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View Full Version : Review: Canon EOS WB shift


Doug Kerr
April 15th, 2008, 06:18 AM
The Canon EOS cameras have a "white balance shift" setting. This provides for setting a "color correction" of up to ±9 units along two axes, designated "amber-blue" and "green-magenta".

I thought it would be interesting to see just what these two axes are and how they relate to the axes of known color spaces.

I tested by having my EOS 20D regard a WhiBal gray card exposed to a consistent ambient illumination. I used the CWB function to get a basic WB correction for the illumination. Then I took test exposures with the WB shift set to all nine possible combinations of -5, 0, and +5 units on each of the two axes (treating the "amber" direction and "green" directions as positive, the convention used in reporting the WB shift setting in the Exif metadata).

I plotted the results on several different coordinate planes, along with vectors showing the "directions" of the chromaticity of the three color space primaries (R, G, and B) and the complementary hues C, Y, and M. The result for the "0,0" setting was used as the reference, arbitrarily plotted at the origin of the coordinate system.

The results on the CIE u-v plane were the most telling. There, as a matter of fact, the six "primary" vectors are almost equally-spaced angularly.

Here we see that plot. The axes are labeled "du" and "dv" to reflect that this is a "differential" plot; the actual origin of the u-v plane is far off this chart.

http://Pumpkin.Annex.home.att.net/figures/20d_wb_shift_uv.jpg

Note that since I read the resulting chromaticities of the color-corrected images on an 8-bit RGB basis, the plot is susceptible to errors from quantizing that can approach 0.001 u-v unit.

The light violet lines connect the extreme points of the constellation of data points, a primitive way to estimate the orientation of the two axes without benefit of formal linear regression analysis.

The two black double-ended arrows show the orientations of these two axes.

We see that the two axes are not quite orthogonal (at right angles) on this plane. The "green-magenta" axis is seen to be closely aligned with the green and magenta primary vectors. The "amber-blue" axis falls between two pairs of primary vectors; thus we see that the labels "amber" and "blue" are somewhat arbitrary. (The two directions are actually an "orange" hue and a "cyan-blue" hue.)

On the larger front, we note that evidently the chromaticity shift produced by this setting "piles onto" the shift established by the "other" white balance setting, regardless of the choice. That is, whatever color shift would be produced by the current WB setting (including a decision made by the camera's AWB algorithm), the "WB shift" adds to that.

The Canon manuals generally indicate that, with respect to settings along the amber-blue axis, one unit of shift corresponds to 5 mireds of color temperature correction.

The mired is a unit of inverse color temperature (now preferably called the "mirek", or even more preferably, the "micro reciprocal Kelvin")

It properly only quantifies chromaticity shift along the "blackbody locus", that is. among the chromaticities produced by a black body radiator at various temperatures. The "amber-blue" axis of the Canon WB shift lies generally in that direction. (Since the locus is a curve on the u-v plane, no axis can truly follow it along its length.)

Similarly, the green-magenta axis, being orthogonal to the amber-blue axis on the CIE u-v plane, is generally the direction along which we measure the departure of a chromaticity from the blackbody locus (the direction reflected by the "tint" parameter in Photoshop).

Michael Desilets
January 11th, 2010, 05:30 PM
i have recently had a conversation with the support people at canon about the issue of value associated with the wb bias of green/magenta on the wb shift function. the blue/amber has a 5 mired value per shift on the blue/amber axis. why is there no known value for the green/magenta shift on the opposed axis?

the support people can't get an answer from the developers at canon, so i pose the question to those who may have used this function. is there anyone out there who has any idea what the value is for the cct of the green/magenta shift?

Doug Kerr
January 11th, 2010, 06:17 PM
Hi, Michael,

i have recently had a conversation with the support people at canon about the issue of value associated with the wb bias of green/magenta on the wb shift function. the blue/amber has a 5 mired value per shift on the blue/amber axis. why is there no known value for the green/magenta shift on the opposed axis?

The mired (today often called the mirek) is a measure of a shift in coordinated color temperature. (Recall that definition is in terms of the unit of coordinated color temperature, the Kelvin.)

A chromaticity shift essentially parallel to the "Planckian locus" on the CIE chromaticity diagram represents a change in correlated color temperature, and thus the amount of such a shift can be properly described in mireds.

The Planckian locus is the path of all points representing the chromaticity of a "blackbody radiator" at different temperatures. Only points on that curve have actual color temperatures. For chromaticities near, but not on, the curve, we can state a "correlated color temperature", which is the temperature of the nearest point on the curve. When we hear of the "color temperature" of a light source, it is usually the correlated color temperature that is actually meant.

The amber-blue axis of the Canon white balance color correction bias adjustment is generally in a direction parallel to the Planckian locus, and thus changes along that axis are reasonably quantified in mireds.

A chromaticity shift perpendicular to the "Planckian locus" on the chromaticity diagram represents no change in correlated color temperature, and thus the amount of such a shift in mireds would always be zero.

The green-magenta axis of the Canon color correction adjustment is in a direction generally perpendicular to the Planckian locus, and thus changes along that axis would basically have a mired measure of zero. (They would have a small mired value, but it would not be meaningful.)

So the real answer to your question is that along the green-magenta axis there is no change in mireds (at least the mired value would not be meaningful). And I guess that's why the Canon guys do not know the answer!

Best regards,

Doug

Cem_Usakligil
January 11th, 2010, 11:00 PM
Hi Doug,

This is a great essay. Although I already knew a lot about color spaces and WB, this essay has helped me understand certain issues much more clearly than I did before. The information presented is generally applicable regardless of the fact that it is about the WB shift of the Canon cameras. For example, it gives a good insight into why WB controls in raw converters have G/M as their tint axis.
At first, I have tried to decipher the graph without reading your text and I have come a long way. This means that the graph is drawn very well and speaks for itself. Your text is, of course, essential. Thanks so much for helping us out with this knowledge, much appreciated.

Cheers,

PS: I have just seen this essay although it was posted almost 20 months ago. Why were there no reactions at that time? Go figure!

Bart_van_der_Wolf
January 12th, 2010, 01:14 AM
A chromaticity shift essentially parallel to the "Planckian locus" on the CIE chromaticity diagram represents a change in correlated color temperature, and thus the amount of such a shift can be properly described in mireds.

The Planckian locus is the path of all points representing the chromaticity of a "blackbody radiator" at different temperatures. Only points on that curve have actual color temperatures. For chromaticities near, but not on, the curve, we can state a "correlated color temperature", which is the temperature of the nearest point on the curve. When we hear of the "color temperature" of a light source, it is usually the correlated color temperature that is actually meant.

The amber-blue axis of the Canon white balance color correction bias adjustment is generally in a direction parallel to the Planckian locus, and thus changes along that axis are reasonably quantified in mireds.

A chromaticity shift perpendicular to the "Planckian locus" on the chromaticity diagram represents no change in correlated color temperature, and thus the amount of such a shift in mireds would always be zero.

The green-magenta axis of the Canon color correction adjustment is in a direction generally perpendicular to the Planckian locus, and thus changes along that axis would basically have a mired measure of zero. (They would have a small mired value, but it would not be meaningful.)


Hi Doug,

My compliments, you've worded that very well.

For those who want to get into a lot more detail, I refer to the excellent site of Bruce Lindbloom (http://www.brucelindbloom.com). He also offers ANSI C program code (http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_T.html) to calculate the Coordinated Color Temperature from an XYZ color coordinate (using Robertson's algorithm), and formulas to convert to and from other coordinates, such as Luv or xyY (click on the "Math" button on his site to get an overview of the available conversions).

Cheers,
Bart

Michael Desilets
January 12th, 2010, 08:58 AM
Thanks guys for the quick attention to this matter. Perhaps, I was not clear in expressing my initial question to the group.

To rephrase:

If I am in a situation where I needed a plus green filter, could I adjust by using the green/magenta bias to the wb/shift program on my Canon camera?

And, by how many steps/shifts along the green/magenta bias (not the blue/amber bias), would equal a Cinegel Tough ˝ Plusgreen filter value of CC15 Green?

I apologize for the previously phrased question.

Regards,

Bart_van_der_Wolf
January 12th, 2010, 10:38 AM
Thanks guys for the quick attention to this matter. Perhaps, I was not clear in expressing my initial question to the group.

To rephrase:

If I am in a situation where I needed a plus green filter, could I adjust by using the green/magenta bias to the wb/shift program on my Canon camera?

There's a difference between shooting through a filter, and changing the whitebalance after the fact. Can't you just include a WhiBal or other gray object in a test shot and do the colorbalancing afterwards? It'll be much more accurate as well.

And, by how many steps/shifts along the green/magenta bias (not the blue/amber bias), would equal a Cinegel Tough ˝ Plusgreen filter value of CC15 Green?

I'd have to look that up and calculate, but it seems like an exercise that's not too useful. It will stiil be postprocessing when you don't alter the colorbalance of the light that will fall on your sensor.

Cheers,
Bart

Cem_Usakligil
January 12th, 2010, 12:03 PM
Thanks guys for the quick attention to this matter. Perhaps, I was not clear in expressing my initial question to the group.

To rephrase:

If I am in a situation where I needed a plus green filter, could I adjust by using the green/magenta bias to the wb/shift program on my Canon camera?

And, by how many steps/shifts along the green/magenta bias (not the blue/amber bias), would equal a Cinegel Tough ˝ Plusgreen filter value of CC15 Green?

I apologize for the previously phrased question.

Regards,

If I understand you correctly Michael (of which I am not certain):
1) There is a shot which should have been taken using a green filter.
2) You took the shot without using the filter.
3) In the raw conversion or post processing, you want to simulate the green filter's effect by making the tint greener.

The other scenario being:
A) There is a shot which was taken using a green filter.
B) In the raw conversion or post processing, you want to nullify the the green filter's effect by making the tint more magenta.

So which scenario are we talking about? 1-2-3 or A-B? Or a third one?

If we are talking about scenario A-B, Bart's advice of using a WhiBal or a gray card is quite sensible.

If we are talking about scenario 1-2-3, there are simulated photo filters in photoshop which add any tint to your image after it has been converted normally via the raw converter. There are many ways to achieve this in the post.

Sorry if I misunderstood this totally :-).


Cheers,

Asher Kelman
January 12th, 2010, 01:35 PM
If I understand you correctly Michael (of which I am not certain):
1) There is a shot which should have taken using a green filter.
2) You took the shot without using the filter.
3) In the raw conversion or post processing, you want to simulate the green filter's effect by making the tint greener.

The other scenario being:
A) There is a shot which was taken using a green filter.
B) In the raw conversion or post processing, you want to nullify the the green filter's effect by making the tint more magenta.

So which scenario are we talking about? 1-2-3 or A-B? Or a third one?


Cem,

The other scenario is that one wishes to take a picture with a green filter, but achieve the same reusult through color balance adjustments. What settings correspond to what filters?

In practice would one take a WB shot, then adjust for for the wanted green filter effect and in processing use the WB shot taken before the effect was added?

Asher



Asher

Cem_Usakligil
January 12th, 2010, 02:02 PM
...The other scenario is that one wishes to take a picture with a green filter, but achieve the same reusult through color balance adjustments. What settings correspond to what filters?

In practice would one take a WB shot, then adjust for for the wanted green filter effect and in processing use the WB shot taken before the effect was added?

Hi Asher,

This is my scenario 1-2-3, please re-read what I wrote. In this scenario, I would set the WB as normal during raw conversion and then afterwards add the green filter effect in photoshop.

Cheers,

UPDATE: Now that I've finally understood what Michael wanted, I also think that your message was actually about the in camera WB correction. If so, then I stand corrected, that was my third scenario indeed :-)

Michael Desilets
January 12th, 2010, 02:27 PM
This example should clear up the use for the green balance shift theory that I have.

Under fluorescent lighting requiring a Cinegel Tough ˝ Plusgreen filter value of CC15 Green to my strobe to balance the light, could I conceivably use the green/magenta bias (knowing the value to the shift) to bring the scene back to a neutral/white balance?

It is just an idea and was wondering if there was a way to work this in camera rather than PP.


I really appreciate all the attention to this matter.

Cem_Usakligil
January 12th, 2010, 02:40 PM
This example should clear up the use for the green balance shift theory that I have.

Under fluorescent lighting requiring a Cinegel Tough ˝ Plusgreen filter value of CC15 Green to my strobe to balance the light, could I conceivably use the green/magenta bias (knowing the value to the shift) to bring the scene back to a neutral/white balance?

It is just an idea and was wondering if there was a way to work this in camera rather than PP.


I really appreciate all the attention to this matter.
This is rather straightforward to achieve in the camera. Just use a gray card or a white sheet to photograph under those conditions you've mentioned (i.e. fluorescent light and flash without the Plusgreen filter) and use that image to set the custom WB setting of the camera. Afterwards, use the custom WB for that shoot. That's pretty much it.

HTH,

Michael Desilets
January 12th, 2010, 03:53 PM
Thank you all for your attention and diligent effort in understanding such a difficult topic.

I look forward to implementing your suggestions and sharing the results.

Regards,

Asher Kelman
January 12th, 2010, 03:57 PM
This is rather straightforward to achieve in the camera. Just use a gray card or a white sheet to photograph under those conditions you've mentioned (i.e. fluorescent light and flash without the Plusgreen filter) and use that image to set the custom WB setting of the camera. Afterwards, use the custom WB for that shoot. That's pretty much it.

HTH,
Cem,

The two scenarios do not give the same mathematics. When one balances the lights with filters, the values of the numbers that are low will be recorded with a certain accuracy depending on how many photons arrive and what the noise level is at various stages. The exposure will have to be longer to account for the light loss of the filter for the wavelengths one wants conserve. This maximized the photon counts for the lower numbers. One, in practice is strengthening that accuracy of counting photons in the image you will finally get. If this is done in PS, then one is boosting the value of lower numbers to achieve the required color balance. That is not as good as having those levels to begin with. So it's seems to me to be always best to expose with the spectral distribution one wants for the type of image one is making.

If the image is bright, the difference might not be great. However, with a full range of tonalities, likely the image will be better where corrections are made at the time of exposure.

Asher

Bart_van_der_Wolf
January 12th, 2010, 04:11 PM
Under fluorescent lighting requiring a Cinegel Tough ˝ Plusgreen filter value of CC15 Green to my strobe to balance the light, could I conceivably use the green/magenta bias (knowing the value to the shift) to bring the scene back to a neutral/white balance?

Given the generally problematic emission spectrum of fluorescent lighting, it is best to modify that at the source. Because of practical reasons (having to fit all fluorescents with daylight sleeves) you instead chose to modify the strobes so they match the fluorescents. If you didn't, you'd have mixed lighting which is a pain to colorbalance afterwards. An internal camera adjustment cannot correct for mixed lighting.

Since you now have an overall greenish lighting (fluorescents and filtered strobes), it is relatively easy to white balance by creating a "Custom White Balance" in your camera and shoot with that. That's much easier than adding an offset to another colorbalance. The overall correction for the greenish lighting should be pretty good for spectrally uniform materials, but for critical colors you may still suffer some color inconsistency or metamerism effects. It is better to have a daylight type of lighting from the onset.

Alternatively you could delay the whole color balancing (from green to neutral) to postprocessing, because there is little difference between the camera applying a (custom) color balance, or the Raw converter on your computer. They both alter the same Raw data, they are both doing postprocessing. The only benefit from doing it in camera is that your embedded thumbnails have decent colors.

Cheers,
Bart

Gina Stephen
March 28th, 2011, 07:52 AM
Hi Doug,

This is a great essay. Although I already knew a lot about color spaces and WB, this essay has helped me understand certain issues much more clearly than I did before. The information presented is generally applicable regardless of the fact that it is about the WB shift of the Canon cameras. For example, it gives a good insight into why WB controls in raw converters have G/M as their tint axis.

I agree with the above statement. As a newbie, it's good for me to read stuff like this. My background is mostly in family portraits and wedding photographer. It's good for me to read things that are slightly over my head, and learn new things about color and features. Thanks for the information.

Doug Kerr
March 28th, 2011, 08:13 AM
Hi, Gina,

I agree with the above statement. As a newbie, it's good for me to read stuff like this. My background is mostly in family portraits and wedding photographer. It's good for me to read things that are slightly over my head, and learn new things about color and features. Thanks for the information.
Thanks for writing.

Best regards,

Doug

Gina Stephen
April 11th, 2011, 08:28 AM
Thanks for the warm welcome Doug! As I said, some of this stuff flies above my head, but I think that is a good thing! I come from the family of a wedding photographer (http://www.angieslist.com/photography/) and a science nerd. I want to learn more about the science of our art so I can do a better job with my pictures!

Nick Pudar
April 24th, 2013, 12:31 PM
Hi everyone. I'm new to this forum, and have been very impressed with the quality of content. I stumbled upon it while researching how to properly use a Sekonic C-500 Color Meter with my Canon camera. I am contemplating getting the C-500, and I want to make sure that it will work for me before I spend any money.

First, Doug, the picture you included in the original post appears to be missing from the hosting site. Would it be possible to re-post the image?

Let me start with my current situation, and why I think the C-500 will help me, and then I will ask the questions that I am trying to understand. I do a lot of indoor gymnasium sports shooting with ambient light. I shoot RAW, but I want the WB setting to be at a good starting point so that I don't have to do much post process color correction. So, to do this, I first take a reading of light with a light meter so that I can set my manual exposure. Then I set my custom white balance function with an X-Rite ColorChecker Passport card. This process generates some great results as a starting point, and it saves me a lot of time in post process.

Sometimes it is awkward and intrusive to shoot the ColorChecker card when there are other public activities going on (I like to take that shot in the middle of the gym floor where the lighting is consistent with the later shots). I started to explore color meters, and was surprised by how expensive they were. But, for that price, I thought that the quality of result (color accuracy) should be superior to my current approach.

So I began to see what I could learn about how to use a C-500 with my Canon camera, and was surprised how little information there was from either company, or from any real people.

Piecing together what I have gleaned, here is how I think it would work, and I would love to get any informed feedback and commentary. The C-500, when reading ambient light, provides the color temperature in Kelvin, and the Green/Magenta shift filter compensation in units they call CC Index values. For my camera, I believe that I would put it in Custom Temperature mode, and inputting the Kelvin reading directly. Then for the CC Index value, I would go into the White Balance Correction screen. Since the Kelvin reading gets the Blue/Amber level right, that would be set at 0, and the Green/Magenta data would be converted to a corresponding value on the vertical axis only. (Does this make sense? I think it does, but I may be missing something.)

The conversion of CC Index value to the steps (0 to 9) on the Canon screen is the part that has no documentation anywhere. The only thing ai could find is a dpreview.com post from five years ago, where kbentsen did his own test and found that 1 Canon step was equal to 4.3 CC Index values.

Assuming that kbentsen is right, let me use this example: I get a reading from the Sekonic C-500 that shows 4600K, and 18G. This means that I should use a corresponding level of green filter on my lights. But I am in a gym and can't control the lights, so I want to make that green adjustment in the camera. So, I enter 4600 in the Custom Temperature setting, and I set the WB Shift 4 steps towards Green (18 / 4.3).

My questions:
1) does my understanding make sense?
2) does anyone have a C-500 and use it like this
3) would this be more accurate than my current ColorChecker card Custom White Balance approach?

Thanks in advance for any insights!
Nick

Nick Pudar
April 25th, 2013, 07:21 AM
...related to my last post...

I am thinking about renting a Sekonic C-500 and just running some tests myself to see if I can answer my own questions.

Any recommendations on how I should design and conduct a meaningful test?

Essentially I want to know how to use the readings from the C-500 used in ambient light mode with my Canon camera in a way that will give me better White Balance results than I could with just a grey card and the Custom White Balance function.

Thanks,
Nick

Cem_Usakligil
April 25th, 2013, 07:35 AM
Hi Nick,

Welcome to OPF, I hope you will like it here. I don't have the answers you are looking for. I am sure that Doug will eventually chime in but I think that he is on the move for the next few days. So don't get discouraged if it takes a while.

Bart_van_der_Wolf
April 25th, 2013, 08:48 AM
Hi everyone. I'm new to this forum, and have been very impressed with the quality of content. I stumbled upon it while researching how to properly use a Sekonic C-500 Color Meter with my Canon camera. I am contemplating getting the C-500, and I want to make sure that it will work for me before I spend any money.

Hi Nick,

Welcome to OPF.

The Sekonic C500 Color Meter is unique in the sense that it not only has a traditional (film based) filter set for measuring the spectrum, but also a more Digital filter set. That should help in getting things a bit more predictable when used with a DSLR or Medium Format Digtal Back.

However, these meters are more suited for balancing the incident and ambient light colors (by the use of light modifier filters and/or camera/lens filters), than as a guide for dialing in a WB setting for postprocessing.

The reason for that is two-fold. For one the Color Temperature and Tint controls between different Raw converters / photo-editors will give different value readouts when you click on something with known spectrally neutral reflection. So input of a given Color temperature will result in different color rendering responses. The other reason is that it is much easier (and cheaper) to use something like a WhiBal or a ColorChecker card to get a neutral White Balance with a mouse click in the Rawprocessor, assuming it is used correctly (i.e at the position of the main subject, and perpendicular to the camera).

I do a lot of indoor gymnasium sports shooting with ambient light. I shoot RAW, but I want the WB setting to be at a good starting point so that I don't have to do much post process color correction. So, to do this, I first take a reading of light with a light meter so that I can set my manual exposure. Then I set my custom white balance function with an X-Rite ColorChecker Passport card. This process generates some great results as a starting point, and it saves me a lot of time in post process.

That's the best way to do it, IMHO, as long as you shoot Raw image files. It also takes into account a certain amount of reflection from the floor color, as long as you measure the WB at approx. eye- or chest level.

Sometimes it is awkward and intrusive to shoot the ColorChecker card when there are other public activities going on (I like to take that shot in the middle of the gym floor where the lighting is consistent with the later shots).

This won't be any different with a Color meter, that too must be used just like an incident light meter, at the subject position.

So the best you can do is set a Color Temperature and Tint in you Camera if you want the JPEG preview to be approximately correct (based on experience, the balance between fluorescents and other light sources, and the reflection from the floor color), or use a camera preset like Fluorescent or Tungsten, although the Autosetting is often not too bad (it will never be accurate with such mixed lighting anyway).

Cheers,
Bart

Nick Pudar
April 25th, 2013, 10:58 AM
Cem, Bart, thanks for the welcome!


However, these meters are more suited for balancing the incident and ambient light colors (by the use of light modifier filters and/or camera/lens filters), than as a guide for dialing in a WB setting for postprocessing.

The reason for that is two-fold. For one the Color Temperature and Tint controls between different Raw converters / photo-editors will give different value readouts when you click on something with known spectrally neutral reflection. So input of a given Color temperature will result in different color rendering responses. The other reason is that it is much easier (and cheaper) to use something like a WhiBal or a ColorChecker card to get a neutral White Balance with a mouse click in the Rawprocessor, assuming it is used correctly (i.e at the position of the main subject, and perpendicular to the camera).


Bart, I appreciate the reminder about the Raw processor affects. That leads me to a different question, which strays slightly from my original post. (I am always amazed by the fact that the more I know, the more there is to learn!)

I currently use the ColorChecker card for my white balance to then set the camera with its Custom White Balance function. I also take a shot of the Gretag Macbeth color chart of the passport for later color correction. I import the photos into my Lightroom Catalog, and I use the X-Rite Plugin to adjust the colors for my camera sensor.

I do not reset the white balance by using the eyedropper on the WhiBal card, since I believe the camera Custom White Balance already took care if that.

My new question is about how the Gretag Macbeth adjustment for my sensor may be messing things up, or whether it is not a factor in the White Balance part of the process.

Anyway, back to my original question. I'm still wondering if the C-500 could (with appropriate testing and translation tables) be used as I am hoping with a quality result.


This won't be any different with a Color meter, that too must be used just like an incident light meter, at the subject position.


Yes, you are right, but it is much easier to walk onto the floor and take a quick reading with the meter than to have to have someone stand and hold the chart for me to take a picture of. Most time, I don't have someone else to hold the card, so I perch it on the floor (at a slight angle to get less of the floor coloring reflection). I realize this is not good, but it is the best I can do in most cases.

I've been thinking a bit more about how I would set up a test with a rented C-500.
I would go to my daughter's high school gym, armed with a large, high quality grey card, a full-size Gretag Macbeth color chart, and my X-Rite ColorChecker Passport. I would replicate (with good annotation) my current process with the Passport, and also with the full size cards. I would have the pictures taken at appropriate height from the floor, and I would also do the floor-perch Passport card to later compare how off that really is. With the camera's Custom White Balance set with each set of cards, I would then take my reference photos of the Grey Card.

I would then, for comparison, use the C-500 to dial in the Custom Temperature and White Balance Shift in the camera. Again, I would then take other reference photo of the Grey Card.

Next, I would import everything into Lightroom. I would open all the photos of the Reference Grey Cards in Photoshop, convert them to Lab color space, and with the eyedroper tool, see where the grey card reads on the a and b channels. A good result should read a 0 on both of those channels at all points of the grey card.

There are some other testing aspects that I have not thought through to convert the C-500 CC Index values into appropriately translated WB Shift steps on the camera. I do realize that I have to solve this before I can do the above gymnasium test.

Does this test approach seem reasonable?

Doug Kerr
April 26th, 2013, 03:51 AM
Hi, Nick,

Welcome to the forum.
Hi everyone. I'm new to this forum, and have been very impressed with the quality of content. I stumbled upon it while researching how to properly use a Sekonic C-500 Color Meter with my Canon camera. I am contemplating getting the C-500, and I want to make sure that it will work for me before I spend any money.

First, Doug, the picture you included in the original post appears to be missing from the hosting site. Would it be possible to re-post the image?
Yes, I changed my web host.

I'm on-the-road until Sunday (2013.04.28), but when I get home I should be able to locate that figure and repost it.

I haven't thought much about this topic for a while, and I will have to revive my thoughts! But a quick look at your message suggests to me that you are on the right track.

Let me review it a little later today (after I really wake up!) and get back to you further.

Again, it is nice to have you with us.

Best regards,

Doug

Doug Kerr
April 26th, 2013, 06:08 AM
Just a small niggle regarding the use of a reflective-neutral card as a target in connection with white balance color correction.

In theory, at least, the card should be at the subject location, and oriented parallel to the surface of the subject for which you are most keen to have the proper color correction.

The reason is that you want the card illuminated the same way "that surface of the subject" is by the various light sources.

Of course, which way it is oriented is only of any consequence if the subject is illuminated by different light sources, with their light arriving from different directions, and with their light having different chromaticities.

Of course, for a non-planar subject, which surface will we be most keen to have "correctly balanced"? Well, we must decide, Is it the model's left cheek, or right cheek, or forehead.

If we can't decide, then our only choice is to hold the card at whatever angle is handy and take "pot luck".

And of course in many cases of mixed light, we would not want "ideal" white balance color correction for all surfaces of the subject. If the subject's left side is illuminated by firelight, for example, we may well want that side to look "reddish-yellowish" (which ideal color correction would overcome).

Rules like "have the card face the camera" or "have the card face a direction halfway between the azimuths to the two principal light sources" may work, but only by accident.

Some may find this article interesting:

http://dougkerr.net/Pumpkin/articles/White_Balance.pdf

Best regards,

Doug

Doug Kerr
April 26th, 2013, 06:25 AM
Just in the interest of greatest precision (and the lack of it here causes no misunderstanding), note that only light whose chromaticity falls on the Planckian locus (so-called blackbody radiation) has a color temperature.

For any other kind of light, the first property reported by a color meter of the type discussed in this thread, on Kelvin, is actually the correlated color temperature (CCT).

Again, there is no real misunderstanding here if we are not precise. Usually.

The second property reported is the distance (on a certain chromaticity plane) between the chromaticity of the light being measured and the chromaticity of the nearest point (reckoned on that plane) on the Planckian locus. And it is the color temperature of that chromaticity (yes it has one) that is called the correlated color temperature of the chromaticity of the light being measured.

I am not sure what the unit is for that quantity (the Planckian offset) as reported by the meter under discussion. (I probably knew once!). The basic scientific unit is the "u-v unit" - the unit of the scale of the axes of the CIE u-v chromaticity plane.

Some may find this article of interest:

http://dougkerr.net/Pumpkin/articles/Color_Temperature.pdf

Best regards,

Doug

John Kirby
April 27th, 2013, 03:23 PM
Hi Nick,

Chiming in a little late here, but have you considered trying an incidence collector filter, such as an ExpoDisc (or a cheaper alternative) to determine your custom white balance? Certainly much easier to carry around than cards/charts, and you don't need any assistance to hold anything. So a darned side quicker to boot.
However, it's still not going to be the magic bullet in mixed light source situations; they will continue to be problematic.

John.

Nick Pudar
April 27th, 2013, 09:06 PM
John,
Thanks, but I do have an ExpoDisc which I used before I got the ColorChecker Passport. It does a reasonable job, but as you said, it can't really compensate for the mixed lighting in the gym, (or the excessive floor coloring).
Nick

Doug Kerr
April 28th, 2013, 05:17 PM
Hi, Nick,

John,
Thanks, but I do have an ExpoDisc which I used before I got the ColorChecker Passport. It does a reasonable job, but as you said, it can't really compensate for the mixed lighting in the gym, (or the excessive floor coloring).
Nor can a neutral target, nor in fact, white balance color correction in general.

Best regards,

Doug

Nick Pudar
April 28th, 2013, 07:17 PM
Doug,
I read the articles you linked to, as well as many more in your photography collection. Really great stuff!! I have a much better understanding of the content. Thank you.

I'm going to most likely rent a Sekonic C-500 to do my own tests as described in an earlier post of mine in this thread. As I mentioned, the important first step is to try to figure out what the transfer function is from the Sekonic CC Index (Wratten Filter designation) and the Canon WB Shift +/- 9 steps in the Green/Magenta axis.

Here is my thinking on that, and I'd appreciate any commentary or advice.
First, I bought some large lots of Wratten Filters on eBay from someone who obviously has no more use with them. There is a large collection of some used, and some unopened, in varying strengths and colors.

I'm thinking of gelling my strobes with varying levels of Green and Magenta and taking shots of a large high quality grey card. I will then remove the gels and take photos of the grey card at all 19 levels of the G/M axis of WB Shift. For all of these shots described above, I will also take a reading from the C-500 to see what the CC Index shift off of the CCT is being measured. I will then figure out how to plot all of this data to see where the equivalency levels are. Hopefully it is some sort of linear response so that future translations are easy to calculate. Once I have this mapping figured out, then I can do my other tests to see how the quality of using the Sekonic is.

Anyway, that's what I'm thinking for now.

Any thoughts would be appreciated.

Best regards,
Nick

Bart_van_der_Wolf
April 29th, 2013, 02:29 AM
Here is my thinking on that, and I'd appreciate any commentary or advice.
First, I bought some large lots of Wratten Filters on eBay from someone who obviously has no more use with them. There is a large collection of some used, and some unopened, in varying strengths and colors.

I'm thinking of gelling my strobes with varying levels of Green and Magenta and taking shots of a large high quality grey card.

Hi Nick,

Just 2 things for consideration.

The Wratten filters are intended to be used in the optical path. I'm not sure if they will survive the strobe's light intensity, and they may significantly change characteristics with such use. You may need to consider some dedicated light modifier gels by Lee (http://www.leefilters.com/lighting/technical-list.html#&filter=conversion) (some of which are available as High Temperature versions) if you want to modify your flash. Of course modifying the flash will also reduce it's output power.

The other thing is that many Grey cards are not necessarily spectrally neutral. Afterall, they are usually intended to determine exposure, not necessarily white-balance. So use a good quality grey card that has a known spectrally neutral reflection. I once measured the WhiBal cards (http://www.openphotographyforums.com/forums/showthread.php?p=44252#post44252), and they seemed to be reasonably uniform, good enough for white balance tests on digital cameras (which have limited UV sensitivity and usually a built-in IR rejection/absorption filter).

Cheers,
Bart

Doug Kerr
April 29th, 2013, 10:26 AM
Hi, Nick,

Doug,
I read the articles you linked to, as well as many more in your photography collection. Really great stuff!! I have a much better understanding of the content. Thank you.
Thank you. This is all fun stuff!

I'm going to most likely rent a Sekonic C-500 to do my own tests as described in an earlier post of mine in this thread. As I mentioned, the important first step is to try to figure out what the transfer function is from the Sekonic CC Index (Wratten Filter designation) and the Canon WB Shift +/- 9 steps in the Green/Magenta axis.

Yes, that would be valuable.

Here is my thinking on that, and I'd appreciate any commentary or advice.
First, I bought some large lots of Wratten Filters on eBay from someone who obviously has no more use with them. There is a large collection of some used, and some unopened, in varying strengths and colors.

I'm thinking of gelling my strobes with varying levels of Green and Magenta and taking shots of a large high quality grey card. I will then remove the gels and take photos of the grey card at all 19 levels of the G/M axis of WB Shift.

It would be even better to move along both axes (even though I know that will be a lot more work).

How will you quantify the results of those test shots? Will you perhaps observe which pair of shift coordinates gives a "neutral" (R=G=B) chrmaticity for the image of the target?

What WB will you have set in the camera (there is no "none") or in the RAW processing (there is no "none")?

For all of these shots described above, I will also take a reading from the C-500 to see what the CC Index shift off of the CCT is being measured. I will then figure out how to plot all of this data to see where the equivalency levels are. Hopefully it is some sort of linear response so that future translations are easy to calculate. Once I have this mapping figured out, then I can do my other tests to see how the quality of using the Sekonic is.

It looks as if that is a good approach. It seems as if you will be able to follow the trail! Be sure to not miss any of the stages (including those I raised questions about above).

Let us know how it unfolds.

Best regards,

Doug

Jerome Marot
April 29th, 2013, 01:48 PM
I had the curiosity to check the manual of my camera (Sony). It also has a compensation scale of ±9. The manual reads:

Based on using the set color temperature as the standard, the color can be compensated to G (Green) or M (Magenta). The color filter value can be selected between G9 and M9. The increment is approximately equivalent to CC filter number 5.

The manual continues with:

Note
Since most of the color meters are designed for film cameras, using the measurements obtained from them may not give correct results under fluorescent/ sodium lamp/mercury lamps. We recommend that you perform the compensation according to a test shooting or use the custom white balance.

On a Nikon manual:

Use the multi selector to fine-tune white balance. White balance can be fine-tuned on the amber (A)–blue (B) axis and the green (G)–magenta (M) axis. The horizontal (amber-blue) axis corresponds to color temperature, with each increment equivalent to about 5 mired. The vertical (green- magenta) axis has the similar effects to the corresponding color compensation (CC) filters.

I don't have a Canon manual to hand.

Nick Pudar
April 29th, 2013, 08:57 PM
Bart,
The Wratten filters are intended to be used in the optical path.
Great point! And this is more of my lack of knowledge showing. That is why the filters are that small -- they are supposed to be put in front of the lens. That also slightly changes my testing procedure, but it hopefully will not degrade the results too badly.

many Grey cards are not necessarily spectrally neutral. Afterall, they are usually intended to determine exposure, not necessarily white-balance. So use a good quality grey card that has a known spectrally neutral reflection.
I think I'm ok here -- I will be using the full grey side of the Sekonic Exposure Profile Target II, which I'm pretty sure is spectrally neutral. (Although I could not find any documentation about that fact anywhere.)

Doug,
It would be even better to move along both axes (even though I know that will be a lot more work).
I will describe my testing process below, but I'm struggling in understanding how the Blue/Amber shift information would help me. The Sekonic C-500R only outputs the Color Temperature (actually it is CCT -- thanks for the tutorials!), and the Color Correction Index (in Wratten Filter numbers) for the Plankian Shift. I would be entering the CCT value which implies a zero shift along the Blue/Amber axis. And my whole testing process is designed to figure out how to convert the Sekonic CC Index to the Canon WB Shift steps.

How will you quantify the results of those test shots? Will you perhaps observe which pair of shift coordinates gives a "neutral" (R=G=B) chrmaticity for the image of the target?
I will be opening the images in Photoshop in Lab mode. Using the eyedropper tool, I will check to see the values of the a and b channels. The b channel is for Blue/Amber, and I should always see zeros for all positions on all of the the grey card photos. The a channel is for the Green/Magenta reading, and it will show the appropriate values based on the filters I was using. When the image should be properly White Balanced, the a channel should also read zeros.

What WB will you have set in the camera (there is no "none") or in the RAW processing (there is no "none")?
This is a great question, and it really got me thinking about whether the whole concept of the test was even possible. But I think it is. I will begin the whole testing process by using the camera's Custom White Balance function with my grey card to establish the camera's interpretation of the correct white balance. (I will measure the a and b channels of that image to ensure that they read all zeros.) Then when I add the green and magenta filters to the front of the lens durin the testing, the CCT won't shift, and I will be doing an offset only in the Green/Magenta axis. I'm really glad you asked that question!

Jerome,
I had the curiosity to check the manual of my camera
Thanks for looking that up. I really appreciate it. In my Canon manual it says that "One level of the blue/amber correction is equivalent to approximately 5 mireds of a color temperature conversion filter." But it does not say anything about the green/magenta axis.

So after much thinking and some great input from OPF members, here is my testing design. Any further input on the testing design would be appreciated.

Phase 1: Establish Base Reference Data

Use Sekonic L-758DR light meter to establish correct exposure settings for camera.
Use Sekonic Exposure Profile Target II grey card to use the camera's Custom White Balance function to set that as the starting reference point White Balance.
With the camera set up, take a reference photo of the grey card.
With the Sekonic C-500R color meter, take a reference reading of the light.
Open the reference images of the grey card in Photoshop in Lab mode, and using the eyedropper tool, confirm that the a and b channels read "0" at all points on the image.
Compare the Custom White Balance settings from the camera with the color meter reading.


It is my expectation (and hope) that this set of reference data and images will be very close to being identical. If it is not, I will have to think about what the offsets mean.

Phase 2: Record and Measure Modified Light

For a full range of Wratten gels (70M, 60M, 50M, ... 10M, 0, 10G, 20G, ... 70G), take a photo of the grey card at each level of light modification.
For each of the gels, also take a reading with the color meter and record the results. (Since the light strobes are not being gelled, cover the color meter sensor with the gels to modify its incoming light.)
Open each image in Photoshop (Lab mode) and record the a and b channel values.


Phase 3: Record and Measure Camera's White Balance Shift Modifications

For each step in Canon's White Balance Shift settings of the Green/Magenta axis (-9, -8, -7,... -1, 0, +1, +2,... +9), take a photo of the grey card (with no modifying gels).
Open each image in Photoshop and record the a and b channel values.


Phase 4: Determine Transfer Function

Plot the data from phases 2 and 3, and determine the transfer function that will convert the color meter's CC Index reading to the equivalent Camera's WB Shift value.


Until I plot the data, I do not know what the transfer function will look like. I am hopeful that it will be reasonably linear in nature, and something like "each step In Canon's WB Shift is equal to 10 CC Index values".

Phase 5: Test the Transfer Function

Go to an environment with different lighting conditions (gymnasium).
Take a light meter reading to correctly set the manual exposure.
Take a color meter reading.
Set the camera's Custom Temperature value to the Kelvin value from the color meter.
Set the camera's WB Shift value after converting the color meter's CC Index value using the transfer function from phase 4. This is for the Green/Magenta axis only.
Take a photo of the grey card with these settings.
Change the White Balance setting of the camera to Custom White Balance mode using the grey card. After establishing the Custom White Balance, take another reference photo of the grey card.
Open the images in Photoshop, and read the respective a and b channel values.


If the conversion went well, I would expect that the a and b channel values would all read "0" (in both images).

If I had time, I would also try all of the above steps at a couple of other ISO values, but I do not think the ISO should make any difference.

Anyway, that is my current thinking of a test design. If the test goes well, someone could probably do the same test with a Nikon camera for Nikon's WB Shift function.

Thanks again for everyone's input.
Best regards,
Nick

Doug Kerr
April 29th, 2013, 09:09 PM
Hi, Nick,



I will describe my testing process below, but I'm struggling in understanding how the Blue/Amber shift information would help me.

I will be opening the images in Photoshop in Lab mode. Using the eyedropper tool, I will check to see the values of the a and b channels. The b channel is for Blue/Amber, and I should always see zeros for all positions on all of the the grey card photos. The a channel is for the Green/Magenta reading, and it will show the appropriate values based on the filters I was using. When the image should be properly White Balanced, the a channel should also read zeros.

Sounds good.

Best regards,

Doug

Doug Kerr
April 29th, 2013, 09:35 PM
Hi, Nick,

Here is a link to (almost) the figure from my earlier post:

http://dougkerr.net/illustrations/20d_wb_shift_uv-pl6000.jpg

In fact, this is just a little different. It shows the Planckian locus based on the premise that the origin of the chart is planted at a blackbody chromaticity at 6000 K.

Note that this figure does not show or imply the L*a*b* a* and b* axes.

Best regards,

Doug

Nick Pudar
April 30th, 2013, 04:23 AM
Doug,
I plotted the results on several different coordinate planes, along with vectors showing the "directions" of the chromaticity of the three color space primaries (R, G, and B) and the complementary hues C, Y, and M. The result for the "0,0" setting was used as the reference, arbitrarily plotted at the origin of the coordinate system.

The results on the CIE u-v plane were the most telling. There, as a matter of fact, the six "primary" vectors are almost equally-spaced angularly.
Thank you for posting that picture. It helps a lot, and is most interesting. I think I have to go back to your articles again, but how did you determine what the du and dv values were for each of the 8 readings around the origin?

If I could plot the results of my test on the CIE u-v plane, it might be the most revealing.

Thanks,
Nick

Doug Kerr
April 30th, 2013, 06:21 AM
Hi, Nick,


Thank you for posting that picture. It helps a lot, and is most interesting. I think I have to go back to your articles again, but how did you determine what the du and dv values were for each of the 8 readings around the origin?

The basic test scheme is described in my post at the head of this thread.

Then, for each setting of the two color shift coordinates (with a uniformly-illuminated target), I read the L*a*b* a* and b* coordinates of the target in the adjusted image in (I think) Photoshop.

Then I converted those into other various other coordinate systems, including CIE u-v, and plotted them using my illustration program. I found the u-v plot the most "illuminating".

A good tool for doing such transforms is Bruce Lindbloom's CIE Color Calculator, available here:

http://www.brucelindbloom.com/index.html?ColorCalculator.html

If you go to Bruce's home page:

http://www.brucelindbloom.com/

and select the Math button, you will get to a page that has a matrix for all transforms from one color space to another, each cell giving links to pages where the equations used to do the transforms are given.

There is much other good info on this site as well.

It has been a while since I worked with any of this, but it will no doubt come back if needed! If you have any problem with the Lindbloom calculator, let me know. (Be sure to follow the "Learn more" link at the foot of the calculator page.)

I think you are on a good track.

Best regards,

Doug

Nick Pudar
April 30th, 2013, 04:42 PM
Then, for each setting of the two color shift coordinates (with a uniformly-illuminated target), I read the L*a*b* a* and b* coordinates of the target in the adjusted image in (I think) Photoshop.

Then I converted those into other various other coordinate systems, including CIE u-v, and plotted them using my illustration program. I found the u-v plot the most "illuminating".

Doug,

Thanks for the great reference links to Lindbloom's work. Great stuff!

I took a look at his CIE Color Calculator, and I'm trying to figure out how I will calculate the delta u-v values from my Lab readings. When you converted your readings to CIE u-v, is that the same thing as Luv on his calculator? (There was no read-out called CIE uv.) Assuming that it is, when I generate all my data points, I will just calculate the delta from the reference grey card and make it the origin on the delta u-v scale as you did.

The other thing that has me perplexed a bit are the Reference White and Adaptation settings. Do I just leave them at his default settings of D50 and Bradford?

Thanks,
Nick

Doug Kerr
May 1st, 2013, 10:13 AM
Hi, Nick,

Thanks for the great reference links to Lindbloom's work. Great stuff!

I took a look at his CIE Color Calculator, and I'm trying to figure out how I will calculate the delta u-v values from my Lab readings. When you converted your readings to CIE u-v, is that the same thing as Luv on his calculator? (There was no read-out called CIE uv.)

Yes, u and v are the u and v of the Luv coordinate system (L being the "quasi-luminance" coordinate). I should have mentioned that.

Assuming that it is, when I generate all my data points, I will just calculate the delta from the reference grey card and make it the origin on the delta u-v scale as you did.

The other thing that has me perplexed a bit are the Reference White and Adaptation settings. Do I just leave them at his default settings of D50 and Bradford?

This is very confusing to me, and I will have to ponder that a bit.

But you should get some practical insight from his page of explanation, which shows the data flow involved in the various transforms.

But when all else fails, go with the defaults!

I'll look at it too when I get a minute.

Thanks.

Best regards,

Doug

Nick Pudar
May 1st, 2013, 11:54 AM
I did some more reading and learned that Photoshop uses then CIE D50 White Point setting, and that the CIE preferred Chromatic Adaptation transform function is the Bradford approach. So the defaults were the safe bet after all.

Incidentally, the fine folk at Sekonic are interested in my test results and have kindly allowed me to borrow a color meter for my tests. This is wonderful since I will not have to rush the work through a 3-day rental period.

Nick

Doug Kerr
May 1st, 2013, 04:28 PM
Hi, Nick,

I did some more reading and learned that Photoshop uses then CIE D50 White Point setting, and that the CIE preferred Chromatic Adaptation transform function is the Bradford approach. So the defaults were the safe bet after all.

That's good.

But insofar as converting from L*a*b* to L*u*v*, the adaptation choice doesn't have any effect.

[quote]Incidentally, the fine folk at Sekonic are interested in my test results and have kindly allowed me to borrow a color meter for my tests. This is wonderful since I will not have to rush the work through a 3-day rental period.

That is terrific.

Let me know how the project unfolds.

Best regards,

Doug

Nick Pudar
May 19th, 2013, 07:12 PM
Doug,

Between lots of business travel, I was able to make some progress on my project. I was able to execute Phases 1 and 3 of my experiment as described earlier in this thread.

Phase 1 is about getting some reference starting points.

Phase 1: Establish Base Reference Data
Use Sekonic L-758DR light meter to establish correct exposure settings for camera.
Use Sekonic Exposure Profile Target II grey card to use the camera's Custom White Balance function to set that as the starting reference point White Balance.
With the camera set up, take a reference photo of the grey card.
With the Sekonic C-500R color meter, take a reference reading of the light.
Open the reference images of the grey card in Photoshop in Lab mode, and using the eyedropper tool, confirm that the a and b channels read "0" at all points on the image.
Compare the Custom White Balance settings from the camera with the color meter reading.

I used the Color Meter to measure my light, and it read 5500K with 0 shift in the Green/Magenta direction. So, after setting the camera to the right exposure based on my Light Meter reading, I set the White Balance to 5500K. Here is a link (http://www.pudar.com/ReferenceStart.jpg)to the photo of my reference card. When I opened the RAW image of the reference gray card in Lightroom, it reported that the White Balance was at 5300K and there was a Tint of +2 (towards Magenta). When I converted the image to L*a*b in Photoshop, the gray card read 44, -2, 0. The -2 of the a channel might correspond to the +2 in Lightroom to make the White Balance look right, but I'm not sure. I decided to go with this initial reference card setup, since I was going to be using offset calculations later anyway. (I am curious how to think about the 5500 vs 5300 White Balance readings. Any ideas?)

Since I do not have sufficient Wratten Gel Filters (still waiting on some to arrive from an order), I decided to move on to Phase 3.

Phase 3: Record and Measure Camera's White Balance Shift Modifications
For each step in Canon's White Balance Shift settings of the Green/Magenta axis (-9, -8, -7,... -1, 0, +1, +2,... +9), take a photo of the grey card (with no modifying gels).
Open each image in Photoshop and record the a and b channel values.

As you had recommended, I took photos for all combinations of White Balance Shift settings, even in the Blue/Amber dimension. I did all that (361 photos!) and recorded all of the L*a*b data from photoshop. I then used Bruce Lindbloom's CIE Color Calculator to convert the L*a*b data to L*u*v data as you had earlier. I then offset all the data points so that the Canon White Balance Shift setting of (0,0) actually fell on the du,dv values of 0,0.

After all that, this link (http://www.pudar.com/dUdVChart.jpg)shows the data plotted. (The red plot points are those where one of the White Balance Shift values are 0 -- to make it easier to read the chart.) It looks similar to the small data set you generated in your earlier test. Although my du and dv units are not exactly the same, and they are off by a factor of 1000. I may be doing something wrong, but I'm not sure what it is.

From here, I just need to wait for some of the filters to arrive. I was only able to get a very small set of pure Green and Magenta filters from eBay (I have lots of Blue and Yellow filters!). I'm buying a few more of the Green and Magenta and I will have a small sample set to run my other experiment phases. There will have to be some extrapolation going on later. Hopefully the filters come in the mail soon, as I only have the Color Meter on loan until the end of June.

While this is all interesting and I'm learning a lot, it certainly feels like the run will be way too long for what may turn out to be a very short slide! I'm keeping at it though.

Nick

Doug Kerr
May 20th, 2013, 08:53 AM
Hi, Nick,

Between lots of business travel, I was able to make some progress on my project. I was able to execute Phases 1 and 3 of my experiment as described earlier in this thread.

Thanks for the update.

Your procedure sounds very good overall.

Phase 1 is about getting some reference starting points.

I used the Color Meter to measure my light, and it read 5500K with 0 shift in the Green/Magenta direction. So, after setting the camera to the right exposure based on my Light Meter reading, I set the White Balance to 5500K. Here is a link (http://www.pudar.com/ReferenceStart.jpg)to the photo of my reference card. When I opened the RAW image of the reference gray card in Lightroom, it reported that the White Balance was at 5300K and there was a Tint of +2 (towards Magenta). When I converted the image to L*a*b in Photoshop, the gray card read 44, -2, 0. The -2 of the a channel might correspond to the +2 in Lightroom to make the White Balance look right, but I'm not sure. I decided to go with this initial reference card setup, since I was going to be using offset calculations later anyway. (I am curious how to think about the 5500 vs 5300 White Balance readings. Any ideas?)

Since I do not have sufficient Wratten Gel Filters (still waiting on some to arrive from an order), I decided to move on to Phase 3.

As you had recommended, I took photos for all combinations of White Balance Shift settings, even in the Blue/Amber dimension. I did all that (361 photos!) and recorded all of the L*a*b data from photoshop.
Yes, the "hundreds of test [photos, files, etc.]" phenomenon is a staple of life here!

I then used Bruce Lindbloom's CIE Color Calculator to convert the L*a*b data to L*u*v data as you had earlier. I then offset all the data points so that the Canon White Balance Shift setting of (0,0) actually fell on the du,dv values of 0,0.

After all that, this link (http://www.pudar.com/dUdVChart.jpg)shows the data plotted. (The red plot points are those where one of the White Balance Shift values are 0 -- to make it easier to read the chart.) It looks similar to the small data set you generated in your earlier test. Although my du and dv units are not exactly the same, and they are off by a factor of 1000. I may be doing something wrong, but I'm not sure what it is.

Yes, there is very good agreement with the general result I reported.

I'm not sure where the 1:1000 discrepancy comes from. If I get a chance I will poke around a little and see what that might come from.

While this is all interesting and I'm learning a lot, it certainly feels like the run will be way too long for what may turn out to be a very short slide!

Important work often has that feel to it. Do not let that feeling derail you in your quest for understanding.

Thanks again for keeping in touch.

Best regards,

Doug

Doug Kerr
May 20th, 2013, 09:12 AM
Hi, Nick,

Just for reference, this shows the CIE u-v plane, showing the scale of the coordinates:

http://dougkerr.net/illustrations/CIE_uv_efg-01.jpg

The figure was taken from this most useful paper:

http://www.efg2.com/Lab/Graphics/Colors/Chromaticity.htm

Best regards,

Doug

Doug Kerr
May 20th, 2013, 09:21 AM
Hi, Nick,

There seems to be something funny in Bruce's calculator re Luv. I'm going to work my way through it and see why it seems funny.

Later.

Best regards,

Doug

Doug Kerr
May 20th, 2013, 09:52 AM
Hi, Nick,

Here is where I led you astray.

The color space called "Luv" in Bruce's calculator is actually L*u*v*.

In some cases, the "*" in a color space designation means a nonlinear coordinate, and here it means that for L*, but here for u* and v* it means something else: that these coordinates are scaled with "L" (so that they essentially define pseudo-chrominance, not chromaticity). (True u and v define chromaticity.) *

The relationships are these (and for rigor I will use L*, u* and v*, the formal designations, for the values reported in Bruce's calculator in the three columns of the "Luv" color space):

u* = 13L*u
v* = 13L*v

where L*, u*, and v* are the three values reported in the Lindbloom calculator for the "Luv" color space, and u and v are the chromaticity coordinates as in the CIE u-v chromaticity plane.

Thus:

u = u*/13L*
v = v*/13L*

*If you are unsure as to the subtlety of the distinction between chromaticity and chrominance, you may find this helpful:

http://dougkerr.net/pumpkin/articles/Chromaticity_Chrominance.pdf

Best regards,

Doug

Nick Pudar
June 23rd, 2013, 05:42 PM
Well, I have run out of time with my experiment, and I had to return the loaner C-500R to Sekonic. My summer will be hectic, and I just do not have the solid blocks of time necessary to do this justice. Part of my dilemma is that I just can't get my hands on the right Wratten Filters. I ordered a lot of large bundles of used filters on eBay, but after a couple hundred dollars, there was not much quality stock of green and magenta filters. (LOTS of blue and amber.) I had also ordered the remaining necessary filters new from B&H Photo, but they seem to be on perpetual backorder, and I fear that I will never get them. Come to think of it, I should really just cancel the order.

I was able to complete most of the foundational elements of the experiment, but the transform function from the Color Meter to the Color Shift parameters in the camera eluded me.

I'm just going to have to stick with the Custom White Balance process with my camera.

Thanks to everyone, especially Doug Kerr for all the fabulous help and encouragement on this little experiment process.

Best regards,
Nick

Doug Kerr
June 23rd, 2013, 07:19 PM
Hi, Nick,

Well, I have run out of time with my experiment, and I had to return the loaner C-500R to Sekonic. My summer will be hectic, and I just do not have the solid blocks of time necessary to do this justice. Part of my dilemma is that I just can't get my hands on the right Wratten Filters. I ordered a lot of large bundles of used filters on eBay, but after a couple hundred dollars, there was not much quality stock of green and magenta filters. (LOTS of blue and amber.) I had also ordered the remaining necessary filters new from B&H Photo, but they seem to be on perpetual backorder, and I fear that I will never get them. Come to think of it, I should really just cancel the order.

I was able to complete most of the foundational elements of the experiment, but the transform function from the Color Meter to the Color Shift parameters in the camera eluded me.

I'm just going to have to stick with the Custom White Balance process with my camera.

Thanks to everyone, especially Doug Kerr for all the fabulous help and encouragement on this little experiment process.

I well understand your situation - I have been there many times. I have this very day in fact put several lines of investigation into the "banked fire" situation you describe.

I know you are disappointed not to be able to bring your investigation to a full conclusion. But take heart in the enormous amount of understanding you have gained so far.

Remember, we never understand anything completely! Our "wisdom" is a "grand triangulation" among many things we understand perhaps 20% or maybe even at best 80%!

Best regards,

Doug