Well lets get something straight. I understand and agree with you that myths get promulgated by repetition... even in academic documents and standards. I make a living in engineering standards and their application, BTW, so I know that better than many. Nobody is suggesting that you repeat incorrect information since that would contradict your thesis. And nobody that I know of is telling you to stop your effort. All I suggest is that literature review is a normal and accepted practice in scientific and engineering documentation like this. It helps address the question many readers have, like "Did the author do his/her homework or just bloviating opinion?" You probably have done it intuitively. But for a nontechnical approach I submit that many people have never noticed that their correct or incorrect use of a grey card because that hasn't made photography, as a whole or as an individual effort, fail in a significant manner. But that doesn't matter... if there is a right way then that should be known. I appreciate this effort. My point was that I looked at a couple of existing "how to use a grey card" sections of books by known and respected authors and noted that (1) "The Exposure Manual" was too technical and resulted in my not really understanding if there is a right way and a wrong way, and (2) "Perfect Exposure" seems to say exactly what you say but without the discussion of H&D etc... and I understood it. I'm not sure that kind of discussion is necessary in a NON-TECHNICAL article. If you do, then do what you want... as said earlier, I don't think anyone is trying to say that you shouldn't. The summation of my comments is this: I'm not sure that I really understood the point you are trying to make until the last page. But maybe I'm not your intended audience since I haven't seriously used a grey card for the last 28 years. And one more thought... how about a statement of who you are and what your qualifications are. That is another "credibility building" aspect of scientific and engineering documentation like this. I have no reason to doubt your credentials, but I don't really know anything about you except for a few postings on internet forums. Good luck with your effort.
Feedback Please: The Kodak 18% Grey Card and Metering, a new look.
-
A
- Thread starter RobC
- Start date
Recent Classifieds
-
For Sale Whole plate film
- Started by Peter Schrager
-
Free Beseler Polycontrast Filters
- Started by mgb74
-
For Sale 2 x Antique dark room lights
- Started by Manual Camera
-
For Sale 2 x Antique dark room lights, Demaria-Lapierre Excelsior (France, 1908-1922)
- Started by Manual Camera
Forum statistics
Well lets get something straight. I understand and agree with you that myths get promulgated by repetition... even in academic documents and standards. I make a living in engineering standards and their application, BTW, so I know that better than many. Nobody is suggesting that you repeat incorrect information since that would contradict your thesis. And nobody that I know of is telling you to stop your effort. All I suggest is that literature review is a normal and accepted practice in scientific and engineering documentation like this. It helps address the question many readers have, like "Did the author do his/her homework or just bloviating opinion?" You probably have done it intuitively. But for a nontechnical approach I submit that many people have never noticed that their correct or incorrect use of a grey card because that hasn't made photography, as a whole or as an individual effort, fail in a significant manner. But that doesn't matter... if there is a right way then that should be known. I appreciate this effort. My point was that I looked at a couple of existing "how to use a grey card" sections of books by known and respected authors and noted that (1) "The Exposure Manual" was too technical and resulted in my not really understanding if there is a right way and a wrong way, and (2) "Perfect Exposure" seems to say exactly what you say but without the discussion of H&D etc... and I understood it. I'm not sure that kind of discussion is necessary in a NON-TECHNICAL article. If you do, then do what you want... as said earlier, I don't think anyone is trying to say that you shouldn't. The summation of my comments is this: I'm not sure that I really understood the point you are trying to make until the last page. But maybe I'm not your intended audience since I haven't seriously used a grey card for the last 28 years. And one more thought... how about a statement of who you are and what your qualifications are. That is another "credibility building" aspect of scientific and engineering documentation like this. I have no reason to doubt your credentials, but I don't really know anything about you except for a few postings on internet forums. Good luck with your effort.
Rob, I already understand this stuff. I was spending my time and effort attempting to help you.
Last edited:
Ralph is correct and my Kodak gray card does measure 0.75C 0.75M and 0.75Y.
RobC!
The unit for the incident meter is lumen per squared meter (lm/m^2 or lux) and not cd/m^2 as stated in your article.
A diffused surface with a reflectivity of 100% is exposed to an illuminance, E, of 1 lm/m^2 (lux) at the plane of the surface the luminance, L, of that surface is E/pi= 1/3.1416 or 0.318 cd/m^2. Yes there is the Pi factor in there.
An 18% gray card is illuminated at 0EV (2.5 lux as per a K250 meter) would have a luminance of 0.1432 cd/m^2 which is 0EV as measured with a meter with a K14.
So the 18% gray card is the relationship between an incident and reflective meters. It's 17.59% for C250 and K14 like Minolta
and 15.70% for C250 and K12.5 like Sekonic meter.
So the 18% is really related to the incident light meter not so much reflected light meter.
RobC!
The unit for the incident meter is lumen per squared meter (lm/m^2 or lux) and not cd/m^2 as stated in your article.
A diffused surface with a reflectivity of 100% is exposed to an illuminance, E, of 1 lm/m^2 (lux) at the plane of the surface the luminance, L, of that surface is E/pi= 1/3.1416 or 0.318 cd/m^2. Yes there is the Pi factor in there.
An 18% gray card is illuminated at 0EV (2.5 lux as per a K250 meter) would have a luminance of 0.1432 cd/m^2 which is 0EV as measured with a meter with a K14.
So the 18% gray card is the relationship between an incident and reflective meters. It's 17.59% for C250 and K14 like Minolta
and 15.70% for C250 and K12.5 like Sekonic meter.
So the 18% is really related to the incident light meter not so much reflected light meter.
Interesting thoughts!
I'm enjoying this thread too but I'm still convinced that a gray Card has 18% and that it has to do with average or typical scene reflectance. Maybe digital photography can help here:if one took ,let's say 100+ landscape/streetscene images and looked at the histogram statistics in Photoshop to find the average luminocity and then calculated the average reflectance;This would be a repeat ofthe Jones and Condit experiment and could get us closer to an answer.
- Joined
- Feb 9, 2010
- Messages
- 9,193
- Format
- 4x5 Format
RalphLambrecht,
I think it started out at 18% before studies were done. And there are K values that will make it so.
But I think once the statistical average was found, I think the values of K were changed to skew slightly towards a darker tone like 12%
8 is a pretty important number - it's used in the definition of film speed - but I don't think our reflectance target is reduced to 8%
I think it started out at 18% before studies were done. And there are K values that will make it so.
But I think once the statistical average was found, I think the values of K were changed to skew slightly towards a darker tone like 12%
8 is a pretty important number - it's used in the definition of film speed - but I don't think our reflectance target is reduced to 8%
- Joined
- Feb 9, 2010
- Messages
- 9,193
- Format
- 4x5 Format
I think the 18% gray card was perfect for the old ASA prior to 1960. There was a safety factor in the film speed before 1960 to allow careless metering or mistakes. The safety factor was mostly taken out in 1960. But before 1960, when using a gray card, you were metering carefully and did not need the safety factor. 18% would have been just right to compute a perfect exposure.
I'm looking at Fifth Edition, 1st 1956 printing of Kodak Data Book, Copying.
Exposure Indexes are given for some fine grain and process films. Plus-X for example: Daylight 80, Tungsten 64. With these EI's you were to use Incident readings or 18% gray card readings directly.
I'm looking at Fifth Edition, 1st 1956 printing of Kodak Data Book, Copying.
Exposure Indexes are given for some fine grain and process films. Plus-X for example: Daylight 80, Tungsten 64. With these EI's you were to use Incident readings or 18% gray card readings directly.
Last edited:
With all this interesting discussion,I forgot what is in question here?
a)Does a gray card have18%reflection? Yes it certainly does!
b)What is the origin of the 18%? I believe it's a statistical average of many average scenes.
I really don't care what its origin is, I'm more interested in stating what it's usefulness is or not or how to compensate for its inaccuracies.
I'm going to completely re-write the article but for now its on hold. I thank all those who gave constructive comment and didn't try and divert attention to their own agenda. Those that did try and hijack the topic with their own agenda can start their own topic and see if anyones interested.
Summing up some of my feedback from Rob's "paper."
· 18% is about the psychophysical middle for the human eye, which makes for a good reference.
· Kodak’s Neutral Gray Card instructs the photographer to open up ½ stop when using the card as a metering device in exterior scenes.
· Reflectance doesn't play much of a factor in exposure determination.
· Exposure meters use Luminance not Reflectance.
· Jack Dunn’s Exposure Manual has 9% as the average scene Reflectance from a 7 stop scene.
· Exposure isn’t intended to be placed in the middle of the B&W film curve.
· The metered exposure point is determined from psychophysical testing and not from an average Reflectance and has to do with Luminance distribution.
· The film is exposed by the camera image and any attempt to interpret exposure should consider such variables as flare.
· When an example is used to illustrate a point, it should be relevant to the point being made. Don’t use the ISO speed determination curve to talk about exposure placement on the film curve. Show where the middle is and where the exposure range should fall.
· The K factor is not an adjustment for the average scene Reflectance. It has to do with the light loss of the camera’s optical system, factors from the exposure meter (spectral sensitivity of the photo cell), and the Luminance distribution of the average scene.
· The difference of K factor with different meters is mostly probably cause by the spectral sensitivity of the photocell or the assumed transmittance of the average lens.
· The effective calibration Reflectance in the ISO standard is 12%.
· Offer proof that a hypothesis is correct. Does it give the desired results? For example, what is the relationship of the value metered to its value at the film plane?
· Include references to source materials.
· 18% is about the psychophysical middle for the human eye, which makes for a good reference.
· Kodak’s Neutral Gray Card instructs the photographer to open up ½ stop when using the card as a metering device in exterior scenes.
· Reflectance doesn't play much of a factor in exposure determination.
· Exposure meters use Luminance not Reflectance.
· Jack Dunn’s Exposure Manual has 9% as the average scene Reflectance from a 7 stop scene.
· Exposure isn’t intended to be placed in the middle of the B&W film curve.
· The metered exposure point is determined from psychophysical testing and not from an average Reflectance and has to do with Luminance distribution.
· The film is exposed by the camera image and any attempt to interpret exposure should consider such variables as flare.
· When an example is used to illustrate a point, it should be relevant to the point being made. Don’t use the ISO speed determination curve to talk about exposure placement on the film curve. Show where the middle is and where the exposure range should fall.
· The K factor is not an adjustment for the average scene Reflectance. It has to do with the light loss of the camera’s optical system, factors from the exposure meter (spectral sensitivity of the photo cell), and the Luminance distribution of the average scene.
· The difference of K factor with different meters is mostly probably cause by the spectral sensitivity of the photocell or the assumed transmittance of the average lens.
· The effective calibration Reflectance in the ISO standard is 12%.
· Offer proof that a hypothesis is correct. Does it give the desired results? For example, what is the relationship of the value metered to its value at the film plane?
· Include references to source materials.
Kodak’s Neutral Gray Card is also the color of the sky most of the year in Rochester New York.
The back side of the card is the color of Rob's dog.
Does someone here has the ISO standard. I can't afford to purchase it from ISO. If so can someone post a quote from the standard about the reflectance standard of 12%?
I agree with you but understanding standards is the first step before questioning them.
I have just about every version going back to the military standard used during WWII. The standard doesn't say 12%. Like I said, Reflectance doesn't play much of a part. It's about the relationship between the calibration Illuminance and calibration Luminance. A short cut is the ratio of K and C; however, I believe using the constants may only be valid only for when the meter's influence is not factored in or basically the standard's calibration "ideal."
I think you can get more out of the Re-evaluation paper that is available online. I believe I gave the link in one of my posts. The paper Calibration Levels is a relevant read.
Oh yeah, if you want a copy of the 1971 ANSI standard with the appendixes and the ISO one from sometime in the 90s, send me your email address.
Just for my personal culture I would like to understand the points above, if anybody can help, with simple words, and no mathematics.
The basis where I start from, and that makes me bulk on certain affirmations (which I don't deny might be right) are as follows:
Tell me which affirmation is wrong, please.
(I only use slides);
Slides can, basically, be developed in one, correct way. There is a standard well-known way to develop a slide. That's the way you are assumed to develop it;
If you take a reflected light measure on an object, of any reflectivity, and you follow the indication of the light meter, you are going to obtain a "middle grey" image (actually, an "average grey", see below).
Be the cat white, black or grey, if you point a spot meter on it, and follow the indication, you are basically going to have a middle grey cat in your image;
That's because a light meter must assume a certain reflectivity of the subject in order to suggest an exposure value. The light meter assumes an "average" reflectivity (be it 18% or 14% we let it apart from the moment) and gives the exposure that, on the given ISO film, gives that shade of grey.
If the cat is twice as dark as the assumption, it will reflect half the light. The light meter will make the camera expose for twice the light. The cat will be twice overexposed, will be grey just as the assumption. The same if the cat is twice as reflective: twice the reflection, half the exposure, cat just as grey as the assumption. So for the light meters all cats are grey, and all cat actually will appear grey if I follow the spot light meter on them).
To insert a little bit of mathematics:
If y is luminance, a is reflectivity, and x is illuminance, the relation should be always valid:
y = a * x.
Now, a is 80% if the cat is white, and 10% if the cat is black, and 18% if the cat is grey.
Correct exposure is the exposure that makes the grey cat grey, the black cat black, the white cat white.
The light meter cannot know the colour of the cat. It tells me: "man, I don't know the colour of the cat. I tell you the value for your ISO sensitivity, and for an middle grey cat. If the cat is white or black, you adjust. If the cat is grey and you want it dark grey, you adjust".
That said, what I really can't understand is:
Exposure is not intended to be placed at the middle of the B&W film curve. Well, exposure, for me, is that value that, in that light, would give a "middle grey" cat in my slide. With B&W you can choose different strategies (because the final print is a product, the result of two factors or more factors, film, exposure, development, paper, etc). But I do assume that with a slide the light meter gives me the "average grey placement" of the point I measure, or else I should begin from page 1 of the manual.
"Correct exposure" if the exposure that renders, on my slide, the cat of its true colour. If the cat is grey, correct exposure is the exposure suggested by the light meter.
Exposure meters use Luminance not Reflectance
Well they cannot measure anything else than luminance, I get that. But they must assume a certain, predetermined reflectance in order to give me an exposure value. They don't know and cannot know reflectance, the colour of the cat, but they must be calibrated for a given reflectance, a middle grey cat (or average grey cat, see below).
The metered exposure point is determined from psychophysical testing and not from an average Reflectance and has to do with Luminance distribution
The metered exposure is, if I get it right, determined from a certain "given by design" reflectance, which is, more or less, that "average subject reflectance" that one might expect to find in real life. If that wasn't that way, all light meters would, on average, give a mistaken exposure. On average, exposure should fit the subject. That's because the exposure is based on the average reflectivity, isn't it?
[Whether this is 18%, 14% (or even 8%) is another matter]
18% is the "psychophysical" "middle grey" perceived by the human eye. 14% might be the "average reflectivity" of the scene (a totally arbitrary "average", that's not something that can be measured really, it's only an industrial practical convention based on a certain number of tests. YMMV and your scene might have a very different average reflectance and reflectance range).
The "average reflectivity" of the world might well be a bit darker than the "psychophysical" "middle grey" perceived by the human eye.
What's wrong in these affirmations?
I ask this as a genuine way to learn.
Last edited:
OMG, Ralph... BRILLIANT! I'm going to quote you on my "real job" where I work with engineering standards all day long and constantly have engineers and program managers pushing back on them, but when asked what exactly they disagree with their response is generally deflection and a question, "Can you tell me where I can get a copy?" or "Can you tell me what's in it?"
That's another thing that I did not get, also in RobC original reasoning.
The scene can have whatever SBR and whatever "average" (for that single subject) reflectance.
But the light meter doesn't know that. It doesn't know whether the scene is 4, 7 or 10 stops.
It only gives me the value that will give me a certain shade of grey, which is decided by design, a priori, and has no direct relation with the average reflectivity of my scene.
The photographer will relate that value to its scene. The scene can be high key, low key, high contrast, low contrast... the light meter always thinks in terms of a very precise grey which must be defined as a certain percentage of reflectivity (be it 12% or 14% or whatever).
So the average reflectance of the scene has no relation, that I can see, with the "average" reflectance that the light meter must assume in order to give me an exposure value.
The original reasoning by RobC related the middle grey (18%) as average in a 5 EV scene, but not average in a 7EV scene (or more). Yes. But the light meter doesn't give me the average of "my" scene. It gives me the exposure for a certain "default" average reflectivity. That has no relation with the average reflectivity of my scene, or with the range of my scene, or with any range at all. I must apply corrections for that (if I want to expose for the average reflectivity of my scene, that is).
Scenes with 7, 8, 9, 10, 11 EV range will have different (decreasing) "average reflectivity" but the light meter will always give me the same shade of grey. The rest is, as I see it, irrelevant for exposure. Your lightmeter will "place" that spot on a 14% grey of your slide. The rest will fall according to its reflectivity, and maybe will fall outside of the film range, or not. But the only grey which I must "learn" and know is the "light meter" grey, which is non correlated to the scene range.
I'm not sure if I get you right, too.
Last edited:
I don't know how to send you a private message. I sure like to read those standards.
MattKing
Moderator
They are called "Conversations" now, so you need to start one.
Either go to the "Inbox" tab at the top right, or click on a member's name. In each case, you are offered a chance to "Start a Conversation"
I think I posted how to do that in previous post. Yes K=14 and C=250 then it's 17.59%.
Last edited:
valuable info;I started my evaluation of average street/landscape shots,and so far. I get an average luminance value of 112,which equals abot Zone 5.1 and 17.7 % reflectance;I think,I'm on to something here.
This resource propose some calculation and arrives to the conclusion that "K=14.32 is the value that “matches” 18 % reflectance".
So lightmeters such as Minolta and Pentax calibrated to K = 14 are actually calibrated to a reflectance of 18% not 14% and not 8% according to this source.
It generally makes much more sense to me that my lightmeter is calibrated to the perceveid middle grey of the human eye, 18%.
http://dpanswers.com/content/tech_kfactor.php
So lightmeters such as Minolta and Pentax calibrated to K = 14 are actually calibrated to a reflectance of 18% not 14% and not 8% according to this source.
It generally makes much more sense to me that my lightmeter is calibrated to the perceveid middle grey of the human eye, 18%.
http://dpanswers.com/content/tech_kfactor.php
| Photrio.com contains affiliate links to products. We may receive a commission for purchases made through these links. To read our full affiliate disclosure statement please click Here. |
PHOTRIO PARTNERS EQUALLY FUNDING OUR COMMUNITY:  |