18% Neutral Gray Paint?

[RobC]

yes I erred on using the percentage sign. I should have said 8%

 

[benjiboy]

B& H sell it http://www.bhphotovideo.com/c/product/263737-REG/GTI_N8_G_Standard_Gray_Neutral_8.html

 

[Bill Burk]

B& H sell it http://www.bhphotovideo.com/c/product/263737-REG/GTI_N8_G_Standard_Gray_Neutral_8.html

OK! Now that's a viewing environment standard! Quite different. A wall painted in that color would be quite appropriate.

Not for meter calibration/zone system tests at all.

Careful, I don't think that's 8% though.

 

[RobC]

Careful, I don't think that's 8% though.

Yeah well everyone says a meter is calibrated to 18% middle grey but the actual adjustment from spot meter reading is 8% (K=12.5)

so nowhere near 18% reflectance until you try and and start doing some maths to see what 8% of reading equates to and even then its not exact. Meters don't work how people think they do. But we've been here before so no point in repeating it in detail as its really not important in everyday photography. The meter manufactures and film manufacturers have factored all into their instructions so we don't need to think about it too deeply.

 

[Bill Burk]

Yeah well everyone says a meter is calibrated to 18% middle grey but the actual adjustment from spot meter reading is 8% (K=12.5)

so nowhere near 18% reflectance until you try and and start doing some maths to see what 8% of reading equates to and even then its not exact. Meters don't work how people think they do. But we've been here before so no point in repeating it in detail as its really not important in everyday photography. The meter manufactures and film manufacturers have factored all into their instructions so we don't need to think about it too deeply.

Let's take this guy's website as a guide for the discussion... I have a 12.7% chip that reads "same as" incident and this website suggests approximately 12%. I don't know any 8% but the paint is a Munsell number 8

12 or 18

 

[RobC]

Let's take this guy's website as a guide for the discussion... I have a 12.7% chip that reads "same as" incident and this website suggests approximately 12%. I don't know any 8% but the paint is a Munsell number 8

12 or 18

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.

If you divide anything by 12.5 (the k factor) the answer is 1/12.5 th of whatever you are dividing into. 1/12.5 = (is the same as) 8% of whatever you are dividng into.

That link you gave has made the same mistakes in using percentage when talking about the K factor and there is mostly total bollocks.

8% of a light reading is approx 3 1/2 stops less than the light reading whatever it is.

The meter calculation does NOT ever consider the reflectance being 18% or 12%. It does its calculation on the assumption that the average subject is 7 1/3 stops in range and calculates the exposure to capture that 7 1/3 stop range on film. Grey cards or 18% never come into it. That is taken care of in the film speed (approximately) and not in the meter except that a meter uses the entered film speed to adjust for films sensitivity.

Thats how I see it for a spot meter. Meters being calibrated to any reflection percentage is pure bollocks.

 

[Bill Burk]

OK I see where you arrive at 8% in discussion. But I don't think you are doing the right math with K.

I'm not saying I know what the math is yet, I have unfinished business.

But here is why you can't just "divide by K" even though the formula has numbers over K...

K = 1.16 footcandles says the same as K = 12.5 candles per square meter... and something divided by 1.16 is about 86% of what you started with.

So we can't take the number and divide as-is without considering the corresponding labels.

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.

 

[RobC]

OK I see where you arrive at 8% in discussion. But I don't think you are doing the right math with K.

I'm not saying I know what the math is yet, I have unfinished business.

But here is why you can't just "divide by K" even though the formula has numbers over K...

K = 1.16 footcandles says the same as K = 12.5 candles per square meter... and something divided by 1.16 is about 86% of what you started with.

So we can't take the number and divide as-is without considering the corresponding labels.

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.

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.

Yes the diffference between 12.5 and 14 and indeed 18 K factor is quite small if they were percentages, but they're not, they are factors and that makes a lot more difference, over a full stop between 12.5 and 18 I think. Not worth worrying about unless you possibly you have an old meter actually using 18. I think nearly all modern meter will be using 12.5 or close.

The maths is the standard formula:

2^EV= (B*S)/K

B= luminance in cd/m2
S=film speed
K = K Factor

The film speed adjusts for sensitivity.

If the meter sensor is factory set to something we don't know about and is missing from the formula then all bets are off. But I assume that the meter hardware/software is able to convert to a correct value for B. Who knows how it actually does that and what calcs are harcoded into it. I certainly don't. The only way a meter could be calibrated to 18% grey is in that conversion internally. However, I don't think its necessary because the film speed has it buried in its own calculation using the ISO test. But not to a specific value of 18%. It will be a ball park middle grey because a) film curves are variable and there is no set requirement for a middle grey negative density in the ISO test and b) negatives don't have reflectance, they have opacity which is something completely different. and c) the only thing that could be dependant on 18% is the value for B and the formula simply doesn't contain anything for that.

Note:
I edited above a tad.

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"

 

[RobC]

Also it may be the case that old meters used different standard formula and therefore different K factor than modern meters. If thats the case you need to find out what that formula was and the K factor it used.

 

[AgX]

Now I wonder why one would need a wall-sized 18%-grey surface.

Am still curious...

 

[Mustafa Umut Sarac]

Am still curious...

My mother always wanted grey walls , we were poor and bought cheapest largest canister of white paint and dropped inside black paint and it turned not grey but beige. Our home now looks like white house presidents work room , böööögggghhhh.

 

[BrianShaw]

We did it for a laboratory. Were doing goniophotometric analysis of rocks. The wall color was for "control" but mostly was to give the lab a professional look for when the sponsors toured.

 

[M Carter]

Back to the paint discussion - I needed a green screen set and didn't want to get raped by buying a can of Roscoe paint. I just took a green screen to Home Depot - they scanned it and mixed the paint. It came out perfect. The technology is very accurate these days.

 

[Mustafa Umut Sarac]

When working with drum scanner , the first thing they teached , c m y would be stronger to weaker format to get the perfect white. c 7 m 5 y 4 or alike.

When Brian told why grey wall been used at lab and that was for perfect white at rock color analysis , I remembered why belgium diamond cutters and traders prefered north window to value a diamond piece ? Because they tell the purest neutral color comes from northern daylight.

Same north daylight advice , comes from watch makers also , they advise to place the table in front of northern daylight.

 

[BrianShaw]

When working with drum scanner , the first thing they teached , c m y would be stronger to weaker format to get the perfect white. c 7 m 5 y 4 or alike.

When Brian told why grey wall been used at lab and that was for perfect white at rock color analysis , I remembered why belgium diamond cutters and traders prefered north window to value a diamond piece ? Because they tell the purest neutral color comes from northern daylight.

Same north daylight advice , comes from watch makers also , they advise to place the table in front of northern daylight.

"Perfect white" calibration on the photometer was done with a calcium carbonate standard.

 

[DREW WILEY]

I'm not even going to read the previous threads. I have batched up a true 18% gray paint. First of all, I had access to a true continuous tone industrial spectrophotometer. It took me a week to make a quart of the stuff, and I was at the time a professional color consultant and color matcher. You can't just walk into a paint store and expect anything resembling a true match of anything. It takes a lot of training,
and the machines can only get you into a vicinity. All the fine-tuning has to be done by eye. The formula cannot be reproduced. I got results within 1% over the entire visual spectrum, and the equivalent of about a full stop into both nonvisible UV and IR too. No commercially printed gray card came even close to it alleged midtone accuracy; and none was color neutral either. The gray patches on an unfaded Macbeath Color Checker Chart were quite good, however. Why did I undertake this harebrained project? Just for the heck of it. Calcium carbonate is the least stable white pigment out there. Technically, it's terms an "adulterant" rather than a colorant. Early spectrophotometers were lined with pure barium sulfate paint. You can still get it for around $200 per half-pint. But all modern spectrophotometers are calibrated to fadeproof baked white ceramic tiles - one very specific tile per machine.

 

[snikulin]

Am still curious...

The idea is to paint all the walls in a small room (SO's approval still pending), install gray-painted table and start training myself basics of composition and Zone System.
Like painters learn: a white ball, an egg, a white cube, a white cylinder.
Gray surrounding eliminates color casts and makes metering easier.

 

[RobC]

The idea is to paint all the walls in a small room (SO's approval still pending), install gray-painted table and start training myself basics of composition and Zone System.
Like painters learn: a white ball, an egg, a white cube, a white cylinder.
Gray surrounding eliminates color casts and makes metering easier.

there's no hope for you.

 

[snikulin]

there's no hope for you.

Hey, a man can dream!

 

[DREW WILEY]

Learning the Zone System is either a LOT more complicated than that; or a LOT easier. I prefer the latter route.

 

[BrianShaw]

"Perfect white" calibration on the photometer was done with a calcium carbonate standard.

Drew is correct that I was wrong. Barium sulfate disk is what my mind should have remembered. Sintered PTFE is also used apparently. I don't know about ceramic though.

Not that this really has much to do with grey or gray paint...

 

[Mr Bill]

I think that if you painted a room grey, it might not stay that way too long. It seems like a good idea, but in a large lab where I worked, we once turned a room grey for the same basic reasons you bring up. The result: none of the color correctors wanted to work there. It seemed dreary, and without a more normal scene to look at, your perception tends to get skewed. We got rid of our "neutral" room.

Something you might consider is to just pick up a few sheets of grey matt board at an art store, then make a small booth on your table. You don't want to use this as a critical grey reference, or anything like that, but it might serve some of your purposes in a cheap, easy way.

 

[removed account4]

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!

do paint stores near you have
the color analysers ?
the ones near me ( ace/true value, home despot, independents, and lowers ) i just need
to bring something in the size of a quarter and it matches the color .
a grey-card is much bigger than a quarter,
you shouldn't have a problem ..

have fun painting !

 

[Bill Burk]

... But all modern spectrophotometers are calibrated to fadeproof baked white ceramic tiles - one very specific tile per machine.

Tiles! Of course...

snikulin... you could be looking for gray tiles!

 

[Bill Burk]

Like painters learn: a white ball, an egg, a white cube, a white cylinder.

This part sounds like it will be time well spent.

18% would be useful for learning to see shades of gray, considering it's about halfway between black and white

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