Where does 18% gray originate: A while back there was a discussion where a graph showed that an 18% gray card reflected 50% of the light falling on it. This makes sense, since it is...

wjones8637 Loc: Burleson, TX

A while back there was a discussion where a graph showed that an 18% gray card reflected 50% of the light falling on it. This makes sense, since it is used for setting mid tones. My question is where did the 18% originate? Is this the percentage of a pure black pigment mixed with a pure white pigment?

A curios mind had time to wander.

Thanks,
Bill

a6k Loc: Detroit & Sanibel

No.
A 18% card reflects 18% of the light falling on it. I will leave it for the next responder to answer your actual question.

wjones8637 Loc: Burleson, TX

a6k wrote:

No.
A 18% card reflects 18% of the light falling on it. I will leave it for the next responder to answer your actual question.

The graph in the post indicated that 50% of the light was absorbed/reflected.

Bill_de Loc: US

wjones8637 wrote:

A while back there was a discussion where a graph showed that an 18% gray card reflected 50% of the light falling on it. This makes sense, since it is used for setting mid tones. My question is where did the 18% originate? Is this the percentage of a pure black pigment mixed with a pure white pigment?

A curios mind had time to wander.

Thanks,
Bill

Curious minds should look to Google when their minds start to wander. More info available than at UHH.

https://www.tonycorbell.com/blog/2016/6/14/18-gray-the-middle-value

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wjones8637 Loc: Burleson, TX

Bill_de wrote:

Curious minds should look to Google when their minds start to wander. More info available than at UHH.

https://www.tonycorbell.com/blog/2016/6/14/18-gray-the-middle-value

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Thank you Bill De, I hadn't considered the log or exponential side of things.

Bill

Wallen Loc: Middle Earth

This might help :-) https://www.diyphotography.net/what-is-middle-grey-and-why-does-it-even-matter/

bleirer

The weird thing is 18% is midpoint between paper white and ink black, but photographic middle is more like 12% or so I read. If one were a stickler one would meter a 18% grey card then overexpose a half stop to get back to 12%. I dont think most people could notice the half stop. Or shoot raw and fix it in post. Or meter the 18% and shift to LAB mode, which is supposed to be based on 18%, or just don't worry about it.

Wallen Loc: Middle Earth

bleirer wrote:

The weird thing is 18% is midpoint between paper white and ink black, but photographic middle is more like 12% or so I read. If one were a stickler one would meter a 18% grey card then overexpose a half stop to get back to 12%. I dont think most people could notice the half stop. Or shoot raw and fix it in post. Or meter the 18% and shift to LAB mode, which is supposed to be based on 18%, or just don't worry about it.

The reality is that this is just a guide preferred and universally adapted. It really is just a starting point as it will show brightness "somewhere in the middle" of the available reflected light. This is because bounced light may vary, not only according to how reflective the grey material is, its angle to the source of light etc. but also to subject being photographed. Reflectors on clothes are a prime example of a material that can mess up exposure.

Distance can also be an issue if the tool is used improperly because light falls off to the square of distance.
So it still boils down to the skill/experience of the user and how he/she uses these tools and what they visualize and want to achieve.

Howard5252 Loc: New York / Florida (now)

The 18% grey card represents what all of the colors of a "typical" scene would look like if they were all mixed together. The camera's meter is set using this 18%; it has no idea what it's actually looking at and "Assumes" the scene to be typical. IF the scene is darker or lighter than typical, you have to let the camera know; that is where exposure compensation comes in.
A lot of folks have had this experience ...
You take a picture of a friend on a beach in bright sun wearing a white bathing suit, the bulk of the photo will be grey and your friend will be a silhouette. The camera didn't know the scene was much brighter than "typical", and you didn't tell it.
Here's a trick from the "Old Days". If you are being lit by the same light source as your friend (the sun), meter your open hand and then open up the exposure one stop. Why? The camera will pick a setting that will make your hand grey in the photo, one stop more should make it just about right. This may not work for every person; you can do a test and then keep that result for a time when it could come in handy.

rfreudenheim

It’s the color of the sky in Rochester, NY (home of Kodak) :)

bleirer

Wallen wrote:

The reality is that this is just a guide preferred... (show quote)

Good answer. I was thinking of camera meters, though, and they shoot for some middle value I think less than 18 but do they tell us the exact number? 12.5 is often given. On my camera there is a setting that effectively moves the middle gray, yours too probably.

That inverse square law makes me reflect (ha) though, because reflected light is a scofflaw, doesn't follow the rules, only the source to subject distance does. For example a flash stand set up 10 feet from the subject is 1/4 the light compared to one 5 feet away, I agree. But the photographer is free to move the camera back or in any amount without changing exposure as long as they don't move the flash. Something about the unit area of light on the sensor also changing. In sunlight one can get close to meter and then move back to shoot without compensating exposure.

Howard5252 Loc: New York / Florida (now)

rfreudenheim wrote:

It’s the color of the sky in Rochester, NY (home of Kodak) :)

This is off the topic but an interesting fact ... Kodak was the largest employer in the US of blind people. Think about it.

Peterfiore Loc: Where DR goes south

rfreudenheim wrote:

It’s the color of the sky in Rochester, NY (home of Kodak) :)

North Sky at Noon...exactly

Peterfiore Loc: Where DR goes south

In truth not the color but the Value.

bleirer

So it made me read more, and I think the bottom line is that it came from ink based printers long ago because they used the card as a reference for middle grey when doing halftone printing, being the geometric mean between 90% white paper and 3% black ink. Ansel Adams used the printer cards as a reference because they were there, but he told us to tilt the card halfway to get the reflectance down to 12%. Cameras with meters started telling people to use the cards but increase exposure by 1/2 stop. I just use exposure simulation in the viewfinder and watch the color histogram to make sure I don't push red over the right edge.