if it's neutral enough.

Indeed, but do we know how neutral those grays are? It may (perhaps) have an average reflectance of 18%, but is it spectrally neutral? I have measured the Babelcolor White target and the WhiBal cards, and my samples are very neutral over the most important part of the visible spectrum (the WhiBal loses some extreme blue reflectivity):


Cheers,
Bart
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MartinCh
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« Reply #3 on: January 18, 2010, 12:22:41 AM »
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Quote from: BartvanderWolf
Indeed, but do we know how neutral those grays are? It may (perhaps) have an average reflectance of 18%, but is it spectrally neutral? I have measured the Babelcolor White target and the WhiBal cards, and my samples are very neutral over the most important part of the visible spectrum (the WhiBal loses some extreme blue reflectivity):


Cheers,
Bart

what might be other good alternatives then in getting more accurate white/color balance from a raw file? from the manual, a custom white balance involves taking a white paper sheet or card, then use the cwb function button. or a white balance lens cap?
is a certain type of white card or paper recommended or a very specific 18% gray card. (Kodak?)

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Jonathan Wienke
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« Reply #4 on: January 18, 2010, 12:34:47 AM »
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Quote from: MartinCh
what might be other good alternatives then in getting more accurate white/color balance from a raw file? from the manual, a custom white balance involves taking a white paper sheet or card, then use the cwb function button. or a white balance lens cap?

First of all, when shooting RAW there is no need to use the camera custom white balance procedure--it is only used for JPEGs created in-camera and does not affect RAW files. All you need to do is photograph your white reference in the same lighting you're using for your actual subject. Then you can simply open that image and click on the white balance reference card in your RAW converter to define your white balance.

But it is critically important that your white balance reference card actually be white. If it isn't, then you will get a complementary color cast when you use it. If your WB reference has a green tint, then using it will cause a magenta cast in your photo, etc. Given the advertised price of the card you mentioned and the clueless-sounding ad copy associated with it, I'd be skeptical that it is as truly white as a WhiBal card.

One more thing: 18% gray card is for setting exposure for film, and is not suitable for setting white balance. A white reference should be white, not 18% gray.
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Henry Goh
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« Reply #5 on: January 18, 2010, 01:01:04 AM »
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Quote from: Jonathan Wienke
First of all, when shooting RAW there is no need to use the camera custom white balance procedure--it is only used for JPEGs created in-camera and does not affect RAW files. All you need to do is photograph your white reference in the same lighting you're using for your actual subject. Then you can simply open that image and click on the white balance reference card in your RAW converter to define your white balance.

But it is critically important that your white balance reference card actually be white. If it isn't, then you will get a complementary color cast when you use it. If your WB reference has a green tint, then using it will cause a magenta cast in your photo, etc. Given the advertised price of the card you mentioned and the clueless-sounding ad copy associated with it, I'd be skeptical that it is as truly white as a WhiBal card.

One more thing: 18% gray card is for setting exposure for film, and is not suitable for setting white balance. A white reference should be white, not 18% gray.


White without clipping of course
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NikoJorj
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« Reply #6 on: January 18, 2010, 05:05:30 AM »
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Quote from: Jonathan Wienke
One more thing: 18% gray card is for setting exposure for film, and is not suitable for setting white balance. A white reference should be white, not 18% gray.
What's the problem with gray?
Lack of precision due to lower RGB numers (but we're in 12 or 14 -> 16bits aren't we?), more chances of aging (I've sometimes heard that the kodak gray card may yellow over time)?
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Nicolas from Grenoble
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Jonathan Wienke
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« Reply #7 on: January 18, 2010, 06:05:27 AM »
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Quote from: Henry Goh
White without clipping of course

And a white balance card that is actually white will make it easier to ensure you're not clipping. If the card is clipped, back off the exposure. If not, you may be able to increase exposure, depending on where the card shows up in the histogram.
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Jonathan Wienke
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« Reply #8 on: January 18, 2010, 06:09:54 AM »
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Quote from: NikoJorj
What's the problem with gray?
Lack of precision due to lower RGB numers (but we're in 12 or 14 -> 16bits aren't we?), more chances of aging (I've sometimes heard that the kodak gray card may yellow over time)?

The precision is the issue; sampling to more bits doesn't help at higher ISOs where only 6-7 bits are not noise to begin with.

The Kodak gray card was never neutral to begin with. It was designed to primarily to be 18% reflective, not to be spectrally neutral.
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Shirley Bracken
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« Reply #9 on: January 18, 2010, 06:23:54 AM »
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Good information, thanks.
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ssbracken.com  (Formerly Bumperjack)
MartinCh
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« Reply #10 on: January 19, 2010, 02:09:08 AM »
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are there suggested specific white balance cards/papers in best using as a reference? (eg. white foam sheet, which doubles as flash bounce)
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Jonathan Wienke
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« Reply #11 on: January 19, 2010, 07:43:06 AM »
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I'd recommend something designed for the purpose, such as a WHiBal card or a Color Checker, which has neutral white patches you can use in addition to other reference color patches.
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digitaldog
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« Reply #12 on: January 19, 2010, 10:10:34 AM »
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Quote from: BartvanderWolf
Indeed, but do we know how neutral those grays are? It may (perhaps) have an average reflectance of 18%, but is it spectrally neutral? I have measured the Babelcolor White target and the WhiBal cards, and my samples are very neutral over the most important part of the visible spectrum (the WhiBal loses some extreme blue reflectivity):

The Bablecolor white target is awesome, has a very high Lstar value (good for gauging ETTR) and as Bart illustrates, very neutral. You get what you pay for.
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Andrew Rodney
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« Reply #13 on: January 19, 2010, 10:13:22 AM »
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Quote from: NikoJorj
What's the problem with gray?
Lack of precision due to lower RGB numers (but we're in 12 or 14 -> 16bits aren't we?), more chances of aging (I've sometimes heard that the kodak gray card may yellow over time)?

Raw is linear encoded so half of all the data is contained (with proper exposure) in the first stop of highlight data. You can GB on Gray but its hardly best practice and depending on the card, result in color shifts. Gray cards are fine for gray balancing gamma corrected images, again assuming the gray is neutral. White is better for WB Raw data.

The X-Rite Passport seems to be a great alternative considering it allows you to build DNG profiles, has a mini ColorChecker and a number of off white patches etc.
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Andrew Rodney
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« Reply #14 on: January 19, 2010, 10:14:28 AM »
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Quote from: Jonathan Wienke
The Kodak gray card was never neutral to begin with.

Kind of like Archival Matt wasn't archival <g>
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Andrew Rodney
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bjanes
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« Reply #15 on: January 19, 2010, 07:34:23 PM »
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Quote from: digitaldog
Raw is linear encoded so half of all the data is contained (with proper exposure) in the first stop of highlight data. You can GB on Gray but its hardly best practice and depending on the card, result in color shifts. Gray cards are fine for gray balancing gamma corrected images, again assuming the gray is neutral. White is better for WB Raw data.
That statement about half the data being in the first f/stop goes back to the rationale for ETTR, but does not take noise into account as Emil Martinec explains on his web site. Shot noise is highest in the brightest f/stop. The actual number of gray levels in the brightest f/stop would be many fewer 8192 for a 14 bit file with 16384 possible levels. Most of those levels consist of noise. For example, the Nikon D3 has a tonal range (Tonal range indicates how many gray levels are distinguishable up to noise in an image) of 8.72 bits according to the DXO measurements. That is 422 levels for the total range, not the 16,384 levels that one would expect from a 14 bit file or even the 4096 levels in an 12 bit file. However, it is sufficient for smooth tonal gradation.

One will get better precision by taking the white balance from a white card rather than a gray one, but how significant is this? One can estimate the difference by considering the shot noise. According to Bill Claff, the full well of the D3 is 65,600 electrons, so the shot noise would be sqrt(65600)=256, giving a c.v. (coefficient of variation) of 0.39% at base ISO. 18% saturation would collect about 8454 electrons, giving a standard deviation of 91.9 for a c.v of 1.09%. One can convert to DNs (data numbers) by dividing by the camera gain, as shown in the table. A c.v of 1% would give a very good white balance figure. At higher ISO, use of a white card would become more critical, as each doubling of ISO would reduce the number of electrons by half.

[attachment=19574:ExcelData.gif]
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Jonathan Wienke
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« Reply #16 on: January 19, 2010, 11:47:39 PM »
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It's not that big a deal at base ISO, but when shooting at higher ISOs, it can make a significant difference. I'd rather use best practice all of the time even if sometimes it's overkill, rather than be sloppy and get burned.
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ErikKaffehr
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« Reply #17 on: January 20, 2010, 12:31:41 AM »
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Hi!

1) Good explanation!

2) Definitively seen the problem at high ISO. Lot of noise different pixels give different WB

3) One thing to be avoided is clipping.

Another issue is exposure. If you expose for a white card it would be gray, Would you expose for a black card it would be gray. I'd suggest that using

- A whibal (which has both white and dark parts)

- Any good white card on gray background

- Or a Xrite color checker would be optimal

Best regards
Erik

Quote from: bjanes
That statement about half the data being in the first f/stop goes back to the rationale for ETTR, but does not take noise into account as Emil Martinec explains on his web site. Shot noise is highest in the brightest f/stop. The actual number of gray levels in the brightest f/stop would be many fewer 8192 for a 14 bit file with 16384 possible levels. Most of those levels consist of noise. For example, the Nikon D3 has a tonal range (Tonal range indicates how many gray levels are distinguishable up to noise in an image) of 8.72 bits according to the DXO measurements. That is 422 levels for the total range, not the 16,384 levels that one would expect from a 14 bit file or even the 4096 levels in an 12 bit file. However, it is sufficient for smooth tonal gradation.

One will get better precision by taking the white balance from a white card rather than a gray one, but how significant is this? One can estimate the difference by considering the shot noise. According to Bill Claff, the full well of the D3 is 65,600 electrons, so the shot noise would be sqrt(65600)=256, giving a c.v. (coefficient of variation) of 0.39% at base ISO. 18% saturation would collect about 8454 electrons, giving a standard deviation of 91.9 for a c.v of 1.09%. One can convert to DNs (data numbers) by dividing by the camera gain, as shown in the table. A c.v of 1% would give a very good white balance figure. At higher ISO, use of a white card would become more critical, as each doubling of ISO would reduce the number of electrons by half.

[attachment=19574:ExcelData.gif]
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MartinCh
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« Reply #18 on: January 20, 2010, 01:15:39 AM »
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so would you get whibal card or color checker? (about twice the price)
does color checker provide a much more accurate color balance than the whibal (or gray kard),
or what justifies the price difference?

rather than just a gray card, do you get a more versatile setup with gray, black white card included?
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NikoJorj
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« Reply #19 on: January 20, 2010, 06:03:27 AM »
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Quote from: Jonathan Wienke
The precision is the issue; sampling to more bits doesn't help at higher ISOs where only 6-7 bits are not noise to begin with.
OK, I didn't thought of high ISOs at first but that sound quite clear now that aiming the WB eyedropper at noise will just send everything overbord.

Thanks for the explanations!
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Nicolas from Grenoble
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