18% gray card
Jan 8, 2016 12:45:18 #
I made a color test card using a Kodak gray card (18% reflectance) and pigment paints. Would it be correct to expect 18% of 256 (=46) for the RGB values under the cursor in Photoshop?
Jan 8, 2016 12:57:49 #
Jan 8, 2016 13:08:20 #
Jan 8, 2016 13:08:27 #
I think it should be R, G and B 128.
John_F wrote:
I made a color test card using a Kodak gray card (18% reflectance) and pigment paints. Would it be correct to expect 18% of 256 (=46) for the RGB values under the cursor in Photoshop?
Jan 8, 2016 13:09:03 #
Los-Angeles-Shooter
Loc: Los Angeles
John_F wrote:
I made a color test card using a Kodak gray card (18% reflectance) and pigment paints. Would it be correct to expect 18% of 256 (=46) for the RGB values under the cursor in Photoshop?
NO. Or as pilots say, "negative." Or as truckers say "negatory."
Also be aware that typical cheap gray cards are not color-neutral.
Jan 8, 2016 14:40:39 #
Jan 8, 2016 15:01:56 #
BebuLamar wrote:
With Gamma 2.2 it should be 117.
Bingo! :thumbup: :thumbup: :thumbup:
Jan 8, 2016 15:38:57 #
Apaflo wrote:
Bingo! :thumbup: :thumbup: :thumbup:
Thanks and I have a question for you. If you shoot a gray card and the color balance is correct then you should get the RGB value of 117,117,117 I got that. But however if the card isn't gray I understand that you should get an equivalent brightness. In another word if I convert it to B&W it should again be 117. The question is how do I calculate this value from the RGB values. I know it's not simply the average of the 3 because I know that the Green channel has more weight but how?
Jan 8, 2016 16:37:33 #
Los-Angeles-Shooter
Loc: Los Angeles
Why not keep it simple? Get a CALIBRATED gray card; use it to make a custom white balance with your strobe(s) and/or ambient light; then shoot. Colors will be nailed dead-on each time. For later fine-tuning, take a snapshot of a HIGH QUALITY color chart at the start, each time you change lights/lighting setup.
Jan 9, 2016 06:43:35 #
You could get an expodisc 2.0 and set the custom white balance with one shot on site in any light.
Jan 9, 2016 07:58:16 #
Re: With Gamma 2.2 it should be 117.
Neat. How do you arrive at that number and what has the Gamma got to do with it?
I've always been currious as to why the camera exposes for 18%. Why not 50%?
Neat. How do you arrive at that number and what has the Gamma got to do with it?
I've always been currious as to why the camera exposes for 18%. Why not 50%?
Jan 9, 2016 08:34:24 #
John_F wrote:
I made a color test card using a Kodak gray card (18% reflectance) and pigment paints. Would it be correct to expect 18% of 256 (=46) for the RGB values under the cursor in Photoshop?
The 18% grey card is not nearly as useful for your purposes as a ColorChecker. From Wikipedia: "The ColorChecker Color Rendition Chart (often referred to by its original name, the Macbeth ColorChecker[1] or simply Macbeth chart[2]) is a color calibration target consisting of a cardboard-framed arrangement of 24 squares of painted samples. The ColorChecker was introduced in a 1976 paper by McCamy, Marcus, and Davidson in the Journal of Applied Photographic Engineering.[3] The chartÂ’s color patches have spectral reflectances intended to mimic those of natural objects such as human skin, foliage, and flowers, to have consistent color appearance under a variety of lighting conditions, especially as detected by typical color photographic film, and to be stable over time."
Later, in the same article, Wikipedia reports this: "X-Rite also sells a 140-patch chart called the ColorChecker Digital SG, and is intended for automated use with computer software to characterize digital cameras and scanners."
Jan 9, 2016 08:53:13 #
peterg
Loc: Santa Rosa, CA
An 18% gray card is used to measure exposure. A light meter will give you the camera settings to produce a middle-toned gray. However, that 18% gray may not be a perfect gray to determine white balance. A good white balance card may have more or less 18% reflectance. Some 18% gray cards may be calibrated in both reflectance and color to calibrate both exposure and white balance.
Jan 9, 2016 09:24:01 #
BebuLamar wrote:
With Gamma 2.2 it should be 117.
Why the value 117?
According to the table in http://en.wikipedia.org/wiki/Middle_gray the values for sRGB and Gamma 2.22 are 128, 128, 128?
Jan 9, 2016 09:28:07 #
No. I love 18 percent gray cards.
John_F wrote:
I made a color test card using a Kodak gray card (18% reflectance) and pigment paints. Would it be correct to expect 18% of 256 (=46) for the RGB values under the cursor in Photoshop?
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