Gray Card – 18%

FM
Posted By
Frank Morris
Feb 15, 2005
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1686
Replies
43
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Closed
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set custom white balance?


Frank Morris

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MR
Mike Russell
Feb 15, 2005
Frank Morris wrote:
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set custom white balance?

In the sRGB and Adobe RGB working spaces the value is approximately RGB(118,118,118).

This question is surprisingly complex for several reasons: the philosophy behind the 18% card is open to interpretation, reflectance and luminance are different concepts, and RGB brightness values usually have a gamma value associated with them. Here is a link to a previous thread on this topic: http://www.forum4designers.com/archive4-2004-3-51951.html

The philosophy behind 18 percent gray is that it is a good aim point for the normal exposure for a normal photograph. Though not really mathmatical in nature, this is a perfectly meaningful concept. It is valid to point your digital camera at a wall, take an image, and read the RGB values in Photoshop’s info palette. This value will be your camera manufacturer’s concept of an "18 percent gray".


Mike Russell
www.curvemeister.com
www.geigy.2y.net
K
Kayaker
Feb 15, 2005
Frank Morris wrote:
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to

set custom white balance?

In the sRGB and Adobe RGB working >>spaces the value is approximately

RGB(118,118,118).

This question is surprisingly complex for >>several reasons: the
philosophy
behind the 18% card is open to >>interpretation, reflectance and
luminance >>are different concepts, and RGB >>brightness values usually have a gamma >>value associated with them. Here is a link >>to a previous thread on this topic:
http://www.forum4designers.com/archive4->>2004-3-51951.html

I understand the 18% gray card is supposed to represent a middle gray, half way between black and white. If in Photoshop, I use the Gradient tool to create a blend from black to white and then using Posterize, and setting the number of gradients to be an uneven number, I can move my curser to the middle gradient, Info says the RGB colors are 127, 127, 127. Why doesn’t this represent 18% gray? It’s the middle gradient of the grays.
Kayaker
MR
Mike Russell
Feb 15, 2005
Kayaker wrote:
[re 18% gray card RGB value]

I understand the 18% gray card is supposed to represent a middle gray, half way between black and white. If in Photoshop, I use the Gradient tool to create a blend from black to white and then using Posterize, and setting the number of gradients to be an uneven number, I can move my curser to the middle gradient, Info says the RGB colors are 127, 127, 127. Why doesn’t this represent 18% gray? It’s the middle gradient of the grays.

There are different definitions of middle. Because of Gamma, the middle of RGB is not the same as the middle of the brightness range. The 118 value is what you get if you select Lab(50,0,0) in Photoshop’s color picker, for an Adobe RGB image. For Apple RGB this value is close to RGB(100,100,100).


Mike Russell
www.curvemeister.com
www.geigy.2y.net
X
xDsrtRat
Feb 16, 2005
18% Gray is more like a middle log value, not an arithmetic value.

It comes from the Zone System of photography where, relating to the finished print, Zone 9 is paper base white (unexposed and developed, fixed paper) and Zone 1 is photopaper has the densest black obtainable (exposed to daylight, developed in bright daylight for a long time* and fixed). Each of the steps is twice the brightness of the preceding step. It works out that the brilliance of the reflectance from Zone 5 is approximately 18% of the light falling on the paper.

For more information, see the works of Ansel Adams and Minor White. There are corresponding reverse values for the negative, but since we are talking about printing here, we’ll leave that alone!

*Photo-purists please note, I am deliberately ignoring the effects of solarization and physical development in my deep black example, I’m just trying to give a visual example most people can relate to.


___________________________________
I just want to know, how can someone claim
to be re-elected when they were not elected
the first time?

"Kayaker" wrote in message
Frank Morris wrote:
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to

set custom white balance?

In the sRGB and Adobe RGB working >>spaces the value is approximately

RGB(118,118,118).

This question is surprisingly complex for >>several reasons: the
philosophy
behind the 18% card is open to >>interpretation, reflectance and
luminance >>are different concepts, and RGB >>brightness values usually have a gamma >>value associated with them. Here is a link >>to a previous thread on this topic:
http://www.forum4designers.com/archive4->>2004-3-51951.html

I understand the 18% gray card is supposed to represent a middle gray, half way between black and white. If in Photoshop, I use the Gradient tool to create a blend from black to white and then using Posterize, and setting the number of gradients to be an uneven number, I can move my curser to the middle gradient, Info says the RGB colors are 127, 127, 127. Why doesn’t this represent 18% gray? It’s the middle gradient of the grays.
Kayaker
R
RSD99
Feb 16, 2005
I was taught that the use of 18% gray goes back way before the days of Minor White, Edward Wesson, Mortensen, or Ansel Adams. I’m a bit "hazy on the details," but IIRC, someone at the NBS, who was doing research on photometrics and color theory, developed the hypothesis that the *average* reflectance of a natural scene was approximately 18%.

Therefore, an 18% gray was chosen as the "midpoint" or exposure aimpoint. This was alleged to have occurred during the late 1920’s to mid 1930’s,

Don’t know if it’s really true, but I’ll bet that a search of the NBS/NIST archives will provide a **lot** of information on the general subject(s) that was developed during the referenced time period.

"xDsrtRat" wrote in message
18% Gray is more like a middle log value, not an arithmetic value.
It comes from the Zone System of photography where, relating to the
finished
print, Zone 9 is paper base white (unexposed and developed, fixed paper)
and
Zone 1 is photopaper has the densest black obtainable (exposed to
daylight,
developed in bright daylight for a long time* and fixed). Each of the
steps
is twice the brightness of the preceding step. It works out that the brilliance of the reflectance from Zone 5 is approximately 18% of the
light
falling on the paper.

For more information, see the works of Ansel Adams and Minor White. There are corresponding reverse values for the negative, but since we are
talking
about printing here, we’ll leave that alone!

*Photo-purists please note, I am deliberately ignoring the effects of solarization and physical development in my deep black example, I’m just trying to give a visual example most people can relate to.

___________________________________
I just want to know, how can someone claim
to be re-elected when they were not elected
the first time?

"Kayaker" wrote in message
Frank Morris wrote:
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to

set custom white balance?

In the sRGB and Adobe RGB working >>spaces the value is approximately

RGB(118,118,118).

This question is surprisingly complex for >>several reasons: the
philosophy
behind the 18% card is open to >>interpretation, reflectance and
luminance >>are different concepts, and RGB >>brightness values usually have a gamma >>value associated with them. Here is a link >>to a previous thread on this topic:
http://www.forum4designers.com/archive4->>2004-3-51951.html

I understand the 18% gray card is supposed to represent a middle gray, half way between black and white. If in Photoshop, I use the Gradient tool to create a blend from black to white and then using Posterize, and setting the number of gradients to be an uneven number, I can move my curser to the middle gradient, Info says the RGB colors are 127, 127, 127. Why doesn’t this represent 18% gray? It’s the middle gradient of the grays.
Kayaker

B
Brian
Feb 16, 2005
RSD99 wrote:

I was taught that the use of 18% gray goes back way before the days of Minor White, Edward Wesson, Mortensen, or Ansel Adams. I’m a bit "hazy on the details," but IIRC, someone at the NBS, who was doing research on photometrics and color theory, developed the hypothesis that the *average* reflectance of a natural scene was approximately 18%.

Therefore, an 18% gray was chosen as the "midpoint" or exposure aimpoint. This was alleged to have occurred during the late 1920’s to mid 1930’s,
Don’t know if it’s really true, but I’ll bet that a search of the NBS/NIST archives will provide a **lot** of information on the general subject(s) that was developed during the referenced time period.

"xDsrtRat" wrote in message

18% Gray is more like a middle log value, not an arithmetic value.
It comes from the Zone System of photography where, relating to the

finished

print, Zone 9 is paper base white (unexposed and developed, fixed paper)

and

Zone 1 is photopaper has the densest black obtainable (exposed to

daylight,

developed in bright daylight for a long time* and fixed). Each of the

steps

is twice the brightness of the preceding step. It works out that the brilliance of the reflectance from Zone 5 is approximately 18% of the

light

falling on the paper.

For more information, see the works of Ansel Adams and Minor White. There are corresponding reverse values for the negative, but since we are

talking

about printing here, we’ll leave that alone!

*Photo-purists please note, I am deliberately ignoring the effects of solarization and physical development in my deep black example, I’m just trying to give a visual example most people can relate to.

___________________________________
I just want to know, how can someone claim
to be re-elected when they were not elected
the first time?

"Kayaker" wrote in message

Frank Morris wrote:

Can anyone tell me the RGB equivalent of 18% Gray as used on cards to

set custom white balance?

In the sRGB and Adobe RGB working >>spaces the value is approximately

RGB(118,118,118).

This question is surprisingly complex for >>several reasons: the

philosophy

behind the 18% card is open to >>interpretation, reflectance and

luminance >>are different concepts, and RGB >>brightness values usually have a gamma >>value associated with them. Here is a link >>to a previous thread on this topic:

http://www.forum4designers.com/archive4->>2004-3-51951.html

I understand the 18% gray card is supposed to represent a middle gray, half way between black and white. If in Photoshop, I use the Gradient tool to create a blend from black to white and then using Posterize, and setting the number of gradients to be an uneven number, I can move my curser to the middle gradient, Info says the RGB colors are 127, 127, 127. Why doesn’t this represent 18% gray? It’s the middle gradient of the grays.
Kayaker
You are quite right RSD, an 18% grey card gives the same refelctance of light as that of an "average scene." So if 100 units of light are hitting an "average" scene/subject, 18 units of light will be reflected off that scene/subject.

An 18% grey card is purely a reference card the photographer uses to work out how much light is reaching a scene, so he can set his camera accordingly (aperture/shutter speed) for a correct exposure.

Brian.
CD
Colin D
Feb 22, 2005
Frank Morris wrote:
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set custom white balance?


Frank Morris

Actually (having read what other posters have said), the choice of an 18% gray for exposure reference *is* derived mathematically. But, it is open to objection, as you will see.

If one postulates a five-stop range for the brightness values in a typical scene, then the brightness range is 2^5, or 1:32. The geometric mean between 1 and 32 is simply the sqrt(32), or 5.66. This says that the range 1:5.66 is the same as the range 5.66:32 – which is right because 5.66 is the geometric mean of 1 and 32.

Ok, the brightness, or ‘grayness’ of 5.66 units as a percentage of 32 units is 5.66*100/32, which is 17.68%, rounded to 18%. Therefore, an 18% gray card represents the geometric mean of a brightness range of 5 stops.

However, what if the brightness range is not 5 stops? What if it is 6 or 7 stops? For a 6-stop range, the brightness range is 2^6, or 1:64. The geometric mean of 64 is 8, and 8 as a percentage of 64 is 8*100/64, = 12.5%

So now we need a 12.5% gray card if our scene brightness covers 6 stops. It is interesting to note that some European camera manufacturers actually calibrated their meters for a 12.5% gray on the grounds that five stops is not enough, particularly for negative films.

Modern films, and certainly digital cameras, can cover much more than 6 stops of brightness, and some scenes can have a brightness range of 1:128.

Let’s go round again for a 7-stop range, and see what happens. 2^7 is 128, and the geometric mean of 128 is 11.31. 11.31*100/128 is 8.84% – so the correct gray card for this 7-stop range would be an 8.8% gray.

So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin
B
Brian
Feb 22, 2005
Colin D wrote:
Frank Morris wrote:

Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set custom white balance?


Frank Morris

Actually (having read what other posters have said), the choice of an 18% gray for exposure reference *is* derived mathematically. But, it is open to objection, as you will see.

If one postulates a five-stop range for the brightness values in a typical scene, then the brightness range is 2^5, or 1:32. The geometric mean between 1 and 32 is simply the sqrt(32), or 5.66. This says that the range 1:5.66 is the same as the range 5.66:32 – which is right because 5.66 is the geometric mean of 1 and 32.

Ok, the brightness, or ‘grayness’ of 5.66 units as a percentage of 32 units is 5.66*100/32, which is 17.68%, rounded to 18%. Therefore, an 18% gray card represents the geometric mean of a brightness range of 5 stops.

However, what if the brightness range is not 5 stops? What if it is 6 or 7 stops? For a 6-stop range, the brightness range is 2^6, or 1:64. The geometric mean of 64 is 8, and 8 as a percentage of 64 is 8*100/64, = 12.5%

So now we need a 12.5% gray card if our scene brightness covers 6 stops. It is interesting to note that some European camera manufacturers actually calibrated their meters for a 12.5% gray on the grounds that five stops is not enough, particularly for negative films.
Modern films, and certainly digital cameras, can cover much more than 6 stops of brightness, and some scenes can have a brightness range of 1:128.

Let’s go round again for a 7-stop range, and see what happens. 2^7 is 128, and the geometric mean of 128 is 11.31. 11.31*100/128 is 8.84% – so the correct gray card for this 7-stop range would be an 8.8% gray.
So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin

Sorry Colin, I think you are misinterpreting what a grey card is all about. The overall brightness range of a scene has nothing to do with it. An 18% grey card simply REFLECTS the same amount of light (on a percentage basis) as what an average scene does. In other words, take an "average" scene and it has been found to reflect 18% of the light landing upon it. A grey card also reflects 18% of the rays landing upon it. It is purely a measure of reflectance, nothing to do with brightness. I could ramble on here and clearly describe what the grey card is really telling you and why, but I don’t want to bore people with photography talk. Brian.
B
Brian
Feb 22, 2005
Brian wrote:

Colin D wrote:

Frank Morris wrote:

Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set
custom white balance?


Frank Morris

Actually (having read what other posters have said), the choice of an 18% gray for exposure reference *is* derived mathematically. But, it is open to objection, as you will see.

If one postulates a five-stop range for the brightness values in a typical scene, then the brightness range is 2^5, or 1:32. The geometric mean between 1 and 32 is simply the sqrt(32), or 5.66. This says that the range 1:5.66 is the same as the range 5.66:32 – which is right because 5.66 is the geometric mean of 1 and 32.

Ok, the brightness, or ‘grayness’ of 5.66 units as a percentage of 32 units is 5.66*100/32, which is 17.68%, rounded to 18%. Therefore, an 18% gray card represents the geometric mean of a brightness range of 5 stops.

However, what if the brightness range is not 5 stops? What if it is 6 or 7 stops? For a 6-stop range, the brightness range is 2^6, or 1:64. The geometric mean of 64 is 8, and 8 as a percentage of 64 is 8*100/64, = 12.5%
So now we need a 12.5% gray card if our scene brightness covers 6 stops. It is interesting to note that some European camera manufacturers actually calibrated their meters for a 12.5% gray on the grounds that five stops is not enough, particularly for negative films.
Modern films, and certainly digital cameras, can cover much more than 6 stops of brightness, and some scenes can have a brightness range of 1:128.

Let’s go round again for a 7-stop range, and see what happens. 2^7 is 128, and the geometric mean of 128 is 11.31. 11.31*100/128 is 8.84% – so the correct gray card for this 7-stop range would be an 8.8% gray.
So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin

Sorry Colin, I think you are misinterpreting what a grey card is all about. The overall brightness range of a scene has nothing to do with it. An 18% grey card simply REFLECTS the same amount of light (on a percentage basis) as what an average scene does. In other words, take an "average" scene and it has been found to reflect 18% of the light landing upon it. A grey card also reflects 18% of the rays landing upon it. It is purely a measure of reflectance, nothing to do with brightness. I could ramble on here and clearly describe what the grey card is really telling you and why, but I don’t want to bore people with photography talk. Brian.

Just to clarify that Colin, you are talking as though an 18% grey card represents the average brightness of a scene. No, it represents the average reflectance of a scene. Totally different issue.

Brian.
B
Brian
Feb 22, 2005
..
So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin

Colin,
I just read your last sentence again. You mentioned contrastier scenes. Let me explain it now. What affects exposure is actually how much light is landing on a scene, not how much light is reflected from a scene. Therefore, the camera meter has to work out how much light is hitting a scene by working back from how much light is reflected from a scene. As an example, lets pretend 100 units of light results in a camera setting of 1/250 sec at aperture F8. It does not matter what coloured t-shirt the subject is wearing, black or white for example, the correct exposure is still obtained in this example using 1/250 at F8 (or any shifted combination).
Camera meters are programmed to assume that what they see is an "average scene" of 18% reflectance. This is incorrect because if the subject wears a white t-shirt he may be reflecting 95% of the light landing upon him, and if he wears black he may only be reflecting 5% (rough figures). So look at each example. For simplicity, let’s pretend it is a 20% grey card, not 18% (just to make calculations easier). The white t-shirt: 100 units hit it, 95 units are relfected, BUT the camera thinks it is seeing 20% of the original light, so it thinks there is 95 X 5= 450 units landing on the subject. The black t-shirt: 100 units of light hit it, 5 units reflect off it. The camera sees 5 units of light and thinks there must be 5 x 5 = 25 units of light hitting the subject. So look what has happened, the camera thinks one subject has 4.5 times more light hitting it than there really is, and the other subject has 1/4 the light hitting it than there really is. It will give 2 "completely" different readings for you to set on your camera……BUT both subjects actually need the same setting to be correctly exposed.
In comes the grey card. It’s reflectance is accurate. Hold it in front of both subjects and point it toward the camera. The grey card reflects 18% (20% for this example) so the camera sees 20 units of light. The camera assumes it is seeing 20% of the light hitting the scene (and it is!!!) and assumes there is 100 units of light landing on the subject. So it gives a correct exposure. Whether the subject was wearing black or white made no difference to the grey card. It does not even see the black or white. It simply points towards the light hitting the subject and reflects 18% back at the camera.
Can you see my point now? What has the bightness range of the scene to do with anything? It does not matter if the scene is dark or light, flat or contrasty, etc, the grey card reflects the "light falling upon the subject" back at the camera with a single reading!!! The correct reading. Where it gets tricky and you need multiple spot readings is where there is not the same light falling on different parts of the scene. Then the grey card would only give an accurate reading for the part of the subject sitting in the same light as the grey card is. Other parts of the scene obscured by shadows from buildings, montains, etc. would be under-exposed. They would have less light on them than the areas sitting in direct sunlight where the grey card is in this example. So sorry to bore everyone,
Brian.
T
tailender
Feb 22, 2005
"Brian" wrote in message
.
So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin

Colin,
I just read your last sentence again. You mentioned contrastier scenes. Let me explain it now. What affects exposure is actually how much light is landing on a scene, not how much light is reflected from a scene. Therefore, the camera meter has to work out how much light is hitting a scene by working back from how much light is reflected from a scene. As an example, lets pretend 100 units of light results in a camera setting of 1/250 sec at aperture F8. It does not matter what coloured t-shirt the subject is wearing, black or white for example, the correct exposure is still obtained in this example using 1/250 at F8 (or any shifted combination).
Camera meters are programmed to assume that what they see is an "average scene" of 18% reflectance. This is incorrect because if the subject wears a white t-shirt he may be reflecting 95% of the light landing upon him, and if he wears black he may only be reflecting 5% (rough figures). So look at each example. For simplicity, let’s pretend it is a 20% grey card, not 18% (just to make calculations easier). The white t-shirt: 100 units hit it, 95 units are relfected, BUT the camera thinks it is seeing 20% of the original light, so it thinks there is 95 X 5= 450 units landing on the subject. The black t-shirt: 100 units of light hit it, 5 units reflect off it. The camera sees 5 units of light and thinks there must be 5 x 5 = 25 units of light hitting the subject. So look what has happened, the camera thinks one subject has 4.5 times more light hitting it than there really is, and the other subject has 1/4 the light hitting it than there really is. It will give 2 "completely" different readings for you to set on your camera……BUT both subjects actually need the same setting to be correctly exposed.
In comes the grey card. It’s reflectance is accurate. Hold it in front of both subjects and point it toward the camera. The grey card reflects 18% (20% for this example) so the camera sees 20 units of light. The camera assumes it is seeing 20% of the light hitting the scene (and it is!!!) and assumes there is 100 units of light landing on the subject. So it gives a correct exposure. Whether the subject was wearing black or white made no difference to the grey card. It does not even see the black or white. It simply points towards the light hitting the subject and reflects 18% back at the camera.
Can you see my point now? What has the bightness range of the scene to do with anything? It does not matter if the scene is dark or light, flat or contrasty, etc, the grey card reflects the "light falling upon the subject" back at the camera with a single reading!!! The correct reading. Where it gets tricky and you need multiple spot readings is where there is not the same light falling on different parts of the scene. Then the grey card would only give an accurate reading for the part of the subject sitting in the same light as the grey card is. Other parts of the scene obscured by shadows from buildings, montains, etc. would be under-exposed. They would have less light on them than the areas sitting in direct sunlight where the grey card is in this example. So sorry to bore everyone,
Brian.

never mind a war! could someone provide a short, simple explanation of how a newbie might benefit by using an 18% grey card with a digital camera

Then all the undoubtedly useful science will have been of benefit to me too and I will feel all improved…sort of

B
H
Hecate
Feb 22, 2005
On Tue, 22 Feb 2005 19:38:09 -0000, "tailender" wrote:

never mind a war! could someone provide a short, simple explanation of how a newbie might benefit by using an 18% grey card with a digital camera
Then all the undoubtedly useful science will have been of benefit to me too and I will feel all improved…sort of
That depends on how much control you have. If you are able to meter off a gray card and hold that value before taking the shot, then you’ll get a well-exposed image in most situations. If you can’t then it’s no use to you at all.



Hecate – The Real One

veni, vidi, reliqui
MR
Mike Russell
Feb 22, 2005
Brian wrote:
.
So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin

Colin,
I just read your last sentence again. You mentioned contrastier scenes. Let me explain it now. What affects exposure is actually how much light is landing on a scene, not how much light is reflected from a scene. Therefore, the camera meter has to work out how much light is hitting a scene by working back from how much light is reflected from a scene. As an example, lets pretend 100 units of light results in a camera setting of 1/250 sec at aperture F8. It does not matter what coloured t-shirt
the subject is wearing, black or white for example, the correct exposure is still obtained in this example using 1/250 at F8 (or any shifted combination).
Camera meters are programmed to assume that what they see is an "average scene" of 18% reflectance. This is incorrect because if the subject
wears a white t-shirt he may be reflecting 95% of the light landing upon him, and if he wears black he may only be reflecting 5% (rough figures). So look at each example. For simplicity, let’s pretend it is a 20% grey card, not 18% (just to make calculations easier). The white t-shirt: 100 units hit it, 95 units are relfected, BUT the camera thinks it is seeing 20% of the original light, so it thinks there is 95 X 5= 450 units landing on the subject. The black t-shirt: 100 units of light hit it, 5 units reflect off it. The camera sees 5 units of light and thinks there must be 5 x 5 = 25 units of light hitting the subject. So look what has happened, the camera thinks one subject has 4.5 times more light hitting it than there really is, and the other subject has 1/4 the light hitting it than there really is. It will give 2 "completely" different readings for you to set on your camera……BUT both subjects actually need the same setting to be correctly exposed.
In comes the grey card. It’s reflectance is accurate. Hold it in front of both subjects and point it toward the camera. The grey card reflects 18% (20% for this example) so the camera sees 20 units of light. The camera assumes it is seeing 20% of the light hitting the scene (and it is!!!) and assumes there is 100 units of light landing on the subject.
So it gives a correct exposure. Whether the subject was wearing black or white made no difference to the grey card. It does not even see the black or white. It simply points towards the light hitting the subject and reflects 18% back at the camera.
Can you see my point now? What has the bightness range of the scene to do with anything? It does not matter if the scene is dark or light, flat or contrasty, etc, the grey card reflects the "light falling upon the subject" back at the camera with a single reading!!! The correct reading. Where it gets tricky and you need multiple spot readings is where there is not the same light falling on different parts of the scene. Then the grey card would only give an accurate reading for the part of the
subject sitting in the same light as the grey card is. Other parts of the scene obscured by shadows from buildings, montains, etc. would be under-exposed. They would have less light on them than the areas sitting in direct sunlight where the grey card is in this example. So sorry to bore everyone,
Brian.

To my reading, you’re recreating what Colin said, just in a different way. Colin summarized the reasoning behind 18 percent gray very well, using math. Your explanation is also correct, and more descriptive in nature.

The short answer is, yes, an 18 percent card can be of use. Use it to set your manual white balance before taking the picture. After capturing the image, set it to the RGB() values mentioned earlier in this thread, and you may improve your image, provided the card and scene are lit uniformly.

But there are limits to the usefulness of a gray card because uniform lighting is rare outside of a studio setting. Most images have two or more subtle color casts in them, and a gray card is of limited use in eliminating them. Because of this, a gray card is a good starting place, and also a good learning tool, but it is not a replacement for knowledgeable color correction.


Mike Russell
www.curvemeister.com
www.geigy.2y.net.
B
Brian
Feb 23, 2005
Frank Morris wrote:
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set custom white balance?
Just reading the original question here (as I should have done in the first place!), a grey card is really for setting "exposure". White balance / colour balance is a totally separate issue, usually dealt with using photographic colour correction filters.

Brian.
B
Brian
Feb 23, 2005
Mike Russell wrote:

To my reading, you’re recreating what Colin said, just in a different way. Colin summarized the reasoning behind 18 percent gray very well, using math. Your explanation is also correct, and more descriptive in nature.

Mike,

I won’t harp on about this topic any more, I have overkilled it already. I disagree with your above comment though. What Colin worked out mathematically had nothing to do with what I was saying. He worked out the average brightness of a scene. That is irrelevant. A grey card purely gives a measure of how much light is falling upon the scene which is the determining factor of exposure.

Brian
TT
Tom Thackrey
Feb 23, 2005
On 22-Feb-2005, Brian wrote:

Frank Morris wrote:
Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set
custom white balance?
Just reading the original question here (as I should have done in the first place!), a grey card is really for setting "exposure". White balance / colour balance is a totally separate issue, usually dealt with using photographic colour correction filters.

White balance is a separate issue from exposure. However, an 18% gray card works perfectly for both.


Tom Thackrey
www.creative-light.com
tom (at) creative (dash) light (dot) com
do NOT send email to (it’s reserved for spammers)
B
Brian
Feb 23, 2005
Tom Thackrey wrote:

On 22-Feb-2005, Brian wrote:

Frank Morris wrote:

Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set
custom white balance?

Just reading the original question here (as I should have done in the first place!), a grey card is really for setting "exposure". White balance / colour balance is a totally separate issue, usually dealt with using photographic colour correction filters.

White balance is a separate issue from exposure. However, an 18% gray card works perfectly for both.
Thanks for that information Tom, working with film still I was unaware of that. I will keep that in mind in the near future when I go digital at long last.

Brian.
S
Stephan
Feb 23, 2005
Hecate wrote:
On Tue, 22 Feb 2005 19:38:09 -0000, "tailender" wrote:

never mind a war! could someone provide a short, simple explanation of how a newbie might benefit by using an 18% grey card with a digital camera
Then all the undoubtedly useful science will have been of benefit to me too and I will feel all improved…sort of

That depends on how much control you have. If you are able to meter off a gray card and hold that value before taking the shot, then you’ll get a well-exposed image in most situations. If you can’t then it’s no use to you at all.

Two main things differentiating a pro from a non pro photographer: The lens shade and the gray card!

Stephan
CD
Colin D
Feb 23, 2005
Brian wrote:
.
So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin

Colin,
I just read your last sentence again. You mentioned contrastier scenes. Let me explain it now. What affects exposure is actually how much light is landing on a scene, not how much light is reflected from a scene. <snipped remainder>

With respect, Brian, I think your point of view has been derived from taking for granted the 18% card as being accurate for all occasions. That is not surprising, given that probably 95% of photographers believe the same. At the risk of boring people – though I think that learning is never boring – I’ll put it another way that might be easier to get hold of.

Let’s say you meter your shots with a spotmeter, and you do it the ‘Ansel Adams’ way, first measuring the brightness of the deepest important shadow, and then the brightest important highlight, not counting specular reflections.

And, let’s say the meter indicates 1/100 at f/4 for the shadows, and 1/100 at f/22 for the highlights. That’s a 5-stop range. Now, the chosen exposure to be used is obviously f/9.5, the median stop between f/4 and f/22, right? So we shoot at 1/100 at f/9.5.

Now, in that scenario, the brightness range of the scene extends 5 stops, and the median stop is 2½ stops away from the deepest shadows, and also 2½ stops away from the brightest highlights. So, the brightness ratio between f/4 and f/9.5 is 2^2.5, which is, according to my calculator, a ratio of 5.66 to 1. Likewise, the ratio between f/9.5 and f/22 is also 2½ stops, i.e. 5.66:1. Magically, if we multiply the two ranges to get the total range, we get 5.66×5.66 = 32.03. The .03 is due to rounding errors, so drop that, and we have the brightness range of 32:1 that we started with.

Finally, the ‘grayness’ of the median exposure, since the shadows are black and the highlights are white, is 5.66/32 of white, and, as in my first post, 5.66*100/32 = 17.69%.

If we had used an 18% gray card, it would have given the exposure we calculated, 1/100 at f/9.5.

Now, if you apply the same logic to a six-stop range, the median gray will be 12.5%, and so on.

Colin
B
Brian
Feb 23, 2005
Colin D wrote:
Brian wrote:

.

So, the much-touted accuracy of an 18% gray card exists only for a scene brightness range of 5 stops. Accurate exposure is best found by spot-meter readings of the darkest and the lightest parts to be recorded in the scene, and the exposure set accordingly. For a scene of 5 stops, the card will do it. For contrastier or flatter scenes, the card will be off.

This’ll probably start a war …

Colin

Colin,
I just read your last sentence again. You mentioned contrastier scenes. Let me explain it now. What affects exposure is actually how much light is landing on a scene, not how much light is reflected from a scene. <snipped remainder>

With respect, Brian, I think your point of view has been derived from taking for granted the 18% card as being accurate for all occasions. That is not surprising, given that probably 95% of photographers believe the same. At the risk of boring people – though I think that learning is never boring – I’ll put it another way that might be easier to get hold of.

Let’s say you meter your shots with a spotmeter, and you do it the ‘Ansel Adams’ way, first measuring the brightness of the deepest important shadow, and then the brightest important highlight, not counting specular reflections.

And, let’s say the meter indicates 1/100 at f/4 for the shadows, and 1/100 at f/22 for the highlights. That’s a 5-stop range. Now, the chosen exposure to be used is obviously f/9.5, the median stop between f/4 and f/22, right? So we shoot at 1/100 at f/9.5.

Now, in that scenario, the brightness range of the scene extends 5 stops, and the median stop is 2½ stops away from the deepest shadows, and also 2½ stops away from the brightest highlights. So, the brightness ratio between f/4 and f/9.5 is 2^2.5, which is, according to my calculator, a ratio of 5.66 to 1. Likewise, the ratio between f/9.5 and f/22 is also 2½ stops, i.e. 5.66:1. Magically, if we multiply the two ranges to get the total range, we get 5.66×5.66 = 32.03. The .03 is due to rounding errors, so drop that, and we have the brightness range of 32:1 that we started with.

Finally, the ‘grayness’ of the median exposure, since the shadows are black and the highlights are white, is 5.66/32 of white, and, as in my first post, 5.66*100/32 = 17.69%.

If we had used an 18% gray card, it would have given the exposure we calculated, 1/100 at f/9.5.

Now, if you apply the same logic to a six-stop range, the median gray will be 12.5%, and so on.

Colin

Colin,

Maybe I am misunderstanding you? Are you giving me a method of working out the exposure of a scene where the light is not even across the whole scene, due to shadows (cast by objects, folds in clothing, etc)? If so, then I can see your point and it makes a lot of sense.
If that is the case, it doesn’t really have anything to do with the accuracy of a grey card. 18% grey is still the "correct" grey for providing accurate metering results. You are actually talking of a different scenario. To do what you are talking about, you would have to use the grey card in several different places in the scene and then apply your averaging methods. This still does not change the fact that 18% is the correct value. It will give a correct metering value for "each" part you meter with it. It is the correct value for each spot reading. I hope you follow me.
Just one other thought….you arrived at the ratio 32:1 a very long way. All you had to say was….there is a 5 stop range in the scene = 2 to the power of 5 = 32 (as we are using a base 2 logarithmic scale).

All the best, and thanks for the thoughts and discussion, Brian
T
tailender
Feb 23, 2005
"Stephan" wrote in message
Hecate wrote:
On Tue, 22 Feb 2005 19:38:09 -0000, "tailender" wrote:

never mind a war! could someone provide a short, simple explanation of
how a
newbie might benefit by using an 18% grey card with a digital camera
Then all the undoubtedly useful science will have been of benefit to me
too
and I will feel all improved…sort of

That depends on how much control you have. If you are able to meter off a gray card and hold that value before taking the shot, then you’ll get a well-exposed image in most situations. If you can’t then it’s no use to you at all.

Two main things differentiating a pro from a non pro photographer: The lens shade and the gray card!

Stephan

Two main things differentiating a balanced human being from a tosser with ego deficiencies:
Inability to type nothing when nothing useful to say!

B
M
Marsupilami
Feb 23, 2005
Two main things differentiating a pro from a non pro photographer: The lens shade and the gray card!

Stephan

The ONLY thing that differentiate a pro from a non pro… The jacket…:-)))
http://www.walkabout.com/shop/coat-ff-vest.asp

(also known as the Canon photographer outfit)
another post will be for the Nikonist garment.


Houba houba.
Marsu.
"Never say never"- Romeo void
S
Stephan
Feb 23, 2005
tailender wrote:

Two main things differentiating a balanced human being from a tosser with ego deficiencies:
Inability to type nothing when nothing useful to say!
Hey thanks for the lesson.
So if I get it right you would be a tosser. Right?

Stephan
H
Hecate
Feb 23, 2005
On Wed, 23 Feb 2005 22:51:51 +0100, "Marsupilami" wrote:

Two main things differentiating a pro from a non pro photographer: The lens shade and the gray card!

Stephan

The ONLY thing that differentiate a pro from a non pro… The jacket…:-)))
http://www.walkabout.com/shop/coat-ff-vest.asp

(also known as the Canon photographer outfit)
another post will be for the Nikonist garment.

Oops! Wrong – I much prefer the Tamrac version 🙂



Hecate – The Real One

veni, vidi, reliqui
R
RSD99
Feb 24, 2005
Or the Domke version … but certainly *NOT* the generic "African Safari Jacket."

"Hecate" wrote in message
On Wed, 23 Feb 2005 22:51:51 +0100, "Marsupilami" wrote:

Two main things differentiating a pro from a non pro photographer: The lens shade and the gray card!

Stephan

The ONLY thing that differentiate a pro from a non pro… The jacket…:-)))
http://www.walkabout.com/shop/coat-ff-vest.asp

(also known as the Canon photographer outfit)
another post will be for the Nikonist garment.

Oops! Wrong – I much prefer the Tamrac version 🙂



Hecate – The Real One

veni, vidi, reliqui
M
Marsupilami
Feb 24, 2005
RSD99 wrote:
Or the Domke version … but certainly *NOT* the generic "African Safari Jacket."

For I saw much of pro photographers with that kind of "safari" stuff… (and many groupies around)
They made me laugh but now I’m still nothin’


Houba houba.
Marsu.
"The future’s so bright I gotta wear shades"
Timbuk 3
H
Hecate
Feb 25, 2005
On Thu, 24 Feb 2005 21:22:42 +0100, "Marsupilami" wrote:

RSD99 wrote:
Or the Domke version … but certainly *NOT* the generic "African Safari Jacket."

For I saw much of pro photographers with that kind of "safari" stuff… (and many groupies around)
They made me laugh but now I’m still nothin’

<g> They’re really useful for air travel amongst other things. With the strict weight limits for carry-on baggage, you can save weight by stuffing a body and 2 or 3 lenses in the jacket. 🙂



Hecate – The Real One

veni, vidi, reliqui
B
Brian
Feb 25, 2005
Hecate wrote:

On Thu, 24 Feb 2005 21:22:42 +0100, "Marsupilami" wrote:

RSD99 wrote:

Or the Domke version … but certainly *NOT* the generic "African Safari Jacket."

For I saw much of pro photographers with that kind of "safari" stuff… (and many groupies around)
They made me laugh but now I’m still nothin’

<g> They’re really useful for air travel amongst other things. With the strict weight limits for carry-on baggage, you can save weight by stuffing a body and 2 or 3 lenses in the jacket. 🙂



Hecate – The Real One

veni, vidi, reliqui

You are crazt, you know that Hecate! Just one question….the real "one". But one what?

Brian 🙂
S
Stephan
Feb 25, 2005
Brian wrote:

You are crazt, you know that Hecate! Just one question….the real "one". But one what?

The Real Hecate of course..
Bt the wat, you ttping is funnt

Stephan
M
Marsupilami
Feb 25, 2005
For I saw much of pro photographers with that kind of "safari" stuff… (and many groupies around)
They made me laugh but now I’m still nothin’

<g> They’re really useful for air travel amongst other things. With the strict weight limits for carry-on baggage, you can save weight by stuffing a body and 2 or 3 lenses in the jacket. 🙂

BTW, I do prefer that sort of nikon outfit
less showing off…
http://minilien.com/?jaZ33fmqM2

Less practical for air travel maybe…
"where the medium is more important than the speaker…"


Houba houba.
Marsu.
"Le num
MR
Mike Russell
Feb 25, 2005

[re photog vest]

<g> They’re really useful for air travel amongst other things. With the strict weight limits for carry-on baggage, you can save weight by stuffing a body and 2 or 3 lenses in the jacket. 🙂

I think the "body" would still be classified as carrion, whether it was stuffed in a vest or not.

Mike Russell
www.curvemeister.com
www.geigy.2y.net
H
Hecate
Feb 25, 2005
On Fri, 25 Feb 2005 18:07:53 +1100, Brian
wrote:

<g> They’re really useful for air travel amongst other things. With the strict weight limits for carry-on baggage, you can save weight by stuffing a body and 2 or 3 lenses in the jacket. 🙂

You are crazt, you know that Hecate! Just one question….the real "one". But one what?

Brian 🙂

The Real One as opposed to the person who was trying to pretend to be me a couple of months back 🙂

Me not crazy, just a little eccentric 😉



Hecate – The Real One

veni, vidi, reliqui
R
RSD99
Feb 25, 2005
I believe that was one of the many incarnations of the person sometimes known as "ArtistMike" … wasn’t it?

"Hecate" wrote in message
The Real One as opposed to the person who was trying to pretend to be me a couple of months back 🙂

Me not crazy, just a little eccentric 😉



Hecate – The Real One

veni, vidi, reliqui
MR
Mike Russell
Feb 25, 2005
RSD99 wrote:
I believe that was one of the many incarnations of the person sometimes known as [shhhh] … wasn’t it?

Yes, a harlequin with a peppermill who, now and then, comes through town and tears us all a new one.


Mike Russell
www.curvemeister.com
www.geigy.2y.net
B
Brian
Feb 26, 2005
Stephan wrote:

Brian wrote:

You are crazt, you know that Hecate! Just one question….the real "one". But one what?

The Real Hecate of course..
Bt the wat, you ttping is funnt

Stephan

Hahaha. The only time my ttping is funnt (grrrr) is when I hop on here VERY late at night and I am almost asleep. That is also the time I am cranky and type messages which seem to annoy people. Sorry about that peeps. I really enjoy this newsgroup and hope that I don’t annoy anyone too much.
I am new to NG’s and I am really impressed how much one can learn in here. I have been working with images for 8 yrs on computer and for 18 yrs or so with cameras. I guess we get a little set in our ways (coz they work), but coming in here we learn even better ways to do things that we either forgot, or were unaware of to begin with. Thanks everyone, cheers…
Brian in Oz.
R
RSD99
Feb 26, 2005
HeHeHeHeHe …

You are being way "too kind" …

"Mike Russell" wrote in message
RSD99 wrote:
I believe that was one of the many incarnations of the person sometimes known as [shhhh] … wasn’t it?

Yes, a harlequin with a peppermill who, now and then, comes through town
and
tears us all a new one.


Mike Russell
www.curvemeister.com
www.geigy.2y.net

H
Hecate
Feb 26, 2005
On Fri, 25 Feb 2005 23:16:03 GMT, "RSD99"
wrote:

I believe that was one of the many incarnations of the person sometimes known as "ArtistMike" … wasn’t it?
Darn it, now you’ve mentioned his name and you know what happens when you do that….



Hecate – The Real One

veni, vidi, reliqui
H
Hecate
Feb 26, 2005
On Fri, 25 Feb 2005 22:42:09 GMT, "Mike Russell" wrote:

[re photog vest]

<g> They’re really useful for air travel amongst other things. With the strict weight limits for carry-on baggage, you can save weight by stuffing a body and 2 or 3 lenses in the jacket. 🙂

I think the "body" would still be classified as carrion, whether it was stuffed in a vest or not.

LOL!



Hecate – The Real One

veni, vidi, reliqui
M
Marsupilami
Feb 27, 2005
I believe that was one of the many incarnations of the person sometimes known as "ArtistMike" … wasn’t it?
Darn it, now you’ve mentioned his name and you know what happens when you do that….

BEETLEJUICE- BEETLEJUICE- BEETLEJUICE!!!!


Houba houba.
Marsu.
"Le num
P
paul
Feb 27, 2005
Brian wrote:

Tom Thackrey wrote:

On 22-Feb-2005, Brian wrote:

Frank Morris wrote:

Can anyone tell me the RGB equivalent of 18% Gray as used on cards to set
custom white balance?

Just reading the original question here (as I should have done in the first place!), a grey card is really for setting "exposure". White balance / colour balance is a totally separate issue, usually dealt with using photographic colour correction filters.

White balance is a separate issue from exposure. However, an 18% gray card
works perfectly for both.
Thanks for that information Tom, working with film still I was unaware of that. I will keep that in mind in the near future when I go digital at long last.

Color balance is probably the more important reason for a gray card with digital. If you shoot RAW format, the safest way to set exposure is to push it as bright as possible without blowing the highlights completely due to RAW files having more information in the highlights & the shadows have less info so cannot be brightened without damage. I guess this is due to the logarithmic math. I suppose the 18% grey value could still be used to do the post processing & get it back to 18% but at that point it is no longer an accurate representation of reality but stretched to get the most dynamic range. Artistic intent may or may not change all this.
S
Stephan
Feb 27, 2005
Marsupilami wrote:

BEETLEJUICE- BEETLEJUICE- BEETLEJUICE!!!!

That’s OK, just don’t say Mike three times.

Houbi houbi

Stephan
M
Marsupilami
Feb 28, 2005
That’s OK, just don’t say Mike three times.

Houbi houbi

Stephan

Houba houba
Noone dared pick me up up that way for years 😉


Houba houba.
Marsu..
I’ll give you candy
Give you diamonds
Give you pills
Give you anything you want
Hundred dollar bills
I’ll even let you watch the shows you wanna see
Just marry me, marry me, marry me

Dramarama – "anything"
S
Stephan
Mar 1, 2005
Marsupilami wrote:
That’s OK, just don’t say Mike three times.

Houbi houbi

Stephan

Houba houba
Noone dared pick me up up that way for years 😉

"Pas glop" and "Bof" generation oblige!

Stephan

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