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If I take a photograph of the 18% Reflectance side of a Kodak grey card and open the image in Photoshop. What values should I expect for RGB in the Info palette?
18% reflectance is 18% L in the LAB mode. If you fill in 18,0,0 in LAB, you’ll get RGB 48,48,48.
"Johan W. Elzenga" wrote in message
18% reflectance is 18% L in the LAB mode. If you fill in 18,0,0 in LAB, you’ll get RGB 48,48,48.
I don’t think that’s right. I think 18% reflectance is 18% *absolute* reflectance, which
corresponds to middle grey, which is a Lab luminance of 50.
I put in a Lab value of 50,0,0 (which I think corresponds to 50% reflectance). If you select "scale XYZ",to
you can see that the resulting value for Y (which is Luminance) is close
18, but not exactly 18.
(I’m not sure why there is this slight discrepancy).
wrote:
If I take a photograph of the 18% Reflectance side of a Kodak grey card and open the image in Photoshop. What values should I expect for RGB in the Info palette?
If you want to cross post a message, do so by filling in all the newsgroups. That way, people who read the same groups, won’t see your message twice. Don’t post the same message separately in many newsgroups. Also, people who answer your message, will only need to write the answer once. I posted the answer in alt.graphics.photoshop:
18% reflectance is 18% L in the LAB mode. If you fill in 18,0,0 in LAB, you’ll get RGB 48,48,48.
Assuming the image is in the sRGB color space, and the image is white balanced accurately,
the RGB values would be about 119,119,119, according to this CIE Color Calculator:
http://www.brucelindbloom.com/ColorCalculator.html
I put in a Lab value of 50,0,0 (which I think corresponds to 50% reflectance). If you select "scale XYZ",
you can see that the resulting value for Y (which is Luminance) is close to 18, but not exactly 18.
(I’m not sure why there is this slight discrepancy).
You also need to select "scale RGB" in order for the resulting values to be scaled to the range 0-255.
The values will change according to the gamma of the color space the image is in.
Greg.
My apologies for the crossposting, thats the first (and last) time I’ll do that.
And, if anyone else needs a reason NOT to crosspost – It’s because you end up with intelligent, knowledgeable people NOT being able to engage in a structured conversation and NOT being able to develop their arguments with each other.
My apologies for the crossposting, thats the first (and last) time I’ll do that.
And, if anyone else needs a reason NOT to crosspost – It’s because you end up with intelligent, knowledgeable people NOT being able to engage in a structured conversation and NOT being able to develop their arguments with each other.
The idea behind the original question was this:
If I expose a roll of film and load the images into Photoshop with one exposure a picture of an 18% reflectance grey card, what should the RGB values be in the Info pallete? The consensus seems to be approx R=119, G=119 and B=119. If I then correct any discrepancies – can I apply the same correction to the rest of the roll of film?
There is a discussion on 18% grey card values at
http://photoshopgurus.info/forum/viewtopic.php?t=4448
and they come up with RGB values of 209,209,209 using some very sound arguements and pictures(especially from Rantin Al)
"nikki" wrote in messageset
There is a discussion on 18% grey card values at
http://photoshopgurus.info/forum/viewtopic.php?t=4448
and they come up with RGB values of 209,209,209 using some very sound arguements and pictures(especially from Rantin Al)
I haven’t tried to follow the whole thread in detail yet, but some of the reasoning just doesn’t seem right.
I’ve just taken a photo of my white wall with my digicam after calibrating the white balance,
and the result is a lot closer to that predicted in our thread here. The wall was not evenly
lit (but not terribly uneven, either). Centre pixel values are about 139,140,138
corner pixel values 100,101,102, and if the entire image is averaged, the result is 122,123,122.
(a simple average like this isn’t totally valid, because the camera was
for centre weightedspace,
average – I didn’t apply any weighting)
The values predicted in that thread, at least for a typical gamma RGB
just seem way too high.
Greg.
"nikki" wrote in message——————————–
There is a discussion on 18% grey card values at
http://photoshopgurus.info/forum/viewtopic.php?t=4448
and they come up with RGB values of 209,209,209 using some very sound arguements and pictures(especially from Rantin Al)
I haven’t tried to follow the whole thread in detail yet, but some of the reasoning just doesn’t seem right.
I’ve just taken a photo of my white wall with my digicam after calibrating the white balance,
and the result is a lot closer to that predicted in our thread here. The wall was not evenly
lit (but not terribly uneven, either). Centre pixel values are about 139,140,138
corner pixel values 100,101,102, and if the entire image is averaged, the result is 122,123,122.
(a simple average like this isn’t totally valid, because the camera was set for centre weighted
average – I didn’t apply any weighting)
The values predicted in that thread, at least for a typical gamma RGB space, just seem way too high.
Greg.
"nikki" wrote in message
There is a discussion on 18% grey card values at
http://photoshopgurus.info/forum/viewtopic.php?t=4448
and they come up with RGB values of 209,209,209 using some very sound arguements and pictures(especially from Rantin Al)
I haven’t tried to follow the whole thread in detail yet, but some of the reasoning just doesn’t seem right.
I’ve just taken a photo of my white wall with my digicam after calibrating the white balance,
and the result is a lot closer to that predicted in our thread here. The wall was not evenly
lit (but not terribly uneven, either). Centre pixel values are about 139,140,138
I thought so too , at first, but the tests I have made indicate the RGB value is higher than the the RGB 122,122,122 you indicate.
I hope there is a Color scientist out there who is also a PS user.
The gurus are very confused.
I’ve always figured 127, 127, 127 as the colors for a gray card. Maybe it varies with a different gamma setting but I use 220 which is the Windows default. If you’re using a Mac, set it to 180.
Try this:
1. Select a rectangular shape
2. Set the default foreground/background
colors by typing a "D".
3. Click on the Gradient tool and drag a
horizontal line across the selection.
4. Image > Adjustments > Posterize
(use an uneven number, like 5).
5. With the info box open, notice the
numbers 127 appear in triplicate. If not,
click on the eyedropper in the Info box
and select "Actual Color".
Lynn
Centre pixel values are aboutset
139,140,138
corner pixel values 100,101,102, and if the entire image is averaged, the result is 122,123,122.
(a simple average like this isn’t totally valid, because the camera was
for centre weighted
average – I didn’t apply any weighting)
There’s no need to do all that. If you can accept that the L* value for middle grey is 50 (on a scale of 0 to 100),
just use the CIE Color calculator (as I said earlier). Punch in 50,0,0 in the L*a*b* fields and you can see that the resulting pixel values in sRGB are 119,119,119. The L* scale is perceptually uniform
– that’s why half
scale (50) represents middle grey, exactly. The fact that the resulting pixel values (in sRGB) are not *exactly*
half scale just means that sRGB is not *exactly* perceptually uniform.
Not really. The 18% was chosen on midway on a logarithmic scale between a very low reflectance value (remember, log(0) isn’t a real number) and an ideal Lambertian reflector. L*a*b* uses a cube root as its nonlinearity, rathern than logarithm. So there’s no reason to think that the middle of the two scales would correspond.
– that’s why half
scale (50) represents middle grey, exactly. The fact that the resulting pixel values (in sRGB) are not *exactly*
half scale just means that sRGB is not *exactly* perceptually uniform.
It wasn’t intended to be. It’s an idealized display characteristic.
"Greg" writes:
There’s no need to do all that. If you can accept that the L* value for middle grey is 50 (on a scale of 0 to 100),
Not really. The 18% was chosen on midway on a logarithmic scale between a very low reflectance value (remember, log(0) isn’t a real number) and an ideal Lambertian reflector. L*a*b* uses a cube root as its nonlinearity, rathern than logarithm. So there’s no reason to think that the middle of the two scales would correspond.
Besides which, making an ideal Lambertian reflector the top of your scale will mean clipping anything brighter than that. And, since the world isn’t Lambertian (i.e. ideally diffuse), there’s plenty out there that’s brighter.in
I think obsessing about this is pointless. After all, it’s just a starting point for variations in exposure a la Ansel Adams.
just use the CIE Color calculator (as I said earlier). Punch in 50,0,0
sRGBthe L*a*b* fields and you can see that the resulting pixel values in
are 119,119,119. The L* scale is perceptually uniform
In an approximate way, just as all mathematical representations of color and brightness are. The Munsell scale is perceptually uniform for diffuse surfaces under certain conditions, but there’s no closed-form mathematical formulation for it.
Really, the nonlinearity of brightness perception has been measured as a number of different nonlinear functions; logarithm, square root, cube root, and others, depending on viewing conditions.
– that’s why half
scale (50) represents middle grey, exactly. The fact that the resulting pixel values (in sRGB) are not *exactly*
half scale just means that sRGB is not *exactly* perceptually uniform.
It wasn’t intended to be. It’s an idealized display characteristic.
<snip>
—
-Stephen H. Westin
Any information or opinions in this message are mine: they do not represent the position of Cornell University or any of its sponsors.
Could the reason that our digital cameras seem to produce a middle grey about a third
to a half a stop (or more for some?) above the theoretical/pure middle grey be something to do with
the dynamic range actually achievable by the camera? I.e, are the camera manufacturers placing
middle grey mid-way between the noise floor and full scale, rather than mid way between zero
and full scale?
In article <40449d75$>,grey
"Greg" writes:
Could the reason that our digital cameras seem to produce a middle grey about a third
to a half a stop (or more for some?) above the theoretical/pure middle
midbe something to do with
the dynamic range actually achievable by the camera? I.e, are the camera manufacturers placing
middle grey mid-way between the noise floor and full scale, rather than
way between zero
and full scale?
Something along those lines seems likely to me.
Most manufacturers calibrate their light meters according to ANSI Standard ANSI/ISO 2720-1974 (R1994), ANSI/NAPM IT3.302-1994, which is not readily available to the public. I don’t know of standards for converting raw data to specific color spaces. —
Warren S. Sarle SAS Institute Inc. The opinions expressed here SAS Campus Drive are mine and not necessarily
(919) 677-8000 Cary, NC 27513, USA those of SAS Institute.
Thanks. I’ve actually just stumbled on this document:
http://members.cox.net/dspielman1/Gray_Card/ANSI_PH3_49_1971 .PDF which actually throws another spanner into the works – apparently camera lightmeters
are typically calibrated for 12 to 13% grey! That would mean that the RGB values
we’re getting are even *further* from the theoretical prediction. Arrgh.
it might be well advised to just throw in the
towel on this one!
"Flycaster" wrote in message
it might be well advised to just throw in the
towel on this one!
You’re probably right. But I’m going to try my luck and email Nikon technical support. ;^)
Why do you need to know 12% (or 18%) grey? Just curious.
Buy the Kodak Grey Card pack. It’s got cards with black, white and "18%" grey. It’s got to be under $20us.it
Also get another card with a tone scale and color scale – and photograph
with your digital camera. If you can make a straight DIGITAL print of the color that is _truly_ representative of those samples, then I’ll pop for beverages of your choice.
My mate just reminded me that grey is spelled G-R-A-Y here. 🙂 Sorry.
My mate just reminded me that grey is spelled G-R-A-Y here. 🙂 Sorry.
"jjs" wrote in messagethe
Why do you need to know 12% (or 18%) grey? Just curious.
Because.
Buy the Kodak Grey Card pack. It’s got cards with black, white and "18%" grey. It’s got to be under $20us.it
Also get another card with a tone scale and color scale – and photograph
with your digital camera. If you can make a straight DIGITAL print of
color that is _truly_ representative of those samples, then I’ll pop for beverages of your choice.
There’s no way I could do that with my consumer digicam, because it is limited to
sRGB to start with. Many patches of an IT8 chart are simply out of the camera’s
gamut.
Greg.
"Greg" wrote in message
Thanks. I’ve actually just stumbled on this document:
http://members.cox.net/dspielman1/Gray_Card/ANSI_PH3_49_1971 .PDF which actually throws another spanner into the works – apparently camera lightmeters
are typically calibrated for 12 to 13% grey! That would mean that the RGB values
we’re getting are even *further* from the theoretical prediction. Arrgh.
Having now followed this same thread in 3 different NG’s, and in 2 high end forums, in which all the supposed cognoscenti and experts weighed in…all drawing sometimes vastly different conclusions resulting in "numbers" literally all over the board, it might be well advised to just throw in the towel on this one!
I mean, does it really matter? (it certainly has no impact on how I shoot or use PS, but perhaps I’m simply ignorant, and blissfully so.)
It’s perfectly plausible to me that
different cameras would put the "properly exposed" gray card at somewhat different numerical values;
"Stephen H. Westin" <westin*> wrote in messageIt’s perfectly plausible to me that
different cameras would put the "properly exposed" gray card at somewhat different numerical values;
Yes, of course, but it is puzzling me a bit as to why the results so far seem to be consistently somewhat
above the theoretical mid-point of the luminance scale. I’m mindful of what you said
about there being different formulas for the perceptually uniform luminance scale – if the
camera manufacturers do use a different scale (to that used in Bruce Lindbloom’s CIE Color Calculator), it would
be interesting to know what it is.
Thread deleted for brevity.
So, I would think that grey/color bar is immaterial for the taking of the shot because you can view the shot on the LCD view finder and tell if the colors are where you want.
I guess you could include the cards
on the edge or one frame in the light you are using for a printing reference if a stranger had to make your print and there was no white with texture to reference to. Also to use the cards to calibrate your CRT and printer.
I have a calibration file from my service bureau that I calibrate my screen and my printer to match their hard copy print they include with the digital file.
How many out there are using their own dark room? I have not been in the dark room to make a print since 1992 and I have both feet in the analogue and digital world.
"jjs" wrote in messageof the
[…]
If you can make a straight DIGITAL print of the
color that is _truly_ representative of those samples, then I’ll pop for beverages of your choice.
There’s no way I could do that with my consumer digicam, because it is limited to sRGB to start with. Many patches of an IT8 chart are simply out
camera’s gamut.
Thread deleted for brevity.shot because you can view
So, I would think that grey/color bar is immaterial for the taking of the
the shot on the LCD view finder and tell if the colors are where you want.
Ansel Adams pointed out that some manufacturers of light meters intentionally miscalibrate their meters to try to create a margin for error for the typical user. […]
"Tom Elliott" wrote in message
Thread deleted for brevity.shot because you can view
So, I would think that grey/color bar is immaterial for the taking of the
the shot on the LCD view finder and tell if the colors are where you want.
[…]
That is far from true. There are profound shortcomings concerning color rendition and calibration of workstation displays, and I suspect hell will freeze over before there’s a good little LCD.
outThere’s no way I could do that with my consumer digicam, because it is limited to sRGB to start with. Many patches of an IT8 chart are simply
of the
camera’s gamut.
Bummer! I’d rather lose the bet than learn that.
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