Kodak grey card usage

[markbarendt]

In thanking wiltw for his illustrative tests, I am pondering that they really show why incident light meter are graciously provided with a lumisphere. Their use would be quite error prone if we had to devise any time the exact proper angle at which to use them. The lumisphere takes a lot of complexity out of this. Gives us the light "around the subject" not the light "mostly perpendicular to a certain plane of the subject".

On the other hand, if a subject is plane, such as a dark wooden table, that we want to photograph while maintaining, in the slide, the dark tone of the wood:

- supposing we are using an incident light meter we would obtain more precise results by using the disc instead of the sphere;

- supposing we are using a grey card for whatever reason, I think we would obtain more precise results by placing the grey card flat on the table, and not at a certain angle. We would then read the spot metering on the grey card laying on the table, and we would have the result we would use if the table was grey. Then we would close 1 stop because we want the wood to be dark.

Using the grey card on the same plane of a plane subject (document, painting, mosaic, fresco, pavement, floor, manhole, table...) corresponds to the use of the disc. If the flare on the card is very different from the flare on the subject, we are going to have a less precise indication.

Orienting the grey card half-way between source and lens-subject axis is an attempt to average the light "around" the subject just like a lumisphere would do. That is also a way to try to minimize flare, probably.

But if the subject IS flat and if it has flare, then the disc or card laying on the plane should give a better reading.

Run those tests and let us know what you find.

 

[Diapositivo]

Run those tests and let us know what you find.

Well, I was just pondering, as I said. I don't deal with document or art reproduction and don't even possess a disc (my incident light meter only has a lumisphere). I am interested in the theoretical aspect. For my usual subjects, I really have no doubts in the use of the incident or spot light meter. I never use the grey card.

I think it would be very interesting if somebody here, who routinely photographs flat subjects such as documents or paintings, would shed some light in the way he uses the disc (or the grey card) and confirm, or deny, the hypothesis above.

The tests would be very easy, there's no need to take pictures! Just place the grey card on the table, the incident light meter with the disc, with the sphere, e compare the readings!

(If you have an incident light meter with a disc you should be able to do the test yourself in a few minutes and confirm or deny my assumption).

 

[Diapositivo]

Mmmh, rethinking it, there's a complication.
If you lay the disc over the table, and the sun is let's say 45°, and you take the picture at 45°, the two readings (disc and grey card) should coincide.
But if, let's say, the sun is very high, and your angle of view is different, then the reading that the flat surface of the disc collects is for the light at an angle which is different from the lens angle with the table.
The two readings should coincide if you photograph the table from above, with the lens perpendicular to the table. That is, if you photograph the table as if it were a document. Only in that case the flat grey card collects the same light that the disc collects.

 

[wiltw]

Well, I was just pondering, as I said. I don't deal with document or art reproduction and don't even possess a disc (my incident light meter only has a lumisphere). I am interested in the theoretical aspect. For my usual subjects, I really have no doubts in the use of the incident or spot light meter. I never use the grey card.

I think it would be very interesting if somebody here, who routinely photographs flat subjects such as documents or paintings, would shed some light in the way he uses the disc (or the grey card) and confirm, or deny, the hypothesis above.

The tests would be very easy, there's no need to take pictures! Just place the grey card on the table, the incident light meter with the disc, with the sphere, e compare the readings!

(If you have an incident light meter with a disc you should be able to do the test yourself in a few minutes and confirm or deny my assumption).

Ok since I have the flat disk and hemisphere...

Here is my 'flat art' on a table, lit by windowlight somewhat like an art reproduction might have light at 45 degrees to the art (but to both sides, not merely one)

The test scenario

• Flat disk reading: ISO 400 1/60 f/4.0 +0.1EV from left of art (Minolta Autometer Vf).
  +0.2EV if taken from right of art rather than from left.
• Grey card spot reading: ISO 400 1/60 f/4.0 +0.6EV (-0.5EV less exposure than indicated by flat disk) with Minolta Spotmeter F

Now the shot, at left as exposed per flat disk ISO 400 1/60 f/4.0 +0.0EV
and on the right, I cloned the photo and reduced exposure in LR postprocessing -0.5EV...the grey reads exactly 50-50-50 (rounding the decimal) with the eyedropper.
The spotmeter and grey card did better, more accurate tonal representation of inherent brightness in art and grey card!

 

[markbarendt]

Giggle.

Mmmh, rethinking it, there's a complication.
If you lay the disc over the table, and the sun is let's say 45°, and you take the picture at 45°, the two readings (disc and grey card) should coincide.
But if, let's say, the sun is very high, and your angle of view is different, then the reading that the flat surface of the disc collects is for the light at an angle which is different from the lens angle with the table.
The two readings should coincide if you photograph the table from above, with the lens perpendicular to the table. That is, if you photograph the table as if it were a document. Only in that case the flat grey card collects the same light that the disc collects.

The readings will not necessarily coincide, light sources in the wild aren't single point one direction sources. There are normally reflections from walls, snow, sky, and other such things. The way all these sources light a scene is different, the angle of view of each tool is different.

Good work can be done with any of these tools but that doesn't mean the meters will spit out matching numbers.

 

[wiltw]

OK guys, speculation from the speculators or experts, about why the post 80 results that I obtained?

• One would think the incident meter reading should result in the 18% grey card being centered on the historgram...but it was shifted to the right of center on the histogram!
• One would think the reflected meter reading on the 18% grey card should need to be increased by +0.33EV to 0.5EV in order for the grey card to be centered in the histogram, but it was spot on!

This ought to really mystify those that use sensitometers and density graphs to prove their point about calibration assumptions for reflected light targets vs. incident meters!

It seems to once again prove "your metering results WILL vary (regardless of meter type!)"

 

[Diapositivo]

Giggle.

The readings will not necessarily coincide, light sources in the wild aren't single point one direction sources. There are normally reflections from walls, snow, sky, and other such things. The way all these sources light a scene is different, the angle of view of each tool is different.

Good work can be done with any of these tools but that doesn't mean the meters will spit out matching numbers.

Yes but all this is necessarily approximated by the fact itself that somebody is using, or considering using, a grey card as an approximation of an incident light reading.

 

[Diapositivo]

Ok since I have the flat disk and hemisphere...

Here is my 'flat art' on a table, lit by windowlight somewhat like an art reproduction might have light at 45 degrees to the art (but to both sides, not merely one)

The test scenario

• Flat disk reading: ISO 400 1/60 f/4.0 +0.1EV from left of art (Minolta Autometer Vf).
  +0.2EV if taken from right of art rather than from left.
• Grey card spot reading: ISO 400 1/60 f/4.0 +0.6EV (-0.5EV less exposure than indicated by flat disk) with Minolta Spotmeter F

Now the shot, at left as exposed per flat disk ISO 400 1/60 f/4.0 +0.0EV
and on the right, I cloned the photo and reduced exposure in LR postprocessing -0.5EV...the grey reads exactly 50-50-50 (rounding the decimal) with the eyedropper.
The spotmeter and grey card did better, more accurate tonal representation of inherent brightness in art and grey card!

That's interesting.
Besides your test with Photoshop, the picture to the right seems to have a more natural rendition of the white of the paper, and the one to the left gives the paper which is - at this monitor - a bit washed up.

I would have expected the same reading in this condition with the two systems.

Maybe Mark is right, the lumisphere would give the same result - as the light falls on the subject from all directions.

I would have thought: the light which I see falls on the flat subject mainly from the specular direction. Many incarnations ago I was taking pictures of a black African marble stair which in picture looked black and white, white where it specularly reflected the light, black where it didn't. An extreme case but I would have thought that, also for opaque objects, the "specular" light (on the other side of the flat object) was the more influent. (we see this in portraits with the "accent" light leaving a light spot only on the "specular" part of the front, or hair, for instance, not that it doesn't enlighten the rest, but less).

The chair near the card and light meter though can create some bias, because the seatback projects somehow an uneven shadow on the subject, and the incident meter might have been influenced in a different way than the grey card.

Maybe if and when you have time, you could repeat the experiment:
without the seatback;
with three methods: sphere, disc, spot on grey card.
I would be very interested (and also those who don't have a disc to play with).

 

[markbarendt]

Ok since I have the flat disk and hemisphere...

Here is my 'flat art' on a table, lit by windowlight somewhat like an art reproduction might have light at 45 degrees to the art (but to both sides, not merely one)

The test scenario

• Flat disk reading: ISO 400 1/60 f/4.0 +0.1EV from left of art (Minolta Autometer Vf).
  +0.2EV if taken from right of art rather than from left.
• Grey card spot reading: ISO 400 1/60 f/4.0 +0.6EV (-0.5EV less exposure than indicated by flat disk) with Minolta Spotmeter F

Now the shot, at left as exposed per flat disk ISO 400 1/60 f/4.0 +0.0EV
and on the right, I cloned the photo and reduced exposure in LR postprocessing -0.5EV...the grey reads exactly 50-50-50 (rounding the decimal) with the eyedropper.
The spotmeter and grey card did better, more accurate tonal representation of inherent brightness in art and grey card!

The gray card is "a" middle tone, not necessarily "the" middle tone. There are also other variables in the digital system so specifics are not directly applicable or comparable. Film behaves differently.

 

[wiltw]

The gray card is "a" middle tone, not necessarily "the" middle tone. There are also other variables in the digital system so specifics are not directly applicable or comparable. Film behaves differently.

Mark, there are numerous discussions which mention the Munsell or other scales of black throught white, NONE of them directly impacting photographic exposure of film and all of them related to simply ink on paper, and the multitude of standardized references to colors...and the midtone on all these scales that I have seen is indeed 'the middle of the steps of the scale'

(there was a url link here which no longer exists)

And to quote Munsell, " The scale of value ranges from 0 for pure black to 10 for pure white. Black, white and the grays (as shown in figure 2) between them are called “neutral colors”. They have no hue. Colors that have a hue are called “chromatic colors.” The value scale applies to chromatic as well as neutral colors. The value scale is illustrated for all neutral colors on the chart labeled Munsell’s Nearly Neutral, included in this book of color." IOW, scale value 5 is merely the middle of the range of brightnesses between 'white' and 'black' without reference to exposure per se (or metering).

Middle grey, looks suspiciously like the 18% grey card.

 

[markbarendt]

Middle gray on paper does not mean the tone of the Kodak gray card in the scene. There are many variables between the scene and the paper/a positive.

For example, contrast adjustments (push/pull, soft/hard paper) which will change the placement/tone of the gray card in the print.

IMO part of the reason there is so much confusion about gray cards is that people default to the thought that the card in the scene should match the card on the print and fall exactly in the middle, that's not a given, more of a myth actually.

 

[DREW WILEY]

I didn't realize anyone except a few commercial halftone printers ever talked about the Munsell system anymore. Anything modern is based upon either three axis or four-axis color mapping, which has been around ever since the 1920's, and is standard now. But is there any actual evidence that the Munsell system was ever officially related to anything sensitometric to begin with?

 

[wiltw]

I didn't realize anyone except a few commercial halftone printers ever talked about the Munsell system anymore. Anything modern is based upon either three axis or four-axis color mapping, which has been around ever since the 1920's, and is standard now. But is there any actual evidence that the Munsell system was ever officially related to anything sensitometric to begin with?

Well, one would not think that an International Standard goes out of date as the basis for everything else, unless a new standard was created.

"Munsell Color Theory Linked to Modern Color Measurement
A subcommittee for the Optical Society of America studied the visual spacing of the scales and published recommended changes in 1943. Those recommendations are called the Munsell renotations. The recommended spacing was specified by the system of color measurement standardized by the International Commission on Illumination (identified by the initials, CIE, of its name in French), using CIE Illuminate C and CIE 1931 (2 degrees) Standard Observer.

The renotations provide a method of converting color measurement data to Munsell notations and the specifications for producing Munsell color standards. The Munsell renotations were standardized by the American Society for Testing and Material in D 1535 Standard Test Method for Specifying Color by the Munsell system."​

"The Munsell color-order system has gained international acceptance. It is described in unabridged dictionaries and encyclopedias as well as in specialized publications on art, design, color photography, television, printing, paint, textiles and plastics. It is recognized as a standard system of color specification in standard Z138.2 of the American National Standards Institute, Japanese Industrial Standard for Color JIS Z 8721, the German Standard Color System, DIN 6164 and several British national standards. The Munsell color-order system has been widely used in many fields of color science, most notably as a model of uniformity for colorimetric spaces and has, itself, been the subject of many scientific studies."
​

Are you aware of anything newer, that has superceded it? I did a quick bit of searching and found this:

"The ICC specification is widely used and is referred to in many International and other de-facto standards. It was first approved as an International Standard, ISO 15076-1, in 2005 and revised in 2010."
​

This paper concludes, https://pdfs.semanticscholar.org/2630/f0fa6901b90588d45c47dbd9ce5baa384562.pdf

"Munsell Space remains the prefered data for a uniform color 3D LUT. It is unique in that it provides data out to the spectrum locus. It is the compilation of 3 million observations, most highly relevent to the problem. There may be some errors in the hue plane placement, but it is not obvious how to improve the process.​

 

[Stephen Benskin]

Has anyone mentioned what Dunn calls the Duplex Method where you average the meter reading between a flat disk pointed at the light source direction and pointed in the camera direction?

 

[DREW WILEY]

For all practical purposes, the CIE standards replaced it long ago. This can be converted via analytic geometry software directly into modern color
mapping, lights years ahead of the Munsell system. Even the military had their own spec system for paint. Munsell is worthless for anything transparent or translucent. As far as British standards are concerned, everyone knows their system of measurement was originally based on how
far a drunken Celtic priest could throw the head of an ox.

 

[wiltw]

Has anyone mentioned what Dunn calls the Duplex Method where you average the meter reading between a flat disk pointed at the light source direction and pointed in the camera direction?

I had not seen that referenced, but I did take measurement out of curiosity when I did my most recent testing with flat art,

• flat disk parallel to flat art: ISO 400 1/60 f/4.0 +0.1EV
• flat disk pointed at window source: ISO 400 1/60 f/5.6 +0.6EV
• average of the forementioned: ISO 400 f/1/60 f/4.0 +0.85EV, which is about 0.75EV darker than the original exposure and 0.25EV darker than the grey card exposure (and rendering the grey card a touch dark)

Here is the earlier series, with third exposure added, as above (adjusted in LR postprocessing -0.75EV)

 

[markbarendt]

Has anyone mentioned what Dunn calls the Duplex Method where you average the meter reading between a flat disk pointed at the light source direction and pointed in the camera direction?

Had not gone there yet. Tis' a fave of mine though.

 

[wiltw]

Maybe if and when you have time, you could repeat the experiment:
without the seatback;
with three methods: sphere, disc, spot on grey card.
I would be very interested (and also those who don't have a disc to play with).

Well, with the chair in place, and putting the meter in the center of the 8.5" wide 'piece of art', I measure at most a 0.1EV change of light intensity as I move the disk from top of page to bottom of page, or across it. Keep in mind most of the light was coming downward at an angle, and not UPward through the chair!
I think the chair has negligible effect on what I shot earlier...as you move the art, your eye cannot detect any shadow cast by the chair. Besides, as the set shot shows, the grey card and the art are both affected by the chair placement, so relative brightness remains 0EV.

 

[wiltw]

For all practical purposes, the CIE standards replaced it long ago. This can be converted via analytic geometry software directly into modern color
mapping, lights years ahead of the Munsell system. Even the military had their own spec system for paint. Munsell is worthless for anything transparent or translucent. As far as British standards are concerned, everyone knows their system of measurement was originally based on how
far a drunken Celtic priest could throw the head of an ox.

Is this not somewhat of a moot issue, given the fact that the 18% grey card midtone was developed in the days when Munsell was indeed the reference standard? No one is saying the 18% grey card is no longer a valid reference because it was invented in the days before CIE and digital data.

 

[markbarendt]

Yes but all this is necessarily approximated by the fact itself that somebody is using, or considering using, a grey card as an approximation of an incident light reading.

Sure, and it's very possible to do.

The only things one needs to know to make any target work is how you use it normally and what the offset is to the speed point. It's not about middle gray, it's about the relationship between the target and the speed point.

As with any form of spot metering this comes with a little practice and testing.

 

[Diapositivo]

Middle gray on paper does not mean the tone of the Kodak gray card in the scene. There are many variables between the scene and the paper/a positive.

For example, contrast adjustments (push/pull, soft/hard paper) which will change the placement/tone of the gray card in the print.

IMO part of the reason there is so much confusion about gray cards is that people default to the thought that the card in the scene should match the card on the print and fall exactly in the middle, that's not a given, more of a myth actually.

Mark, I disagree. I verbously expressed my thought in another post in one of those concurrent threands that are taking place on APUG regarding exposure and middle grey and k factors etc.
Basically, those like you who develop and print themselves do not consider that the world at large does not develop nor print themselves. You are "too evolved" and have lost the simple basic truths of this industry.

Normal people take slides, or colour negatives, bring them to the laboratory, where they are processed in an industry standard way, and they expect them to be exposed correctly which means, briefly, that middle grey on reality looks middle grey on the final product if they follow the indication of their light meter. Normal people have no idea what the "speed point" is and they don't need to have it. They don't need to know sensitometry to take "correctly exposed" pictures.

18% is the visual "middle point" of human vision, and corresponds to 128/256 (or 127/255 if we have a representation with the zero) in the digital world.
The scale from 0 to 255 used in the digital world is a "perceptual" one.

The assumption by wiltw that by taking a picture of a grey card he should obtain a colour expressed in RGB as 128, 128, 128 looks to me quite correct. In a slide film 100 ISO he should obtain a density of -1.

Now, as we know, theory and practice differ: flare on the card and other factors might cause a deviation. But in principle, in theory, 18% grey is 128, 128, 128 and that is the "correct" exposure for a grey which is supposed to be perceptually middle (which does not mean it is the exposure the photographer choses, for artistic reasons or for any reason connected to its work-flow).

Stop shaking all my certitudes!

PS You know there is, in digital, an exposure technique known as "exposing to the right". That's a certainly appropriate way to expose a frame and some people such as myself use it routinely. That gives the best results but only if you "develop" yourself (starting from raw). It's not the "correct" exposure but certainly is an effective way to expose a frame if one has full control of the workflow (like you have in B&W developing and printing). For my lightmeter, all those frames are overexposed for my ISO speed. And they, literally, are! They are overexposed and underdeveloped, which gives a better result. But the straight "industry standard" JPEG derived from it is overexposed. The "correct exposure" is the one that gives me middle grey (128,128,128) on the JPEG developed in the default (industry standard) way for my ISO speed. The light meter calculates the exposure for the default JPEG.
It's as if you and some other people shoot digital and spent so much time developing raw images that you forgot that default JPEG exist and said: the correct exposure is the one that avoids clipping highlights and the correct development/printing is the one that gives the desired tone on the final image. Yes, of course. But also no.

 

[markbarendt]

Diapositivo, one thing that truly needs to be understood. Digital plays by a different subset of the rules of physics than film.

In short the hinge point of the film curve is at the shadow end of the curve, around the speed point. Digital is not constrained by physics in that manner, it's hinge point can center on middle gray.

The geometry and adjustments work differently.

 

[markbarendt]

On to negatives for a moment. Negatives don't need perfect camera exposure to get great prints/positives. In general most negative films can produce very good print results if the negative has received exposure in a range from -1 to +2 full stops. This is well documented in both photographic theory and practical, day-to-day photography. Think about all the shots in the world that have been taken on cameras with fixed exposure settings: box cameras, disposables, ... The positive is simply fixed during printing.

The prints the average Joe gets back from the lab have no direct connection to the original camera exposure.

 

[markbarendt]

Now to slides/trannies.

In this one case you can theoretically place a gray card specifically at the point you prefer, if the processing is pre defined.

That still doesn't mean it will fall dead center. 1 isn't the middle between 0 and 1.8.

 

[wiltw]

Sure, and it's very possible to do.

The only things one needs to know to make any target work is how you use it normally and what the offset is to the speed point. It's not about middle gray, it's about the relationship between the target and the speed point.

As with any form of spot metering this comes with a little practice and testing.

Please explain the concept of 'the speed point', hopefully in a manner which does not require pages filled with a long explanation requiring an engineering degree and understanding of calculus.