Metering tonality...18% or 12%?

OK, so there you go, in practical terms you and Wilt agree -- meter an 18% gray card up by 1/2 stop and it will show up close to the "center" of the histogram. But, as we know, "18% gray" is nothing to do with "one half white"!

tonylong wrote in post #12181516
OK, so there you go, in practical terms you and Wilt agree -- meter an 18% gray card up by 1/2 stop and it will show up close to the "center" of the histogram. But, as we no, "18% gray" is nothing to do with "one half white"!

Right.

You've probably heard the old joke...

I wear plaid pajamas, because it keeps the elephants away at night.
But there are no elephants anywhere on this continent.
Right, See?


18% being midpoint is about the same. It coincidentally seems to about work, in the same way, but is of course, the wrong idea. 18% is not the middle of anything digital.

The 18% being middle gray is entirely in the human mind. Literally, it is how our brain's perception responds to 18%. But it is not about histograms.

WayneF wrote in post #12182644
Right.

You've probably heard the old joke...

I wear plaid pajamas, because it keeps the elephants away at night.
But there are no elephants anywhere on this continent.
Right, See?


18% being midpoint is about the same. It coincidentally seems to about work, in the same way, but is of course, the wrong idea. 18% is not the middle of anything digital.

The 18% being middle gray is entirely in the human mind. Literally, it is how our brain's perception responds to 18%. But it is not about histograms.

I found this.

"Warning: this is a long, somewhat technical post that includes some math (but when you get past the superscripts and such, it's ultimately pretty simple math).

Some time ago, Thom Hogan published an article:

http://www.bythom.com/​graycards.htm

that claims meters in Nikon digital cameras are calibrated for a mid-level grey that corresponds to 12% reflectance rather than the 18% grey of most standard grey cards.

Unfortunately, while the title and opening paragraph of the article are quite emphatic about 18% being a “myth”, the remainder of the article fails to provide much factual basis for this claim. Here’s what Thom gives as the basis for his statements:

ANSI standards (which, unfortunately, are not publically published--you have to pay big bucks to have access to them), calibrate meters using luminance, not reflection. For an ANSI calibrated meter, the most commonly published information I've seen is that the luminance value used translates into a reflectance of 12%. I've also seen 12.5% and 13% (so where the heck does Sekonic's 14% come from?), but 12% seems to be correct--one half stop lighter than 18%, by the way. I haven't seen anyone claim that ANSI calibration translates into a reflectance of 18%.

In the end, he seems to have no real basis for his claims, merely a statement that “12% seems to be correct,” with no real evidence, or even information about why he considers this correct. Despite this, however, this article is now widely cited on various photographically oriented web sites (among other places) as if it were absolute and indisputable fact.

Since this issue seems to be of interest to a fair number of photographers, I decided to see if I could find some real facts with evidence to support them. The first step in this journey was to find the standard in question. Doing some searching, I found the relevant standard. Contrary to Thom’s implication above, this is really published by the ISO rather than ANSI. This may be trivial to most, but when I was looking for the standard it was somewhat important – I put in a fair amount of work trying to find an ANSI standard that apparently does not exist. In the end, however, I found the relevant ISO standard: ISO 2720-1974, “Photography - General purpose photographic exposure meters (photoelectric type) - Guide to product specification (First edition - 1974-08-15)”.

I also found that Thom was (at least from my viewpoint) quite mistaken about prices as well – a copy of this standard costs only $65 US. This didn't strike me as "big bucks" -- in fact, it seemed like a fair price to pay for some real enlightenment (pun noted by not really intended) on the subject.

The standard confirmed part of what Thom had to say, such as calibrating meters directly from sources that emit light rather than from reflected light. Unfortunately, other parts of what Thom had to say are not quite so closely aligned with the content of the standard. For example, at the conclusion of his article, he includes a comment from “lance” that mentioned a "'K' factor", without specifying its exact meaning or purpose. Thom replied by saying: “No manufacturer I've talked to knows anything about a K factor, though, and they all speak specifically about the ANSI standard as their criteria for building and testing meters.”

As stated, this may not be exactly wrong – but it’s certainly misleading at best. In reality, a large part of the ISO standard is devoted to the K factor. Much of the rest is devoted to the C factor, which corresponds to the K factor, but is used for incident light meters instead (the K factor applies only to reflected light meters). It would be utterly impossible to follow the standard (at least with respect to a reflected light meter) without knowing (quite a lot) about the K factor.

The standard specifies that: “The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a number of observers, of a number of Photographs, for which the exposure was known, obtained under various conditions of subject matter and over a range of Iuminances.”

The standard also specifies a range within which the K factor must fall. The numbers for the range depend on the method used for measuring/rating film speed (or its equivalent with a digital sensor). For the moment, I’m going to ignore the DIN-style speeds, and look only at the ASA-style speed ratings. For this system, the allowable range for the K factor is 10.6 to 13.4. These numbers do not correspond directly to reflectance values (e.g. 10.6 doesn't imply a 10.6% grey card as mid-level grey), but they do correspond to different levels of illumination that will be metered as mid-level grey. In other words, there is not one specific level of reflectance that is required to be metered as mid-level grey – rather, any value within the specified range is allowable.

The K factor is related to a measured exposure by the following formula:

K = LtS / A2

Where:

K = K factor
L = Luminance in cd/m2
A = f-number
t = effective shutter speed
S = film speed

Using this formula and a calibrated monitor, we can find the K factor for a specific camera. For example, I have a Sony Alpha 700 camera and a monitor that’s calibrated for a brightness of 100 cd/m2. Doing a quick check, my camera meters the screen (displaying its idea of pure white) with no other visible light sources, at an exposure of 1/200th of a second at f/2. Running this through the formula, gives a K factor of 12.5 – just above the middle of the range allowed by the standard.

The next step is to figure up what level of “grey” on a card that corresponds to. Let’s do that based on the sunny f/16 rule, which says a proper exposure under bright sunlight is f/16 with a shutter speed that’s the reciprocal of the film speed. We can mathematically transform the formula above to:

L = A2K/tS

Let’s work things out for ISO 100 film:

L = 16x16xK/.01x100

The .01 and 100 cancel (and they will always cancel since the rule is that the exposure time is the reciprocal of the film speed), so this simplifies to: L = 256K.

Working the numbers for the lowest and highest allowable values for the K factor gives 2714 and 3430 respectively.

Now, we run into the reason the ISO standard specifies light levels rather than reflectance of a surface – even though we’ve all seen and heard the sunny f/16 rule, the reality is that clear sunlight varies over a considerable range, depending on season, latitude, etc. Clear sunlight has brightness anywhere from about 32000 to 100000 lux. The average of that range is about 66000 lux, so we’ll work the numbers on that basis. This has to be multiplied by the reflectance to give a luminance – but the result from that comes out in units of “apostilbs” rather than cd/m2. To convert from apostilbs to cd/m2, we multiply by 0.318:

L = I x R x 0.318.

Where:

R = reflectance
I = Illuminance (in Lux)
L = luminance (in cd/m2)

We already have the values for L that we care about, so we’ll rearrange this to give the values of R:

R = L / 0.318 I

Plugging in our minimum and maximum values for I, we get:

R1 = L / 10176
R2 = L / 31800

Then we plug in the two values for L to define our allowable range for R:

R1,1 = 2714 / 10176
R1,2 = 2714 / 31800
R2,1 = 3430 / 10176
R2,2 = 3430 / 31800

R1,1 = .27
R1,2 = .085
R2,1 = .34
R2,2 = .11

In other words, between the range of brightness of the sun and the range of K factors allowed by the ISO standard, a reflectance anywhere from about 8.5% to about 34% can fall within the requirements of the standard. This is obviously a very wide range of values – and one that clearly includes both the 12% Thom advocates and the 18% of a typical grey card.

To narrow the range a bit, let’s consider just the arithmetic and geometric mean of the range of brightness from the sun: 66000 and 56569 lux respectively. Plugging these into the formula for the range of possible reflectance values gives:

R1,1 = 2714 / 20988
R1,2 = 2714 / 17989
R2,1 = 3430 / 20988
R2,2 = 3430 / 17989

The results from those are:

R1,1 = .13
R1,2 = .15
R2,1 = .16
R2,2 = .19

An 18% grey card is close to one end of this range, but still falls within the range. A 12% grey card falls outside the range; we have to assume an above-average light level for it to work out. If we average the four numbers above together, we get a value of about 16% grey as being the "ideal" – one that should work out reasonably well under almost any condition.

To summarize:

The ISO standard allows a range of calibrations, not just one level
Normal daylight brightness covers a fairly wide range as well
18% grey is justifiable based on average light levels
12% grey is not justifiable based on average light levels
Based on average light levels, the ideal value for a grey card would be about 16%
You meter might be calibrated to 18%, but probably isn't (and shouldn't be) calibrated to 12%."

writen by jrista

airfrogusmc wrote in post #12182703
In the end, he seems to have no real basis for his claims, merely a statement that “12% seems to be correct,” with no real evidence, or even information about why he considers this correct.

See message #44 in this thread, regarding the Sekonic website specifications. Sekonic says their K = 12.5

Kodak also says open 1/2 stop from 18%, which is 12%.

I don't have a grey card (I really do not need one), but I just took my Sekonic L-758D out into the garden on a lovely, bright, sunny day and dialing in f/16 and 100 ISO I got a reading of 1/100 and one tenth when facing the light meter horizontally back towards camera position, with the sun above and directly behind the camera.

Spot metering my wide open palm with the Sekonic at f/16 and 100 ISO gave me a reading of 1/250 dead, exactly 1.3 stops above the "Sunny 16" guideline and 1.2 stops above the incident reading. In other words, spot metering from my palm at +1.3 stops is just as effective as taking an incident light reading.

The reason I have the light meter at all is for metering flash. I don't use it for ambient reading, since there really is no need. When I want to perform mid-tone ambient metering, as opposed to metering highlights at +3 stops, I often use my own palm as a substitute for a grey card and spot meter it with my cameras at +1.3 stops. Based on the confirmation above that seems completely valid to me. A good job too, since I've been using that technique for around three years following the immensely helpful "Exposure crutch" tip by PhotosGuy.

Secure in the knowledge that my palm is indeed 1.3 stops brighter (OK, 1.2 stops) as a reflective target than an incident measurement indicates I thought I'd double check my cameras - a 1D3, 7D and 5D2. With a variety of L lenses I set each camera to 1/100, f/16, 100 ISO and repeated spot metering of my palm in the same light. The 1D3 meter said my palm was at +1 with those settings and the 7D and 5D2 said it was at +1.3. For my purposes I can live with +1.3 as my "mid tone" target for all three cameras. I really don't think I need a grey card of 12%, 12.5%, 18% or anything else to get me by. My palm at +1.3 is perfectly satisfactory as a metering target, and should always allow me to nail the exposure for skin tones, regardless of what else is going on in the scene.

p.s. In the same light my nice green lawn meters at 1/60, or -2/3 if you prefer.

Gamma? What's it got to do with metering? (Rhetorical)

tdodd wrote in post #12183188
Gamma? What's it got to do with metering? (Rhetorical)

Absolutely nothing. But if you plan to inspect your histogram, and claim "this tone is at 128 because ...", then to avoid saying something dumb, it would be wise to understand that the histogram data is gamma encoded.

WayneF wrote in post #12183226
Absolutely nothing. But if you plan to inspect your histogram, and claim "this tone is at 128 because ...", then to avoid saying something dumb, it would be wise to understand that the histogram data is gamma encoded.

I have no intention of doing such a thing.

I'm far more interested in what's going on at the left and right hand sides of the histogram than at the middle and even then I certainly don't pay any attention to what numeric values I might find along the way. Is this information we need, I wonder? I struggle to imagine why we would.

Clipping points, a general impression from the histogram and a pleasing aesthetic on a calibrated monitor is what I'm interested in.

tdodd wrote in post #12183256
I have no intention of doing such a thing.

I'm far more interested in what's going on at the left and right hand sides of the histogram than at the middle and even then I certainly don't pay any attention to what numeric values I might find along the way. Is this information we need, I wonder? I struggle to imagine why we would.

Clipping points, a general impression from the histogram and a pleasing aesthetic on a calibrated monitor is what I'm interested in.

I fully agree with you, and it is a realistic understanding.

The problem comes when some people hear that the 18% gray card referred to as "midpoint". They don't hear the part that this midpoint is only inside the human brain, our logarithmic perception of how we see 18%. It has nothing to do with digital or gamma. It has to do with our eyes viewing reflections from ink on paper.

They also hear that 128 is midpoint of the histogram, not hearing the part about it referring to linear RAW data at the sensor (which we cannot see).

Obviously, since all words Middle must have exactly the same meaning, therefore, they conclude metering a gray card ought to come out at 128 on the histogram that they can see. It does not of course. 1) 18% is not 50%, and 2) the histogram shows gamma data. But some people even recalibrate their light meter so that it does. Fortunately, it is a small error which can be compensated.

WayneF wrote in post #12182786
See message #44 in this thread, regarding the Sekonic website specifications. Sekonic says their K = 12.5

Kodak also says open 1/2 stop from 18%, which is 12%.

Adams disagreed.

WayneF wrote in post #12183590
I fully agree with you, and it is a realistic understanding.

The problem comes when some people hear that the 18% gray card referred to as "midpoint". They don't hear the part that this midpoint is only inside the human brain, our logarithmic perception of how we see 18%. It has nothing to do with digital or gamma. It has to do with our eyes viewing reflections from ink on paper.

They also hear that 128 is midpoint of the histogram, not hearing the part about it referring to linear RAW data at the sensor (which we cannot see).

Obviously, since all words Middle must have exactly the same meaning, therefore, they conclude metering a gray card ought to come out at 128 on the histogram that they can see. It does not of course. 1) 18% is not 50%, and 2) the histogram shows gamma data. But some people even recalibrate their light meter so that it does. Fortunately, it is a small error which can be compensated.

Actually its was tones of gray on photographic paper and in those terms it is relevant.

airfrogusmc wrote in post #12183742
Adams disagreed.

Adams did but he also thought that this "K" factor was something the manufacturers introduced to save us from over exposing and not a value that was supposed to be determined by the manufacturer based on emperical testing.

I'm not saying I wouldn't prefer a reflective meter to work the way Ansel wanted it to but I think he was mistaken in believing the manufacturers were deliberately miscalibrating them as some sort of safety measure.

e-k

E-K wrote in post #12188781
Adams did but he also thought that this "K" factor was something the manufacturers introduced to save us from over exposing and not a value that was supposed to be determined by the manufacturer based on emperical testing.

I'm not saying I wouldn't prefer a reflective meter to work the way Ansel wanted it to but I think he was mistaken in believing the manufacturers were deliberately miscalibrating them as some sort of safety measure.

e-k

Thats why it didn't much matter to Adams because for the zone system to work properly one has to do the tests.