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Archive 2012 · Best Grey Card / Panel for luminance and color accuracy?
  
 
jzucker
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p.1 #1 · p.1 #1 · Best Grey Card / Panel for luminance and color accuracy?


Looking to calibrate my meter and camera for accurate color balancing and exposure. I've heard that the standard "kodak" 18% gray card isn't really as accurate as it should be.

What is the most accurate in terms of color and luminance



Mar 22, 2012 at 06:46 PM
cgardner
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p.1 #2 · p.1 #2 · Best Grey Card / Panel for luminance and color accuracy?


In the distant past Kodak 18% cards weren't color neutral but in recent years they have been made under contract by Tiffen, and currently in conjunction with x-rite and are Munsel Gray neutral. So while there are more expensive and more durable gray balance targets on the market the Kodak card will work for shooting to a technically neutral R=G=B starting baseline for color balance. A you probably know that starting baseline isn't always the most perceptually pleasing color or what you want in the final image to convey a warmer or cooler vibe, just a consistent baseline for judging the color when first opening the file on the computer to evaluate it.

As for calibrating your meter? An 18% gray card isn't the best tool for that task with digital for a couple reasons. You might be assuming the 18% card when correctly exposed will be dead center in the histogram when correctly exposed, but it won't be because digital cameras and meters are now calibrated for a 12-13% reflectance mid-point. Secondly even if you have a 12% reflectance card and center the histogram that may not mean your highlights are correctly exposed. By way of analogy metering to the middle is like scratching your belly if you nose itches: if you are concerned about highlight exposure you need to use a highlight target to calibrate the meter, not a middle gray one.

Gray cards worked for negative film because the negative had a longer linear section of it's curve than the print. You could err on the side of overexposure of +1-2 stops above what it took to put shadow detail on the negative and the lab when making the print compensated by with longer exposure of the print.

But with transparencies and digital, if meter readings which correctly expose white clothing, teeth, skin highlights, etc. is your goal the more logical target to use for calibrating the meter is a piece of white textured cloth like a terry towel.

Try this:

Drape a white towel or shirt over a light stand and illuminated with a single flash above the camera at about a 45 downward (butterfly) angle to create some modeling on it. Take a reading with the incident meter dome out, dome pointed at camera not light for the exposure reading. Let's say the reading is 5.6 on the meter. Bracket exposures from f/4 to f/8 in 1/3 stop increments. Open the 7 files on your computer and look at how the highlights in the towel are reproduced. Odds are the one with the best highlight exposure will not be the f/5.6 file with the best highlight detail in the towel or shirt.

Take the three best exposures (what looks perfect in the RAW, 1/3 stop less, 1/3 stop more) and using your normal PP workflow make 600 x 800 pixel JPGs what you might typically use on a web page and evaluate the highlights again. The point of this second evaluation is so you allow enough exposure "headroom" at capture per the compensated meter reading so you don't wind up clipping the whites or blowing the red channel in the skin highlights when converting the 16-bit Photoshop files to 8-bit sRGB JPG.

The difference between the meter reading (e.g. 5.6) and the JPG which looks best would be the factor you'd enter into the meter. My Canon bodies (20D and 50D) require about 1/3 stop adjustment, meaning the file shot at f/6.3 looked better exposed in the highlights. That's because ISO100 set on the camera is actually around 120 per the way the meter is calibrated. The same lighting with a .3 compensation factor entered in the meter results in the meter display changing from 5.6 to 6.3 and the meter reading producing perfectly exposed highlights.

Where does the spike of the gray card fall? I don't really care because I control the tone of the midtones and shadows with the lighting ratio and it will vary with the ratio I use.








Mar 22, 2012 at 08:35 PM
kenyee
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p.1 #3 · p.1 #3 · Best Grey Card / Panel for luminance and color accuracy?


Get a WhiBal card. They check them individually w/ a color meter before shipping them. It's durable because the color goes all the way through. It has black/white points on the card.



Mar 22, 2012 at 08:57 PM
jzucker
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p.1 #4 · p.1 #4 · Best Grey Card / Panel for luminance and color accuracy?


the problem with evaluating pictures on a screen is that it's subjective IMO. The only accurate method has to involve exposing a known grey color and then testing with the eyedropper in photoshop. If the card isn't a true 128,128,128 grey then you're screwed I guess. From an engineering perspective, the problem with the method you describe is that the white detail method is subjective and dependent on the output device calibration and how much light source it's exposed to as well as your own perception of detail.

cgardner wrote:
In the distant past Kodak 18% cards weren't color neutral but in recent years they have been made under contract by Tiffen, and currently in conjunction with x-rite and are Munsel Gray neutral. So while there are more expensive and more durable gray balance targets on the market the Kodak card will work for shooting to a technically neutral R=G=B starting baseline for color balance. A you probably know that starting baseline isn't always the most perceptually pleasing color or what you want in the final image to convey a warmer or cooler vibe, just a consistent baseline for judging
...Show more



Mar 22, 2012 at 09:57 PM
BrianO
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p.1 #5 · p.1 #5 · Best Grey Card / Panel for luminance and color accuracy?


jzucker wrote:
Looking to calibrate my meter... What is the most accurate in terms of color and luminance


Which meter do you have?

For exposure calibration, Sekonic has a calibration target and software specifically designed to use for calibrating the L-758DR meter like I have (as well as the L-758Cine).

http://www.sekonic.com/Products/L-758DR/Accessories/Sekonic-Exposure-Target-II.aspx

It's not cheap, but in addition to buying one, there are places where you can rent them. You can download the software free from the Sekonic Web site.

http://www.sekonic.com/Support/Downloads/DTSSoftwareforMACandWindows.aspx

Dodd Camera in Cleveland sells the target, or you can rent one from Borrow Lenses:

http://www.borrowlenses.com/product/Lightmeters/Sekonic_Exposure_Profile_Target_II

For meters other than the L-758DR, the same target may be helpful in establishing manual "calibration" (compensation) charts.

For any here who are not familiar with the software-based calibration, here's a video:

http://www.youtube.com/watch?v=tdyosItw3Mk

For white balancing, I also like the WhiBal cards that kenyee mentioned, for their pocket-sized version, but I use the big SpyderCheckr for demanding work like camera/monitor color management.

http://www.youtube.com/watch?v=4-XGpIVGxTA



Mar 22, 2012 at 11:06 PM
jzucker
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p.1 #6 · p.1 #6 · Best Grey Card / Panel for luminance and color accuracy?


I have the minolta autometer V


Mar 22, 2012 at 11:18 PM
BrianO
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p.1 #7 · p.1 #7 · Best Grey Card / Panel for luminance and color accuracy?


jzucker wrote:
I have the minolta autometer V


No need for the Sekonic target, then. I'd get a WhiBal card, a SpyderChekr (or the similar Color Checker Passport), and/or a SpyderCube.



Mar 22, 2012 at 11:45 PM
jzucker
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p.1 #8 · p.1 #8 · Best Grey Card / Panel for luminance and color accuracy?


I think i'm going to go with the lastolite 12" gray card

http://www.amazon.com/Lastolite-LL-LR1250-12-Inch-Ezybalance/dp/B0009QZDL6



Mar 22, 2012 at 11:54 PM
 

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cgardner
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p.1 #9 · p.1 #9 · Best Grey Card / Panel for luminance and color accuracy?


jzucker wrote:
the problem with evaluating pictures on a screen is that it's subjective IMO. The only accurate method has to involve exposing a known grey color and then testing with the eyedropper in photoshop. If the card isn't a true 128,128,128 grey then you're screwed I guess. From an engineering perspective, the problem with the method you describe is that the white detail method is subjective and dependent on the output device calibration and how much light source it's exposed to as well as your own perception of detail.



You are confusing what I suggest for EXPOSURE CALIBRATION with gray balance. Two separate tools for two different tasks.

Gray Balance Reference:

Yes you need a known R=G=B target for setting custom WB and "snapping" color to neutral in Photoshop as a starting baseline. But there is no such thing as a "128,128,128" gray card. Seting custom WB off the card will result in R=B=G. That the actual eye dropper readings will wind up will depend on how you expose the shot containing the card with ISO/shutter/aperture of camera. That's exactly why the gray card is of limited use for calibrating your EXPOSURE meter TO YOUR CAMERA ISO.

Exposure Meter Compensation:

What you are "calibrating" is actually a compensation factor between you incident EXPOSURE meter's ISO setting per factory calibration at ISO 100 and what your camera's sensor sensitivity is when set at ISO 100.

If you can meter a scene containing a white object and the white object (and skintones) are perfectly exposed in your images VISUALLY and per detail and per Photoshop eye dropper readings then no compensation is needed. For example a white card in the scene when correctly exposued should have eyedropper readings of around 250 with only specular reflections off shiny objects at 255. Logicailly to see that distinction between 255 specular and 250 solid white you'd want a white target that is both solid and specular, not a gray card.

If the EXPOSURE meter out of the box doesn't create a 250 value on a white card that tells you the meter ISO and actual camera sensitivity are not in sync. You can't adjust the camera sensitivity so you change the meter reading to match the cameara using the complensation factor or adjust the ISO speed of the meter (if it does not have a compensation feature) until the meter reading does accurately render the highlight values of solid white objects at 250.

If you try what I suggest it will become obvious why I suggest doing it that way.





Mar 23, 2012 at 11:43 AM
cgardner
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p.1 #10 · p.1 #10 · Best Grey Card / Panel for luminance and color accuracy?


[duplicate message removed]

Edited on Mar 23, 2012 at 12:08 PM · View previous versions



Mar 23, 2012 at 11:43 AM
jzucker
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p.1 #11 · p.1 #11 · Best Grey Card / Panel for luminance and color accuracy?


no, i'm not confusing grey balance. The idea of calibrating as close to 128 as possible is the goal. I'm not talking about color. I'm talking about luminance calibration.

again, i'm not saying your method is invalid but if it involves an output device and not a reading from the colors in the file directly it will be biased by the output device rendering.



Mar 23, 2012 at 11:59 AM
Skarkowtsky
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p.1 #12 · p.1 #12 · Best Grey Card / Panel for luminance and color accuracy?


kenyee wrote:
Get a WhiBal card. They check them individually w/ a color meter before shipping them. It's durable because the color goes all the way through. It has black/white points on the card.



+1



Mar 23, 2012 at 12:05 PM
cgardner
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p.1 #13 · p.1 #13 · Best Grey Card / Panel for luminance and color accuracy?


Why is 128 on the card your goal?

Will that ensure your highlights are correctly exposed in the scene containing that 128 exposed gray card?



Mar 23, 2012 at 12:06 PM
jzucker
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p.1 #14 · p.1 #14 · Best Grey Card / Panel for luminance and color accuracy?


cgardner wrote:
Why is 128 on the card your goal?

Will that ensure your highlights are correctly exposed in the scene containing that 128 exposed gray card?


No, it ensures the central point for the camera and the meter being synchronized. Once I do that, I would need to do a test like what you are recommending in order to understand the real-world, working dynamic range but particularly for flash I think it's important to have the meter and camera synchronized on the midpoints.



Mar 23, 2012 at 01:55 PM
jzucker
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p.1 #15 · p.1 #15 · Best Grey Card / Panel for luminance and color accuracy?


by the way, this is the method I'm using roughly :

http://www.frankdoorhof.com/site/2011/08/calibrating-the-light-meter-some-quick-notes/




Mar 23, 2012 at 04:01 PM





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