18% Neutral Gray Paint?

snikulin · Oct 7, 2015

Bill Burk said:
12% would be easier to meter, considering it's about what the meter is calibrated to.

It's Yashica 12 '1967, about 48 years old.
I doubt its meter was calibrated seriously during production to begin with and now the needle spring is sure off.
Also the current air-zinc battery is not a precise substitute for an original mercury one.
Nevertheless the meter does match my Canon 6D + 50mm/1.4 lens in averaging mode.

RobC · Oct 7, 2015

Bill Burk said:
12% would be easier to meter, considering it's about what the meter is calibrated to.

meters are not calibrated to a reflectance value. They are calibrated to roughly the midpoint of the log exposure range of the film speed. That means they are are calibrated to a Lux value. You should be able to work that out by knowing that an EV value has a corresponding lux value and then calculating back using the standard meter formula to check if thats what is happening. The midpoint lux value can be read from the ISO film speed diagram in wikipedia. Get your calculator out.

DREW WILEY · Oct 7, 2015

If you know anything about paint stores, you're lucky to find anyone sober in one. And no, the kind of color analyzers they use won't be able
to make a true 18% gray paint, if they even have a clue what you are talking about (they won't). At one time Benjamin Moore published the
reflectance values of their color chips. But printed samples were made with inks, not paint itself, so are never precisely representative of
what will be in a can. And you'd want a matte finish (flat - nonreflective, yet washable). But this is getting silly anyway. I'd just put a big
gray card or Kodak gray scale chart in the scene. The Zone System is all relative anyway. God didn't create the world in eight zones to begin with. The Zone System just an artificial shorthand tool. Take from it what you need, tweak it for your kind of subject matter, and be done with it. It's like playing chord on a piano. Once you actually learn how to play, you forget all that beginner stuff and start making real
music instead.

BrianShaw · Oct 7, 2015

How exact do you think it needs to be to achieve the goal the OP stated in post #43?

DREW WILEY · Oct 7, 2015

That's the whole point. One could merely go out and buy a sheet of medium gray matboard. Pick out a relatively neutral gray paint chip and
visually compare it to a small gray card. Whatever. The problem with too much gray around you is that it fatigues the eye. It's a chameleon
that becomes just the opposite of any strong color around it. "Simultaneous contrast" is the physiological and artistic term for it. That's why color pros use matching booths instead of whole rooms of the damn stuff. What on earth does this have to do with basic black and white technique? Well, once one starts getting a tad proficient at printing (necessary to any real progress in the Zone System), they'll inevitably want to fiddle around toning for permanence, which inherently introduces toning relative to hue. Try to get good at that without some eye
fatigue. Time to repaint. But at least gray paint is better than gray paint with Damien Hirst polka dots all over the wall.

snikulin · Oct 7, 2015

Drew, I looked all the stuff you refer too and I think you are right and the whole gray room might be not a healthy idea.
Apparently the problem is in our wetware.
Well, it's back to my old set of Kodak cards (I have the "pearl necklace" one).

Thanks!

Ken Nadvornick · Oct 7, 2015

BrianShaw said:
How exact do you think it needs to be to achieve the goal the OP stated in post #43?

Good point, regardless of the direction the OP chooses to go.

I suspect the color analyzer at the home improvement store would be just fine, if not overkill. I know it worked OK for me. My testing meter read the same. And if the spectral response was not exquisitely identical, it certainly fell within the overall error spread for the entire system I was working with. Meaning, it would not have been an outlier data point, and would also probably have been mitigated elsewhere by a different opposing variable.

I've posted before concerning the desirability and wisdom of finding the sweet spot in systems. That point being at least one step back from the beginning of diminishing returns.

There is a reason they don't chalk a baseball batters box using laser beams.

Ken

Bill Burk · Oct 7, 2015

snikulin,

I think since you are planning to learn composition with this, using the painted wall as a neutral background for portrait and still life arrangements... The gray you choose is important if you are in tight quarters... which it sounds like you are.

Here's advice from William Mortensen in Pictorial Lighting, Chapter 3, Equipment, The Background...

"The background should be no smaller than ten feet wide and eight feet high. (For general use. For portrait heads only it may be somewhat smaller.)"

"It should be either matt-white or matt-gray, depending on the size of the shooting space. If there is room to pull the camera back thirty feet from the background, make your background white."

"Eighteen by ten, with the background across the end, is the smallest feasible space."

"The side walls and ceiling, at least immediately adjacent to the background, should be light in color; tan or gray rather than white. If possible, the floor also should be white or light grey."

Bill Burk · Oct 7, 2015

A couple things I didn't mean to sound like I was saying (when I said the gray you choose is important)... I don't think the shade of gray must be a particular percent. I think 18% is fine. I don't think you need it to be neutral across the entire visible spectrum (Unless you can. The paint DREW WILEY made would be wonderful but is the formula available?)

You're just learning, so a little non-neutrality and inaccuracy of shade can be tolerated. I learned a lot with my two-tone painted Zone Board and it's neither neutral nor accurate (at least not now 8 years later - it started out 16% and 36% but I doubt it's still those shades)

I meant to say it's important in a straightforward sense, whether the wall is white or gray... Since you might not have thirty feet in your small room to back up (for those occasions when you want a dark background)... Because it will be easier to work with a middle gray because you will only have to drop its value by a couple f/stops to make it black, and you will only have to raise its value by a couple f/stops to make it white.

For a meter target, I'd take advantage of the recent Internet lore that meters see 12% and that's a better target, closer to what the meter reads than 18%. But don't paint the background 12%... Make a small portable target or gray card that's 12% and use that at the subject position to take a meter reading (12% is better to help you learn to verify your understanding of metering, 18% will just give you readings that don't agree with any other meter readings you might take).

Bill Burk · Oct 8, 2015

RobC said:
meters are not calibrated to a reflectance value. They are calibrated to roughly the midpoint of the log exposure range of the film speed. That means they are are calibrated to a Lux value. You should be able to work that out by knowing that an EV value has a corresponding lux value and then calculating back using the standard meter formula to check if thats what is happening. The midpoint lux value can be read from the ISO film speed diagram in wikipedia. Get your calculator out.

I don't mean that the meter is calibrated to 12% reflectance.

I mean that 12% reflectance is a better test target than 18% when you want to take meter readings off a test target. I verified this with a light meter that has both spot and incident modes and a test target with several target patches 1/6 stop apart surrounding 18% gray. The 12.7% target patch reading in spot mode closely matches the reading in incident mode.

My test doesn't prove that 12.7% is the theoretical correct percent, it just gives me a patch that's pretty close to the right percent to cause the meter to agree from one mode to the other. There's a 14.3% patch that's "too light" for the two modes to agree.

I'm using this as a sanity check on any math that I'm playing with, because any result far off of 12% needs to be explained.

Bill Burk · Oct 8, 2015

I followed the discussions behind wikipedia and found Jeff Conrad was behind some of the edits.

He wrote an article on the homepage of LFPhoto.info that covers the idea of 18% gray and K and C.

Jeff Conrad's article about exposure metering

While yesterday I claimed that 12% is close... Now I found a formula that shows the implied relationship between incident light and reflected light.

Reflectance = ( K x pi ) / C

It's interesting that it works out near 18% when you use C for the flat disc on an incident meter and close to 12% when you use C for the hemisphere disc.

So maybe it boils down to 18% being correct for flat copy flatly illuminated. And 12% is correct for 3-D subjects.

(Sekonic L-758DR Lumisphere C = 340, Flat diffuser C = 250 and K = 12.5)

(12.5 x 3.14) / 340 = 11.5% and (12.5 x 3.14) / 250 = 15.7%

Very interesting...

RobC · Oct 8, 2015

read following carefully

https://en.wikipedia.org/wiki/Exposure_value

Bill Burk · Oct 8, 2015

RobC said:
read following carefully

https://en.wikipedia.org/wiki/Exposure_value

This is the wikipedia article I was reading, says about the same

https://en.wikipedia.org/wiki/Light_meter

Bill Burk · Oct 8, 2015

RobC said:
"K = 1.16 footcandles says the same as K = 12.5 candles per square meter" is nonsense. The K Factor is a factor. It is not a value of cd/m^2 or foot candles. I have no idea where you get 1.16 from but if you have converted from 12.5 then you are in error since 12.5 is not a value of cd/m^2. It is a reduction "factor"

I figured out what's going on here... they are numeric factors without units, but... K is 1.16 when the units of luminance in the equation are footcandles, and K is 12.5 when the units are candles per square meter.

RobC · Oct 9, 2015

Bill Burk said:
I figured out what's going on here... they are numeric factors without units, but... K is 1.16 when the units of luminance in the equation are footcandles, and K is 12.5 when the units are candles per square meter.

No, K is whatever the meter manufacturer says it is and you can't change that. It is a "CONSTANT"

I think you and maths are mutually exclusive.

Which meter are you using and when was it made?

DREW WILEY · Oct 9, 2015

That's why I've stuck with either Pentax or Minolta spotmeters. They're apparently calibrated to exactly the same standard.

RobC · Oct 9, 2015

Michael R 1974 said:
This is how it is laid out in Richard Henry's book. I can't say whether or not he was correct, except that he based the meter calibration section of his book on correspondence with H. Kondo, who was THE guy in Japan at the time on their meter standards and testing committee etc.

The Minolta and Pentax meters at the time seem to have used K=14. Apparently as of the mid 1980s, new Japanese meter models were supposed to switch to 1.16 (12.5) to come in at the most recent recommended ANSI standard. I don't know when/if that change occurred. I bought my Minolta Spotmeter F in the early 1990s, and the manual seemed to have had its last revision in 1986, but K was still 14 for that meter.

Without knowing which formula the meter is using for its calculations its impossible to say what the K factor for it is unless the K factor for the meter was published in its specs.

I saw one older formula which said K= 3.66 ( If memory serves me correctly) but it was different formula from todays formula for spot meters.

Stephen Benskin · Oct 9, 2015

Bill Burk said:
When done I expect to might find that a K of 12.5 versus a K of 14 are less than one-third stop apart.

Bill, this might help.

Stephen Benskin · Oct 9, 2015

RobC said:
Yes you need to convert to the correct units before doing the maths otherwise you get wrong answer. B in the standard formula (see below) is in cd/m^2 so you can't divide by 1.16.

Also "K = 1.16 footcandles says the same as K = 12.5 candles per square meter" is nonsense. The K Factor is a factor. It is not a value of cd/m^2 or foot candles. I have no idea where you get 1.16 from but if you have converted from 12.5 then you are in error since 12.5 is not a value of cd/m^2. It is a reduction "factor"

And here from ANSI PH3.49-1971

I saw one older formula which said K= 3.66 ( If memory serves me correctly) but it was different formula from todays formula for spot meters.

K = 3.33 footlamberts (fL)

You can also find 3.33 in the Units and Values of Exposure Parameters table above.

There is 100% confusion about the K factor being a percentage when infact it is a FACTOR. Even though I know this I still make the same error that everyone else does and and put in the % sign when I shouldn't as I just did in post above.

The factor is used to divide the meter reading value which is in cd/m^2 units to approximate the middle of the 7 1/3 stop average SBR range.

It is a factor, but it can be thought of as a light loss factor. The meter needs to take into consideration the optical system of the camera. The K equation (see above) can be broken down into three parts, the optical system (light loss through the optics), the exposure constant, and variables related to the meter. The light loss portion is the constant "q" in the exposure equation. The relationship between the three constants K, P (exposure constant) an q can be expressed as P / q = K.

Tommy Isaacson · Dec 20, 2017

I would recommend reading this post before painting any walls in the studio. I explained all colors. Read it till the end and at the very bottom you will see that I actually changed my mind and explained why. Here is the link https://www.isa-aydin.com/articles/photographic-studio-what-color-to-paint-the-walls/

wiltw · Dec 21, 2017

benjiboy said:
18% grey is the mid point between deep black and pure white, so if you mix equal quantity of each paint together you shouldn't be too far off.

I once made a checkerboard pattern of alternating black squares and white spaces, with an even number of each on a page. Printed it on my printer on photo paper and I took a reflected light reading of that and it DID NOT MATCH a reflected light reading from an 18% tonality card in the same light.

RalphLambrecht · Dec 21, 2017

snikulin said:
I need a really big (say, a wall in a small room) 18% neutral gray "card".
I wonder how precise are common paint stores like Kelly Moore and paint departments at Home Depot, Ace and Lowe's?
Will they be able to measure my standard Kodak 8x10 card and produce good enough neutral 18% gray paint mix?
What to go for, acrylic, oil?

Thanks!

I used some flat white and black paint, and painted small patches with a 50/50 mix then added black in 10% increments and measured the samples with a densitometer. after a bit trial and error, I had the perfect ratio of black-and white paint to make an 18% gray.took a bit of effort and patience but worked in the end.of course you have to do that again using different paints.

chriscrawfordphoto · Dec 21, 2017

wiltw said:
I once made a checkerboard pattern of alternating black squares and white spaces, with an even number of each on a page. Printed it on my printer on photo paper and I took a reflected light reading of that and it DID NOT MATCH a reflected light reading from an 18% tonality card in the same light.

That's because a checkerboard is not 18% reflectance; its 50%. For 18% reflectance, you'd need 100 squares, with only 18 of them white!

macfred · Dec 21, 2017

chriscrawfordphoto said:
That's because a checkerboard is not 18% reflectance; its 50%. For 18% reflectance, you'd need 100 squares, with only 18 of them white!

Interesting - I would have thought it's just the other way round : 100 squares with 18 of them black ...

sepiareverb · Dec 21, 2017

macfred said:
Interesting - I would have thought it's just the other way round : 100 squares with 18 of them black ...

Not when making negatives.

18% Neutral Gray Paint?

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