Gray Card Basics

Stephen Benskin said:

I actually referenced Jones, and all the information I listed comes from the photographic scientific community. I'm sorry, I don't understand the point you are attempting to make.

Click to expand...

The point i am making is that they started (had to) analyzing what films had to cope with.
They did. They all found a range of a typical scene, and simply formed the mid point (the middle grey value) by taking the mean reflective value in it.

The math, the attempts to make sense of it - though very pertinent and important - all came later.

CPorter said:

The main thing you should remember about the use of a gray card is that it cannot compensate for shadow values in the scene, which is why it is best used when all reflective surfaces in the scene are equally illuminated. With the gray card, you are "placing" the exposure on a known middle value and then letting all other reflectances or luminance values "fall" on the gray scale relative to the luminance of the gray card. But the card can fail you totally in some shadow areas----and it may even fail you in other shadow areas that seem to have developed with some density but perhaps not enough to suit you. The shadows, as you know, are not illuminated equally.

Click to expand...

The card doesn't fail you then.
We do when we don't do what you start this reply with: remember that the card is there to take the unknown reflective value of the subject out of the equation.
The card is not meant (because it can't) to take care of difference in illuminance too. Would be not good if it did.
So just like meters cannot get fooled, cards can not fail.

Q.G. said:

The point i am making is that they started (had to) analyzing what films had to cope with.
They did. They all found a range of a typical scene, and simply formed the mid point (the middle grey value) by taking the mean reflective value in it.

The math, the attempts to make sense of it - though very pertinent and important - all came later.

Click to expand...

I happen to have all of those papers, plus all of the major B&W speed standards, plus all of the meter standards. You've oversimplified the process and the math didn't come later. It was an integral part. Jones was a physicist. He didn't take a dump without using math (to paraphrase Hunt for Red October).

The paper Brightness Scale of Exterior Scenes and the Computation of Correct Photographic Exposure is 37 pages long. Sunlight and Skylight as Determinants of Photographic Exposure part 1 is 58 pages with 41 pages for part 2. There wasn't anything simple about it.

Do you believe that the math created a 'reality' it then helped to describe?

Jones didn't take a dump, and with it produced the sun, the earth, and things that reflect light. He, and others like him, found those already present, and started measuring things.

The middle grey, i.e. the 'middle' reflective value was what they found to be, quite literally, the middle value of the range they encountered in what they each identified as a typical scene.
The theory behind our understanding of tone reproduction, the complicated maths (the thing you got entangled in) came after that. Had to, no other way.

That is quite simply, yes.


My point still is that all this talk, though quite interesting, is of not much use when answering the OPs question about the grey card.
Overly complicated, making things rather harder to grasp.

CPorter said:

With the gray card, you are "placing" the exposure on a known middle value and then letting all other reflectances or luminance values "fall" on the gray scale relative to the luminance of the gray card.

Click to expand...

All exposure is place and fall. You can only have one exposure setting. Exposure theory and film speed theory is based on an assumption of a given set of values, generally statistically average; however, they also take into account how systems vary. Of course, by metering various portions of the scene, you are more knowledgeable in regards to the scene and are then able to make an informed decision in order to accommodate any condition that falls outside the norm. No matter what, you are ultimately only picking one point to place the exposure and all other tones will fall on the curve accordingly.

Loyd A. Jones, in his exhaustive testing at Kodak in the 1930s and 40s, found the average luminance range to be 1:160 (2.2 log units) with a standard divination of 0.38. This means that 68% of the sample population (± 1σ) falls within a ± 1¼ stop luminance range from the average, while 95% falls within a ± 2½ stop range (2σ). A gray card will be adequate enough for a large percentage of situations as well as a point and shoot camera that doesn't have a meter and has a fix f/stop and shutter speed.

Q.G. said:

Do you believe that the math created a 'reality' it then helped to describe?

My point still is that all this talk, though quite interesting, is of not much use when answering the OPs question about the grey card.
Overly complicated, making things rather harder to grasp.

Click to expand...

I finally understand your point. Of course not, but you have to define it. What else is science but an explanation of the natural world? "Sun is bright" isn't as descriptive as 12000 footcandles at sea level. But let's not forget photography isn't only about the physical world. Exposure has to balance the physical with the psychophysical. The aim to to produce a quality image and that is a psychological response. Exposure is about more than just figuring out the mean (which is a mathematical calculation).

One's overly complicated is another's precise. Yes, it is getting complicated, and the question was about gray card basics (basics can also be read as fundamentals). My first response was to say that meters don't see reflectance and that if you meter a gray card for exterior scenes, you should open up 1/2 stop. That answered the original poster's question. My other posts were in response to the inaccurate information posted in the referenced online articles. My use of math was a way to avoid a he said / she said argument. I could claim that the Zone System produces inaccurate film speeds (which it does), but without evidence, it's simply an opinion.

How about the reference I made to the Delta-X criterion and that film's really only have one speed? What a claim and with little explanation and it didn't even illicit a peep. Was it because it was sufficient enough to have changed the way everyone perceive the concept of speed or was too insufficient an explanation for anyone to form an opinion about or an argument against? For an exchange of ideas to happen, they have to include information. And there are people at many different levels of experience on this forum who might appreciate something not written for the common denominator.

What most people know about the K-factor comes from Ansel Adams' book, and what he had to say about it was basically conspiratorial gibberish. He claimed it as an arbitrary safety value factored into the meter. Therefore, meters aren't accurate. They don't read actual illuminance values and we have to do tests to "recalibrate" the meter or to calibrate one's equipment to the film (sound familiar?). Totally wrong, but how many people believe this and have repeated it? Let's say we have a situation where a young photographer wants to make an exposure by taking a reading off a gray card. Having read Adams or heard about the infamous K-factor, he wonders if he can trust his meter or if he should adjust his exposure to "fix" the K-factor influence. Shouldn't we make an attempt to correct misinformation whenever possible?

Adams was wrong about meters reading 18% too, although he isn't the only one. There is a persistent myth that meters read 18% gray. Maybe if more people actually attempted to explain it, there wouldn't be such confusion. What is middle gray? 18%? 12%? 9%? Which one is correct? All of them actually. 18% is basically the psychophysical middle as determined by Munsell. Later Jones chaired the Optical Society of America's Committee on Colorometry and they tweaked it 19 point something (sorry I'll have to look that one up). 9% comes from the statistically average scene of the physical world. 12% comes from the optical representation of the 9% physical world at the film plane. Veiling flare takes off around a stop from the luminance range of the scene (flare adds illuminance to the shadows), thus shifting the mean up. I actually was writing a manuscript on this for PHOTO Techniques, but they though it was too complicated.

Oh, and here's an interesting observation. Through the analysis of the camera image, film curve, and paper curve using a four quadrant reproduction curve, I found that while the meter effectively meters 12% reflectance, for an average 7 1/3 stop scene (yes, not 7), the metered value will most likely fall around a print density of 0.74 or 18% reflectance.

Q.G. said:

The card doesn't fail you then.
We do when we don't do what you start this reply with: remember that the card is there to take the unknown reflective value of the subject out of the equation.
The card is not meant (because it can't) to take care of difference in illuminance too. Would be not good if it did.
So just like meters cannot get fooled, cards can not fail.

Click to expand...

Better said to say that one's expectations will fall short if the limitations of the card are not understood. So, I agree that the card doesn't fail.

Never said/implied that the card is meant to take care of differences in illumination. When the card is used to determine the exposure, then all reflective surfaces receiving the same light will be exposed relative to the luminance of the card. This is what the OP would do well to remember.

Stephen Benskin said:

You can only have one exposure setting.

Click to expand...

Yeah, that's pretty much what I said.

Stephen Benskin said:

How about the reference I made to the Delta-X criterion and that film's really only have one speed? What a claim and with little explanation and it didn't even illicit a peep. Was it because it was sufficient enough to have changed the way everyone perceive the concept of speed or was too insufficient an explanation for anyone to form an opinion about or an argument against? For an exchange of ideas to happen, they have to include information. And there are people at many different levels of experience on this forum who might appreciate something not written for the common denominator.

Click to expand...

I suspect it didn't elicit a peep because probably nobody following a thread entitled "Gray Card Basics", started by someone who has just got a gray card for the first time, knows what the Delta-X criterion is. Or a Lambertian surface. And they probably don't yet care, and likely never will. I don't think anyone is waiting for a fuller explanation. I am not trying to offend - I just think that long tracts of gratuitously complex information have a tendency to kill threads like this one. Interested readers can always start a new thread or send a PM if they want to discuss obscure points further.

Stephen Benskin said:

I finally understand your point. Of course not, but you have to define it.

Click to expand...

I'm glad that you do.

But then, you would understand that you do not have to "define" an observation.
You make sense of it. Try to understand it by constructing an underlying theory that links it to things that we already (think we) understand.

What else is science but an explanation of the natural world? "Sun is bright" isn't as descriptive as 12000 footcandles at sea level.

Click to expand...

And both are observations, that do not require, nor are the result of mathematical considerations.

But let's not forget photography isn't only about the physical world.[...]

Click to expand...

Now that is a matter of definition!

But i do agree.

One's overly complicated is another's precise.

Click to expand...

Trying to calculate an observation is not the same as being precise.
It is believing that mathematics is creative. But "Jones didn't take a dump, and with it produced the sun, the earth, and things that reflect light. [etc.]"

Yes, it is getting complicated, and the question was about gray card basics (basics can also be read as fundamentals). My first response was to say that meters don't see reflectance

Click to expand...

That's true. But only meaningful if you first assume that meters do things, and are not used.
Meters are used (in this thread at least) to measure reflectance.

And who doesn't know that? So with that distraction about 'seeing' meters put aside, the discussion about, meters, reflectance and grey cards can again proceed.

and that if you meter a gray card for exterior scenes, you should open up 1/2 stop. That answered the original poster's question. [...]

Click to expand...

Indeed.

It also contains an observation, being that meters are not calibrated to middle grey.
Is that a correct observation? Or rather a believe arrived at by doing maths?

How about the reference I made to the Delta-X criterion and that film's really only have one speed? What a claim and with little explanation and it didn't even illicit a peep. Was it because it was sufficient enough to have changed the way everyone perceive the concept of speed or was too insufficient an explanation for anyone to form an opinion about or an argument against?

Click to expand...

Or was it as inconsequential as the observation that meters do not see reflectance? A distraction without any bearing on the subject in hand, understood as such?

[...] What most people know about the K-factor comes from Ansel Adams' book, and what he had to say about it was basically conspiratorial gibberish. [...] Shouldn't we make an attempt to correct misinformation whenever possible?

Click to expand...

I agree. And yes.

But should we be the ones who first provide that misinformation, so that we can correct it again?

[...] Veiling flare takes off around a stop from the luminance range of the scene (flare adds illuminance to the shadows), thus shifting the mean up. I actually was writing a manuscript on this for PHOTO Techniques, but they though it was too complicated.

Click to expand...

How did that great philosopher put it again? "It's deja vu, all over again?"


Or was it because they thought that this phenomenon too was discussed, understood and accounted for extensively already almost 100 years ago?

Oh, and here's an interesting observation. Through the analysis of the camera image, film curve, and paper curve using a four quadrant reproduction curve, I found that while the meter effectively meters 12% reflectance, [...]

Click to expand...

Now it is my turn to point out that a meter does not meter a degree of reflectance?

...and yet, despite how bad and incorrect the grey card/incident methods or zone system methods supposedly are in articles, arguments, and laboratories, they somehow give me what I want nearly down to a T...and probably most of the photographers who's PICTURES (yes; pictures) I admire used more "primitive" methods than either of these.

So, grey cards work. Plainly, simply, and basically. They work, and they work quickly, accurately (at the very least "accurately enough'), and conveniently. They work because scientists provided them as tools for artists and others. They did this so that I don't have to. I thank them for their efforts, past, present, and future. They came up some some shit that works. Let us now STFU about quests for perfection and what is wrong with them in a laboratory (because we already know that there has to be something wrong with everything, as nothing is perfect or ideal), and talk about how the OP can use them well to make pictures that are technically sound enough to get what is desired. It is good to share all aspects and opinions of their use, but at this point it is a distraction from the basic learning process, IMO...especially in a post entitled "Gray Card Basics", in which I would think that the OP just needs to know how to use the damned thing quickly, accurately [enough], conveniently, and reliably.

CPorter said:

Never said/implied that the card is meant to take care of differences in illumination.

Click to expand...

It's the use of the word shadow.

Something that is a shadow, or is in a shadow, does not receive the same illumination as something that is not.
Hold the card in the light, and objects having the same reflecting properties as the card will not be rendered equally bright, when one is in that light, the other in a shadow.

I don't know whether it is what you meant to say (thought you did, though. If not, my apologies.), but wanted to point out that this is not due to a failure of the card.

iandavid said:

I suspect it didn't elicit a peep because probably nobody following a thread entitled "Gray Card Basics", started by someone who has just got a gray card for the first time, knows what the Delta-X criterion is. Or a Lambertian surface. And they probably don't yet care, and likely never will. I don't think anyone is waiting for a fuller explanation. I am not trying to offend - I just think that long tracts of gratuitously complex information have a tendency to kill threads like this one. Interested readers can always start a new thread or send a PM if they want to discuss obscure points further.

Click to expand...

I didn't know there was a rule to keep each thread at a consistent skill level. I did answer the posters question first. The more technical response was to a post by ic-racer's where he referenced an article which was fairly very to technical (ic-racer also appears to have broken thread etiquette). Knowing that ic-racer is very technical himself, I addressed my response to the article on a higher level.

The Lambertian surface, also know as a perfect diffuser, is pertinent because a standard gray card isn't a perfect diffuser and will have hot spots which can affect the meter reading. Delta-X was an response to a point made in the article. As film speed, metering, and exposure are related, it seemed relevant to correct a misstatement about film speed.

Nobody is forcing anyone to learn. It seems that if people want to remain ignorant, they wouldn't visit a forum dedicated to information in the first place.

Stephen Benskin said:

Then why do most get it wrong?

Click to expand...

That's life.

I thought explaining things that have been known to those who are unaware is called teaching. By your reasoning, it must be long past time to shut down this forum because everything has been discussed and accounted for?

Click to expand...

Now how and where did you get the notion that that would be my reasoning?

Teaching, by the way, does not equate to flooding someone with as much information as possible. That doesn't work.

Except for the Zone System, of course. That certainly hasn't been covered ad infinitum. Tell me again how meters see Zone V and how to achieve my very own personal film speed.

Click to expand...

Indeed.
I too find it rather alarming how often people start talking about that ZS.
Apropos teaching: the ZS is a great teaching tool. But judging by how often it is mentioned when discussing very simple things that have nothing to do with the ZS, it would appear that it is very little understood.

While were at it, after we shut down APUG, let's get rid of those bothersome schools and universities too.

Click to expand...

Well ... they teach.

I also wasn't aware that the 1940s were almost 100 years ago, but then there's that math again. Should I have smugly added a winking icon here too?

Click to expand...

No. I will.
Your math skills should have let you know what period is almost 100 year ago.
So here it is:

As film speed is a major factor in exposure, I figured it had a bearing.

Click to expand...

Not pulling out a dark slide is also a major factor in exposure.
Why did you not mention that?

As you have probably ascertained, I've come to the conclusion you aren't serious about participating in rational discourse which is insulting to me and disrespectful of everyone's sincere efforts to share and learn on these forums. Plus, it's a waste of everyone's time.

Click to expand...

I have ascertained that you are rather fond of letting people know how learned you are.
So much so, that you don't care about helping people by giving a simple answer to a simple question.

And yes, that is a waste of everyone's time.

I'm not interested in reading the back and forth crap, nor do I imagine that anyone else is. At this point, can it not go to PM mode?

I am, however, very interested in hearing more about the "open 1/2 stop" from a grey card reading.

Keep it to a moderate level of technicality please ()

I basically understand the reason you stated for doing so: An 18% grey card is not actually middle grey, but a half step darker on an eleven-step scale.

However, how does this apply when using the grey card as a testing tool, rather than to determine exposure?

A reflected meter will tell one how to make any surface it meters middle grey, in theory, and closely enough for government work in practice. Therefore any solidly-toned, neutrally-colored sheet of material without too much shininess will work for calibrating ones meter to ones working methods...or so it seems to me.

So, the way I reason it, the open up 1/2 stop thing has to do only with using a grey card as an alternative to an incident meter. OK. That makes sense to me. Infortunately, none of my grey cards are Kodak grey cards, nor has anyone ever taught me this information (via book or any other method), so I had never heard this. I suppose I am lucky that I use an incident meter instead of a grey card for actual shooting!

But since a grey card is not actually *middle* grey, does this mean that in theory, using basic terminology understood by the masses who are familiar with what a certain departure from a grey card will cause to be rendered in a print (AKA "placement"), one should start losing defined detail in a surface that measures at two-and-a-half EVs under a grey card reading instead of three? Or does it mean that one actually gets detail at up to three-and-a-half steps below a true middle grey, and three steps below a grey card?

2F/2F said:

I'm not interested in reading you all's crap, but I am interested in hearing more about the "open 1/2 stop" from a grey card reading.

Keep it to a moderate level of technicality please ()

Click to expand...

My apology. I've deleted the posts. This should be a place for ideas.

Stephen Benskin said:

My apology. I've deleted the posts. This should be a place for ideas.

Click to expand...

Thanks. But you don't need to be sorry about anything, IMO. Arguments are easy to get into and hard to get out of on the Internet. I am really curious about the 1/2 stop thing.

2F/2F said:

Thanks. But you don't need to be sorry about anything, IMO. Arguments are easy to get into and hard to get out of on the Internet. I am really curious about the 1/2 stop thing.

Click to expand...

You're right about the need only when using it as an alternative to an incident meter. Like I said before, 18% is how we see middle, so it's an excellent reference. Our brains compress the lower tones, so they aren't very good for a reference. While the lighter tones are expanded, our brains also have a tendency to be relativistic with tones. One tone will look different when placed next to another. It's call simultaneous contrast. The ten step gray scale from Munsell are made up of visually even spaced tones. That means the blacks have to be more separated and the whites less. Even before Munsell chose 18% from his testing for his step 5, he instinctively used it as his neutral background of his comparison tests. We are hard wired to 18-20% reflectance.

The 12% doesn't come from a different numbered scale like the 11 step one you refer to, but from the camera image of the average scene's contrast range. Munsell's 18% is from 2-D paint swatches evenly lit. The swatches simply can't achieve the same contrast range. The two reflectances come from different disciplines. One is for the eye and the other for photography.

A light meter is calibrated to the same light value that is produced when the assumed average illumination or incident light is reflected off a surface equal to 12% reflectance. An incident meter has an artificial 12% with the dome. They both want to place the exposure at the same place.

As for exposure, close enough generally is. The 1/2 stop is about making sure you hold the shadows. It's just like an incident light meter. You assume a luminance range (contrast range). It's all about place and fall. The meter wants to place everything at a certain exposure based on the film speed. The average conditions that are assumed is that the shadows fall 4 1/3 stops under the meter reading. Flare is considered to be 1 stop for exposure consideration; however, in reality it is slightly higher. That means the shadow falls 3 1/3 stops under the meter reading. As the b&w speed point is located 3 1/3 stops below where the meter wants to place the exposure, the shadows should fall within an acceptable range.

While the gray card is metered in the light, the average range takes into consideration that part of the subject will be in shade or not in direct sunlight. That is really what makes the difference between the contrast ranges of Munsell's two dimensional gray scale and the three dimensional outdoor scene. If you photographed two gray scales with one in the sun and the other in the shade, you will have the full contrast range of the average scene with the combined gray scales.

Everything is all tied into the perception of quality. Quality means different things to different people. That's one of the reasons why there's so much slop in photography. Close enough is usually good enough. How else can a disposable camera with a fix f/stop and shutter produce acceptable images most of the time?

In effect, if you want the gray card to match the exposure value you'd get using an incident meter, you'll will need to open up 1/2 stop.

Thanks, Stephen. I understand.

The eleven step scale to which I was referring is the classic zone system scale on which I learned. 11 print values from maximum black to paper white.

I learned exposure the zone system way, straight out of The Negative, right after taking up photography. It was part of the 4x5 class that I took after basic photo. They taught us both incident and ZS methods in the class, and I settled on ZS, as to me it seemed more "concrete" and exact at the time. I still have all my notes and such because this method works quite well. Just because you come up with an EI that works for that system does not mean that the ISO is "wrong". But now, I approach it differently, especially when using roll film. I usually use an incident meter and a spot meter combined, calibrating with a printing test shot in a real-world-esque composition, aiming for the goal of being able to get reliable placement of low-toned detail and texture (as opposed to aiming for a set density to assign to zone I). I use the incident to measure the amount of light present and determine the average exposure for the scene. I use the spot meter to measure the luminance range, and/or the tonal fall of certain areas of the composition, so I can make adjustments to the incident reading if necessary. Using knowledge from the ZS methods, I guess at pluses and minuses by using percentages, and am happy to get close enough over a whole roll that I can go up or down a filter if needed (while I use graded paper for the ZS).

I calibrated my spot meter to my incident meter using a grey card. They match perfectly. (60 year old incident meter matches a Pentax Digital Spotmeter...I was pretty surprised.) This must actually mean that they are 1/2 a full rating off from each other, and I should subtract 2 EIs from my spot meter ratings? I haven't had any noticeable issues getting detail and texture where I want it, but if I do need to make an adjustment, I would want to do so. No matter how a spot meter is calibrated to a grey card, it will work to measure luminance range. What I am concerned about is the placement of texture and detail, and how this might be improved by lowering the EI I use on my spot meter.

Well, all the chit chat back and forth about the Zone system and gray cards has overlooked the fact that the MacBeth checker is a mini zone system and gray card combined and the Kodak chart is also both with a gray card included. The MacBeth is a 7 step zone system and the Kodak chart is an 11 step zone system.

Both are shown in the attached test. RZ67, Auto, ISO 160, Portra 160 VC.

That is why we used them! Lots of data, even in B&W.

PE

MacBeth Color Checkers are great tools. I have never owned my own, unfortunately, but used the school's for all sorts of color testing, filter demonstrations for b/w and color, film emulsion examples, cross processing examples, and the grey card for ZS calibration, etc. We used either flash or diffused daylight to light them.

BetterSense said:

Is that necessarily true? Film has a limited latitude. I can envision a scenario where you take a reading off a grey card, but the detail in the snow is so far (more than 4 stops) above that that you can't actually capture any detail of the snow.

Click to expand...

SLIDE film is as limited in latitude as digital
Color negative film and black and white film are quite wide in latitude. Color neg, in particular, very well handles up to +3EV of overexposure (and only -1EV underexposure, where color rendition gets 'muddy')

wiltw said:

SLIDE film is as limited in latitude as digital
Color negative film and black and white film are quite wide in latitude. Color neg, in particular, very well handles up to +3EV of overexposure (and only -1EV underexposure, where color rendition gets 'muddy')

Click to expand...

Well, I would say 2 EV underexposure, but otherwise you are correct. Current 160 films (nominally 100) can be exposed as high as 400 with good color rendition.

PE

2F/2F said:

Thanks, Stephen. I understand.

I calibrated my spot meter to my incident meter using a grey card. They match perfectly. (60 year old incident meter matches a Pentax Digital Spotmeter...I was pretty surprised.) This must actually mean that they are 1/2 a full rating off from each other, and I should subtract 2 EIs from my spot meter ratings? I haven't had any noticeable issues getting detail and texture where I want it, but if I do need to make an adjustment, I would want to do so. No matter how a spot meter is calibrated to a grey card, it will work to measure luminance range. What I am concerned about is the placement of texture and detail, and how this might be improved by lowering the EI I use on my spot meter.

Click to expand...

The 1/2 difference could be the result of number of factors. The age of the incident meter's photo cell, the spectral sensitivity of the meter's photo cell (I believe this is the primary reason why two meters never seem to agree), and the angle of the gray card in relation to the light source are a few factors that come to mind.

The concept about the placement of texture and detail all depends. In the prints tests by Loyd Jones, he would grade the quality of the results as first choice, second choice, and so on. In all of the cases people found the second choice prints to be totally acceptable and only found them to be lacking when compared to the first choice prints. Sometimes the choice of first choice and second choice would change with different groups of people. How much better will the detail and tones be with the 1/2 stop difference? Depends. Let's not forget tones aren't judged in a vacuum. They work in relation to each other. There's also there's the influence of flare and how different levels of flare can compress or expand the tones in the shadows as well as add or decrease density to the shadows. This can have a hugh effect on what we judge as quality or desirable in a print.

A lot of factors are involved. That 1/2 stop exposure difference won't make much of a difference for a flatter than statistically average scene. If the shadows don't fall far enough down to be degraded or lost by what would only be a technical 1/2 underexposure, can it even be considered underexposed? In this situation will the difference even matter? I believe it can't and it doesn't.

One last thing. It has to do with the use of the term "placement" as a concept of where the values fall on the characteristic curve. Maybe you've noticed this. I've found the use of the term "placement" can create the false impression with some people learning the Zone System to believe there is only one specific negative density for a given Zone and that once it is there, you're stuck with a specific print density. I know that's not what you mean, but I had to take the opportunity to vent on a personal pet peeve.

Here is a very pertinent quote this from Loyd Jones. According to Jones, "From the standpoint of tone reproduction theory there seems to be no justification for the adoption of any value of density as a significant criterion of the speed of a photographic negative material. The primary function of the negative material is to record brightness (today called luminance) differences existing in the scene. Density, per se, has no significance as an indication of the ability of the photographic material to perform this function. The value of negative density by which any particular object brightness is rendered, as, for instance, the deepest shadow, is of no consequence except insofar as it may have some bearing on the exposure time required to make a print from the negative."

2F/F,

I missed something in my explanation about the opening up 1/2 stop for a reflected meter in order to make it in agreement with an incident. The post will all the math shows it but doesn't spell it out completely. If you take value of the incident light (illuminance) that the incident light meter is calibrated to and compare it to the reflected light (luminance) the reflected light meter is calibrated to, the difference is 12%.

Steve