Kodak grey card usage

[Stephen Benskin]

That would also mean that, in fact, also for negative film, when following light meter indication, one obtains an Hg which corresponds to the new 8 / ISO equation, that is, less exposure!

Missed this part earlier. Fortunately, it doesn't mean that. By changing the speed constant in the reversal color speed equation, only reversal color film is affected.

 

[Stephen Benskin]

It is my understanding that, because it was close to 1, it was outstripped from published formulas of the time. That's why Saint Ansel was very surprised in finding, one day, that there was this K stuff which was, to him, an unnecessary distortion.
In reality, the K factor was already there, but was close to 1 in US literature, and 10.64 in International System literature.
When the K factor was raised to 12.5 (I think, or thought, this was because of the recognising of the higher light transmission of modern lenses) the K factor appeared in all formulas, both US and IS. This disturbed Adams but, in fact, what was actually disturbing him was the raise from 10.64 to 12.5. That's around 1/5 of EV but complicates the calculations that Adams used to do and the formulas he published in his books, I believe.
That's the conjecture by Conrad, 2003, http://www.largeformatphotography.info/articles/conrad-meter-cal.pdf
and I found it quoted here: http://dpanswers.com/content/tech_kfactor.php#adams2

And I thought, but I think Stephen told me I'm wrong, that this same modification caused the revision of the Hg for slide film, setting it 1/3 EV away from the slide film speed point.

A number of years ago I calculated the values of K with the various standards going back to 1944. This is what I found:

1944 - 12.34
1948 - 11.31
1957 - 11.31
1961 - 10.46

According to Allen Stimson in An Interpretation of Current Exposure Meter Technology, "The new value K = 3.333 is used when the meters are calibrated at 4700k. The former value K = 3.6, which has been used when the meters were calibrated at 2700k, is 10% greater because the average cell is about 10% less sensitive at this color temperature. Consequently, no radical change in calibration of reputable American-made meters is anticipated."

I think both you and Conrad have given Adams too much credit. If you carefully evaluate Adams' testing methods and his explanations, he clearly isn't that familiar with sensitometry, exposure theory, or tone reproduction theory. He practically claimed in The Negative that K was some type of conspiracy of manufacturers. He had no idea what K was.

 

[Bill Burk]

According to Allen Stimson in An Interpretation of Current Exposure Meter Technology, "The new value K = 3.333 is used when the meters are calibrated at 4700k. The former value K = 3.6, which has been used when the meters were calibrated at 2700k, is 10% greater because the average cell is about 10% less sensitive at this color temperature. Consequently, no radical change in calibration of reputable American-made meters is anticipated."

Even more fun! I'm enjoying collecting these examples of changes to K that do not change the meter indication.

You can change from K = 3.6 to K = 3.333 while changing the color temperature of the test light and expect the meters to indicate the same before and after.

 

[Diapositivo]

Meters are not calibration for a Reflectance. It's an extrapolation using the calibration Luminance and Illuminance. I don't think it's as important as you might think.

I don't understand this, altough I hear it repeated often, and I would like a clarification on this.

(From Wikipedia)
The constants

and

shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.

(Constant K is for reflected light, C is for incident light measuring, so the same reasoning appears to apply to both cases).

Let's treat the 2 cases separately.

Incident light measuring: the manufacturer will calibrate the instrument with the above method, as described in the ANSI literature.

That will work for incident light metering. You take a large number of images of cats, of difference reflectance, under different luminance conditions, you measure the incident light in front of the cat, and you find the value that reproduce the cat satisfactorily: a white cat must appear white, a black cat must appear black, and a grey cat must appear grey. If you find the right exposure (i.e. the right C, or the acceptable C, as there is some minor slack), the three cats, one near the other, will all appear correctly rendered. The black cat will reflect less light and will appear black, and the white one will reflect more light and will appear white.

Reflected light spot measuring: you do the same as above. You take a large number of pictures of cats, of different reflectance, you measure the luminance of each cat, and you use that value to get the picture, and you examine which is the one which is satisfactory. What is the result? The white cat appears grey and is not satisfactory, the black cat appears grey and it is not satisfactory. The grey cat depending on its shade of grey might appear slightly different from what you saw.

You can take millions of pictures and the result will always be the same. There will always be, in theory, one and only one shade of grey which is "exactly" reproduced by the spot metering. You'd be better off talking, after some million pictures, with the manufacturer and ask him: "Please say which is the shade of cat which is reproduced correctly. Knowing that, I will be able to "adjust" exposure for the other cats, by "placing" the exposure on some "white fur point" or "black fur point" of the film curve. But in order to place it, I must know, in advance, where the "target grey fur point" is".
If the grey point is 18%, and you have an 18% cat, and in the same picture you have a black cat (a 6% cat) and a white cat (a 70% cat) if you take the spot metering on the grey cat you will have all three cat placed on your film curve where they are "true" to what you saw.

Knowing the grey point is absolutely fundamental for placing points when working with slide film.
If the target grey point is, say, 8% (which certainly it is not), in order to place some white marble which, let's say, reflects 80% of light, on the "white marble" point of the curve I must measure it with the spot meter and give more exposure by 3.3 EV.
If the grey point is, say, 18% (which very likely is), in order to place some white marble on the same point, I need to measure the marble with the spot meter and open it a little bit more than 2 EV.
That's a world of difference.

"Placing highlights" with slide film is absolutely a precision work. In order to explain myself better I will give some real life example:

All the images that follow were NOT bracketed. I measured the highlights that I wanted to retain on slide film, I "placed" them (assuming 18% middle grey and IIRC some 2.5 or 2.7 placement above middle grey) and I got those images.

For this colonnade, I measured the roof near the lamp to the left, under the "entrance". I knew the light source would obviously have come out burned, but I wanted the roof to be rendered correctly, i.e. not burned.

http://fineartamerica.com/featured/berninis-colonnade-fabrizio-ruggeri.html

So the high point is placed there, and the contrast range of the film will capture what it may in the shadows and the intermediate tones.

Had i not placed the high tone with the spot meter, I would have obtained a much higher "black point", a lot of black in the picture, much less detail.

By the same token:

http://fineartamerica.com/featured/st-peters-basilica-by-night-fabrizio-ruggeri.html

placing the highlight correctly allows to preserve the shadows while maintaining a natural (not washed up) rendition of highlight tones. 0.5 EV of mistake in a picture like this either burn the highlights or, block the shadows too much.

I will show you a picture where the highlights I think were not well placed:
http://fineartamerica.com/featured/st-peters-square-fountain-by-night-fabrizio-ruggeri.html

With the spot meter I knew the top water jet was to result burned. The picture is not well executed because the front of the central body of the fountain begins being burned. This picture should have been taken with 0.2 or so EV less exposure.
More "closing" than that, and you begin losing the parapet at the base of the fountain.

To sum up: precision is important and knowing precisely where is the middle grey where the spot meter places Hg is fundamental.

I understand that a negative-print guy is not sensible to this kind of problems.

 

[RobC]

which only goes to show that since all the manufacturers are doing is testing to see what works in an average situation, that if you want more precision you have to do your own testing with your own tools and materials so that you know what your own numbers are. You could have done this by now and fully calibrated your exposure with a spot meter. But as usual the idiots who are up themselves with formula have sucked someone wanting to do practical photography down the tubes into an endless discussion about something which won't get you what you want. Go and do the tests to prove to yourself that the numbers you are using actually provide what you want otherwise you are wasting your time and others are helping you to waste it.

Oh, and since you haven't got a densitometer you can never prove anything and if you think that by magic the formulas will produce exactly the tone you want then you are deluded.

 

[Diapositivo]

Rob, a light meter manufacturer (especially a manufacturer of a spot light meter) must give me the grey placement of its instrument just like a film producer must give me the ISO speed of its material.
"Testing" is a very abstract word. One would have to take many pictures, and find the right subjects, in order to know how would the system respond in that situation.
Besides, if I now buy some Provia film, new to me, and do some testing, I must know, first, how the lightmeter works.
If I had a new lightmeter, and a known film, I could test the lightmeter.
You can only test only 1 variable at a time!

 

[Diapositivo]

When I was young, I read an article on an Italian magazine, which basically said this very simple, very basic concept which I find correct.

In simple words, a reflected light meter measures "incident" light by "figuring it out" from the reflected light, knowing reflectance of the object.

If Y is reflected light, and a is the reflectance percentage, and X is the incident light, the reflected light meter knows Y, assumes k, and derives X. X is the exposure that the incident light meter would give.

Y = kX

As anybody knows, the couple kX gives infinite solutions for each value of Y. But if you keep k fixed, you have a first grade function, a determined value of X for each value of Y and viceversa.

The incident meter measures X and gives you an exposure.
The reflected light meter (be it spot, or average, doesn't matter) measures Y.

If what you measure is of the same reflectivity of k, the measure will be like the incident one.
If what you measure is of a different reflectivity, the reflected measure will be incorrect.

We introduce now a new measure of light (luminance and illuminance) and we call it Minnie.

The incident light meter measures a certain scene over an 72% white cat and it is 100 Minnies.
You set your camera according to the relative exposure of the incident reading, and the cat comes out correctly 72% white (because it will reflect, toward your camera, 72 Minnies).

You measure the cat with the reflected light meter.
Result is: 72 Minnies.

The lightmeter doesn't know the cat is white. The lightmeter thinks all the world to be of grey with reflective coefficient k.
Le'ts say k is 0.18, 4 times less reflective than the white cat.

The lightmeter sees: 72 = 0.18 X -> X = 400 Minnies.

The lightmeter will give you the exposure as if the 72 Minnies were four times more light reflected by a four times less reflective cat, than real. He will close 2 EV more than appropriate. You will have a cat which is 4 times less reflective on slide than real. That is, you will get an 18% grey cat. But the cat is white.

If the cat is actually 0.18, and he receives 100 Minnies, he will reflect 18 Minnies.

The lightmeter sees: 18 = 0.18X -> X = 100 Minnies.
The lightmeter will give you the right exposure (because actually 100 Minnies are falling on the cat) and you will get the right shade of grey.

By this time, you ask the manufacturer why your white cat, and your black cat, always come out all grey cats. He must tell you the value of k. He cannot tell you that light meter does not measure reflectance. They assume a certain reflectance.

k in this example is not the K factor in lightmeter, is the reflectance of the subject that the reflected light meter assumes in order to give you a correct exposure.

There must be a k (as in Y = kX) in each lightmeter. This k is a coefficient of reflectivity or, if you prefer, a shade of grey. All that you measure with the reflected light meter will come out k, as shown above.
There must be a fixed relation between k and a grey on a slide.

 

[RobC]

but a manufacturer has ZERO control over the multitude of variables that go into film type, manufacturing of film, development process, your metering technique etc etc. YOU MUST CALIBRATE YOUR OWN SYSTEM if you want accuracy and since that is a must it means all the theory is only ball park.
And your assumption that a meter manuafacturer MUST give you grey placement is 100% flawed. All a manufacturer can do is state their numbers. You have a fixation on Provia but you tell me how can a manufacturer claim his meter will give exact correct grey on Provia, Velvia50 and Velvia100. They can't becasue they will all give a different grey for the one reading you take with your meter. So it should be obvious to anyone that they must do their own calibration of their own system since the manufacturer has no idea of exactly what your system is. They can only give generic advice based on their own system which may or may not conform to some standard but doesn't conform to your own standard which may also be attempting to conform to some standard. But I guarantee you that they will vary. Common sense should tell you that you must calibrate your own system.

Take 6 exposed films all of the same images exposed at exactly the same time lighting etc and give them to 6 different labs. You tell me, will they all come back with exactly the colour/density of a grey card? Don't say yes becasue I won't believe you.

Do you get it now? No matter how much time you spend disappearing up yourself with formula it won't gaurantee you anything when it comes to actually producing correct colour. The only way it would, is if you calibrate your own system to produce what the numbers say it should produce. But in order to do that you MUST do your own calibration and if you are going to do that then you can forget all the formula, go out and do some tests and tweak your system until a grey card matches a grey card on film. And for that you will need densitometer. So what do you think you are playing at worrying about formulas?

 

[Diapositivo]

I don't a fixation with Provia. That's the film curve that is readily available now in the conversation.
I do have a fixation with the concept that, given a certain ISO and an exposure for that ISO, all film will reproduce the same tones. There will be differences in the extreme part of the film curve, but the central part must be "ISO well behaved" so to speak.

And I expect the 6 laboratories to give me 6 images around a certain theoretical middle grey, with a certain dispersion around the mean. Actually for the 6 laboratories I expect the mean to be very near the correct grey.

The manufacturer will give me the ISO speed of the film, given a certain well-known development over which he, and I, have control.

I don't deny doing tests, but you seem to refuse the concept of standard, or of "measure". There is an objective aspect to all this. Photography it's an industrial process, it's not black magic!

 

[RobC]

but have you entertained the thought that your expectations and reality might not match.

 

[Diapositivo]

but have you entertained the thought that your expectations and reality might not match.

Absolutely! Then I would examine my "reality" (my results) and adjust them so that they match expectations. If you know precisely the behaviour of your light meter, your film, your development, you can foresee the results. If they don't match, you reassess your knowledge of their behaviour and/or your practice until you arrive to the point when you can foresee your results.

Some testing might be necessary but there must be a theoretical exactitude around which you revolve until you get there.

 

[RobC]

but you don't need to waste your time on the minutia of the theory, you can go straight to the practical evaluation(calibration) and save yourself a lot of fruitless time which is what I'm proposing.

 

[Bill Burk]

Meters are not calibration for a Reflectance. It's an extrapolation using the calibration Luminance and Illuminance. I don't think it's as important as you might think.

I don't understand this, altough I hear it repeated often, and I would like a clarification on this.

The actual calibration is done using a backlit screen a blue filter (like 80B) and a tungsten light bulb. So it's a luminance, not a reflectance. wiltw demonstrated how bad reflectance is as a target, it's constantly changing unless you get the angles just right. (Reading reflectance was a very difficult problem to solve for many years, I think I read somewhere that PE spent many years on the problem.)

(From Wikipedia)
The constants

and

shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.

I am sure this is the paragraph that rankled Ansel Adams (I assume he was shown a copy of the ANSI standards).

I don't even know exactly where the meter is aiming Em. As I see it, there is room in the recommended values for K for the placement to fall within a range of 0.4 EV. Sure it's "approximately" 10 times the speed point. But did the speed point and it's relationship by delta-X to the 0.3 gradient incorporate the findings of the print studies (where people looked at thousands of pictures of willow ponds and picked their favorite shots)? Or was there a tweak after the speed point was picked, to account for the print studies?

 

[RobC]

So, we now have 265 posts in this topic from all the people trying or claiming to be experts on the topic. Which of you experts can tell me what is the correct way to meter an 18% reflectance grey card to make it reproduce on colour slide film the exact correct tone? My bet is none of you and I also bet the formula kiddies can't do it.

 

[Bill Burk]

Haaa. You said it in your first post.

Place the card precisely as you have described.

Then use an incident meter with dome. Average the readings from subject to camera... and from subject to brightest light source...

Take the photograph and the gray card will be where it belongs.

 

[markbarendt]

So, we now have 265 posts in this topic from all the people trying or claiming to be experts on the topic. Which of you experts can tell me what is the correct way to meter an 18% reflectance grey card to make it reproduce on colour slide film the exact correct tone? My bet is none of you and I also bet the formula kiddies can't do it.

I think you are right Rob.

To be able to place a specific tone for viewing, one must also have control of the way the slide is viewed. The problem is that the display setting isn't always known and is rarely IMO considered, I might even suggest it's the exception rather than the rule. Light tables differ from projectors and not all light tables or projectors are the same brightness and they may vignette and then there's the issue of the surround. The tones look different in darkened rooms vs in lit rooms vs when held up to a lamp vs when held in front of a window.

This variability issue is actually true of prints too, change the lighting on the print and you change the contrast range that a paper actually reflects and in so doing you change the midpoint, highlights can be brightened and more shadow detail viewable by adjusting the lighting. It is not that uncommon for high end printers to print two prints purposefully a bit different to fit into brighter or darker lighting situations.

 

[Diapositivo]

This variability issue is actually true of prints too, change the lighting on the print and you change the contrast range that a paper actually reflects and in so doing you change the midpoint, highlights can be brightened and more shadow detail viewable by adjusting the lighting. It is not that uncommon for high end printers to print two prints purposefully a bit different to fit into brighter or darker lighting situations.

Mark, the variability of the light in which you see a print is way bigger than the variability of the light through which you see a transparency.
Yet, your print doesn't change. That grey in your print is the same if you measure it with the reflective densitometer. You can then turn the light off and say that you only have black in your print, with the light turned off .

Of course there are different light conditions in which you see a print or a slide. But your eye/brain is smart enough to compensate, as far as possible, for the variations and you end up with a mental image of that image which is always the same.

 

[RobC]

Mark, the variability of the light in which you see a print is way bigger than the variability of the light through which you see a transparency.
Yet, your print doesn't change. That grey in your print is the same if you measure it with the reflective densitometer. You can then turn the light off and say that you only have black in your print, with the light turned off .

Of course there are different light conditions in which you see a print or a slide. But your eye/brain is smart enough to compensate, as far as possible, for the variations and you end up with a mental image of that image which is always the same.

and what reading does your reflection densitometer give when you turn off its light which is used for the reading? And yes they do have a light in them. And its a calibrated source directed at a specific angle towards whatever you are taking a reading from and the reading is taken at a specific angle so what densitometer tells you is based on that.

 

[Diapositivo]

The actual calibration is done using a backlit screen a blue filter (like 80B) and a tungsten light bulb. So it's a luminance, not a reflectance.

Reflectance is the percentage of the light reflected by a body. It's a property of that body (we can approximate it with its tone).
Illuminance in photography is the light received by a subject.
Luminance in photography is the light received by the film (presumably after having being reflected by the subject).
Luminance can be light reflected by a subject, or light emitted by a subject (if the subject is a light source, such as a neon light). If the light is emitted by the subject, the reflected light meter sees the light the subject emits.
If the light is reflected by the subject, the reflected light meter sees the light the subject reflects. The light the subject reflects is what the light meter measures. There is a reason why it is called "reflect light metering" after all!

A light meter does not even know the difference between them. A light meter is a light meter is a light meter.
Light meters measure the light falling on the sensor, the cell.
The cell doesn't know if it is "illuminance" or "luminance", you know. They only see light. Luminance or Illuminance are only, in photography, terms meaning "light hitting the subject" or "light hitting the film". Light meters don't know what they are aimed at.

You can calibrate a light meter with a known light source but that will only simulate the light which is emitted by your subject (if your subject is a light source), reflected by your subject (if your subject is opaque and you use the instrument as a reflected light meter) or the light received by the subject (if your subject is opaque and you use the instrument it like an incident light meter).

The cell works exactly the same. Any incident light meter is also a reflected light meter and the other way round.

You can screw a lumisphere on the front screw of your lens and you can use your internal TTL meter to measure incident light (such device exists).
In my Gossen I just move the lumisphere in front of the cell, or move it away. The cell doesn't know whether there is a lumisphere in front of it. Only I know. The cell only measures light falling on it.
It's up to me to interpret it as luminance or illuminance depending on where I collected that light, whether I had the lumisphere in front of it or not, where I aimed it at, etc.

The only way you can measure reflectance of a body is to throw at it a certain known quantity of light and see how much of it it reflects towards you (that's what a reflective densitometer does, but the lamp must be turned on ).
(And I understand that for certain - especially flat - surfaces the angle at which the light is received and reflected is very very important in the final result. We should use a grey cat instead of a grey card ).
Yet, a white cat will always reflects more light than a black cat, under whichever angle and whatever light you throw at it.

 

[Diapositivo]

but you don't need to waste your time on the minutia of the theory, you can go straight to the practical evaluation(calibration) and save yourself a lot of fruitless time which is what I'm proposing.

Rob, if it helps you, see it as an intellectual exercise. Some people like theory just like some people like apples, and some don't. I like the theoretical aspect of all the process, and the theoretical "foreseeability" of the results of the process. All in theory.

 

[markbarendt]

Mark, the variability of the light in which you see a print is way bigger than the variability of the light through which you see a transparency.
Yet, your print doesn't change. That grey in your print is the same if you measure it with the reflective densitometer. You can then turn the light off and say that you only have black in your print, with the light turned off .

Of course there are different light conditions in which you see a print or a slide. But your eye/brain is smart enough to compensate, as far as possible, for the variations and you end up with a mental image of that image which is always the same.

I don't think our brains are that smart.

The lighting shining on, through, and around our photos truly changes the way the tones look to us.

When I'm printing and narrowing in on the settings for the final print I'll actually leave the darkroom and go try that print in various lighting situations and they can look very different here or there.

 

[Bill Burk]

We should use a grey cat instead of a grey card.

Or both...

My point is that the meter is calibrated to a backlit screen, and that explains why it's literally not calibrated to reflectance.
All the discussion about the calibrated reflectance is us trying to figure out the equivalent.

 

[RobC]

you forgot to answer my question which was rather important in making you understand that the grey a reflection densitomter reads is wholly dependant on the light inside the densitomter . i.e. your answer to mark is flawed becasue what the densitomter sees changes with the light used.

Right, so now I know, you're not interested in how to actually do it but rather how to pontificate about it.

arrivederci

 

[Diapositivo]

you forgot to answer my question which was rather important in making you understand that the grey a reflection densitomter reads is wholly dependant on the light inside the densitomter . i.e. your answer to mark is flawed becasue what the densitomter sees changes with the light used.

Right, so now I know, you're not interested in how to actually do it but rather how to pontificate about it.

arrivederci

Rob, it's obvious. The densitometer works by throwing a certain, known amount of light to a subject, and measuring, through a light meter, how much light comes back. If 50% of the light comes back to the cell, than the subject has a 50% reflectivity, which is a light tone. If you turn the lamp off, you cannot measure anything and you cannot say that the cat is black.
They say that when it's night all cows are black, but, believe me, it ain't true!

Just like for a sensitometer the light falling on the subject must be known in order to derive the reflectivity of the subject, so for a reflected light meter the reflectivity of the subject must be known in order to derive the light falling on the subject. That will give the right exposure for all tones of the subject (like an incident light meter) within the dynamic range limits of your photographic material.

[A reflected densitometer is an object that works with the same equation as the reflected light meter:
Y = kX
only, for the densitometer k is unknown, and X is known.
So you could write it as Y = xK, because x (illuminance) is a fixed parameter and K, reflectance, is the unknown function of Y which is light, you can call it luminance, the cell doesn't care, it only knows the physical phenomenon known as "light".]

But the light meter doesn't know the reflectivity of the subject. It must assume a certain reflectivity, k. And you must aim it either at something which is exactly k if you want a precise reading, or at something that is "on average" k (which is your average scene). And because your average scene has reflectivity = k, your TTL reflective meter works.

That's because people making light meters use k taking the average reflectivity of the average scene. That's 16.5% - 18% depending from the maker.

And the demonstration of this is that if you use your TTL meter to meter a white cat in the snow you obtain, with slides, a grey cat on grey snow with absolute certainty. Find some snow, a white cat, and do the test! (so that you are content ).
The cat will be grey, and the snow will be grey, no matter how much light you throw through the slide!
Once you tested this, ask yourself why you obtain a grey cat and a grey snow.

Do the same test with a black cat on a bunch of very black coal. You will obtain on your slide a grey cat on some grey stuff. Absolutely and invariably. Try that!

Now try that with different shades of cat and material. I know, it's a bit boring, but you must test it.
You must arrive to a certain shade of cat and material which is correctly rendered! More or less halfway between the two extremes there is ONE cat exactly rendered. Only ONE.
You then put the cat on the densitometer, turn the light on, and the reflectivity of the cat will tell you k sperimentally!

 

[wiltw]

RobC and Diapositivo,
I see both of you agreeing with elements of what I have said in this thread, and now I am seeing you disagreeing about something, but I am confused as to WHAT you are disagreeing about! Perhaps the two of you could agree about what you are disagreeing about, and fill me in?!

In post #268 Diapositivo made a statement, which I fundamentally agree with and provide an illustration of the point: "Of course there are different light conditions in which you see a print or a slide. But your eye/brain is smart enough to compensate, as far as possible, for the variations and you end up with a mental image of that image which is always the same."

Where previously (post #204) I had four shots, exhibited at different contrast levels but all had 18% grey Macbeth square at 50% density, now I offer the same four shots with the 18% grey square portrayed at 50% density (to mimic the Macbeth card) but also with 1.0EV offset in 'the wrong direction' ...yet the subjective assessment by our brains of the middle tone is nevertheless 'in the middle between perceived black and perceived white'...what Diapositivo posits in post #268! In the objective measurement, 'middle' in tonal scale is maintained, even when the actual density quantitiy is some different value than the 'school solution'. And this remains true even when some of us have our monitors adjusted too bright (the way they came from the store) and normally do not use our monitors for anything except posting discussions on APUG photo forums.