Do You Use the Gray Card for the Reason It Is Meant For?

Do You Use the Gray Card for the Reason It Is Meant For?

Do you have a gray card? A piece of foldable card that has a gray tone which is called 18% gray? If you do, there is a chance you have used it to set the white balance. But that is not where the 18% gray card is meant for.

A gray card is also called 18% gray. I have one in my bag, a foldable card from Lastolite. If white balance became critical, I used this card to have a reference point. I remember I tried to use it with the Lee Big Stopper also, to get rid of the infamous blue color cast. For that I took a second image with the card in view, so I could correct the color cast with a single click of a button in Lightroom.

However, did you know the 18% gray card is not meant for white balance? In reality it is a calibration value for the built-in light meter.

The Lastolite foldable 18% Gray Card.

The Lastolite foldable 18% Gray Card.

The Real 18% Gray

In reality 18% gray is a gray tone that reflects 18% of the light that falls on it. When you look at a gray scale, this gray tone is located exactly in the middle between black and white. From that point of view, this gray tone should rather be called 50% black.

Confused? You shouldn’t be. It becomes very clear if you look at the gray scale I made for you. The gray tone that reflects 18% of the light that falls on it, is exactly in the middle.

The Reference for a Light Meter Inside Your Camera

The light meter that is built into your camera is called a reflection meter. This meter will measure the amount of light that is reflected from your subject. It is not measuring the amount of light that is present. This is important to realize, because it tells us why the light meter will give a wrong value in some cases.

The reflection meter inside the camera is calibrated for 18% light reflection. It looks at the amount of light that is reflected from our subject, calculates the average and expect this to be 18%. In most cases this is very close to the reality, but not always.

What If the Reality Isn’t 18%  

Not every object will reflect 18% of the light. Dark objects will reflect less light, while bright objects reflect more than 18% of the light. If you have mainly one of these objects in the frame, the average amount of light will deviate, and the light meter will give the wrong value.

If you have ever photographed in the snow, you know how wrong a light meter can read the scenery. Although the snow reflects a lot more light than 18%, the light meter will assume it is 18% and set the exposure accordingly. The result is an image which is one or two stops too dark.

An example of a show covered landscape during daylight. The light meter made 18% gray from the white snow. Look at the histogram; it is situated in the middle.

An example of a show covered landscape during daylight. The light meter made 18% gray from the white snow. Look at the histogram; it is situated in the middle.

This also happens when a lot of dark objects are in the frame. In that case the light meter will make the exposure too long. If you would look at both situations, the light meter tries to equalize the exposure, making it as close as possible to 18% gray.

A Test to See How the Light Meter Works

In normal situations we don’t think of it too much. We compensate the exposure according to the first photo, or the histogram that is visible on the LCD screen. It might even be difficult to see the effect of a lot of dark or light subjects, because the subject is almost never complete white, or completely dark.

Let’s do a test: photograph a white sheet and a dark sheet, with aperture priority in a controlled light environment. We’ll keep aperture at f/11 with ISO 200 and let the shutter speed be determined by the light meter. The results might surprise you.

Black becomes gray if you leave it up to the bluit-in light meter. The shutter speed indicates it is a dark surface if you compare it to the next image.

Black becomes gray if you leave it up to the bluit-in light meter. The shutter speed indicates it is a dark surface if you compare it to the next image.

The photo of the white sheet also becomes gray. It is exactly the same tone as the black sheet. Just look at the shutter speed, and compare it to the previous image for reference.

The photo of the white sheet also becomes gray. It is exactly the same tone as the black sheet. Just look at the shutter speed, and compare it to the previous image for reference.

When you look at the results, both images look very similar. The shutter speed is the only indication which photo is from the white sheet, and which is from the black sheet. Take a look at the histogram, and you see how the pixels are in the middle of the histogram, the location of the 18% gray tone.

How to Correct the Light Meter From a Wrong Exposure

The reason why this experiment has this result, is because of the light meter assumes it is 18% reflected light. In reality it is quite different. Black reflects much less light. White reflects a lot. How can we tell the difference if the image shows only gray?

If we use a subject that is reflecting exactly 18% of the light, the light meter will measure a correct exposure value. And yes, for that we must use the 18% gray card. If we have determined the correct exposure, we can use it for every photo we take, as long as the light remains exactly the same.

When I use a 18% Gray Card the light meter measures the correct amount of reflected light. With this shutter speed I the white and black sheets will look... white and black.

When I use a 18% Gray Card the light meter measures the correct amount of reflected light. With this shutter speed I the white and black sheets will look... white and black.

With the shutter speed measured on the gray card, the photo of the black sheet will be exposed correctly, not influenced by the dark tones of the sheet.

With the shutter speed measured on the gray card, the photo of the black sheet will be exposed correctly, not influenced by the dark tones of the sheet.

The photo of the white sheet will also be exposed correctly, not influenced by the bright tones of the sheet.

The photo of the white sheet will also be exposed correctly, not influenced by the bright tones of the sheet.

The Real Reason to Use a Gray Card

As you can see, the 18% gray card is meant to calibrate the light meter inside your camera. As a matter of fact, it is a sort of substitute for a hand held light meter, that measures the light that falls on the subject itself.

If you use the gray card for calibrating the white balance, just continue to do so. It works in most situations. But if you want to do it correctly, use a card that is made for white balance calibration. Often it is a white piece of card or, in my case, the back of the Lastolite foldable gray card. You can also use an Expo disk for that.

Just remember, the 18% gray card is not for white balance, but for a calibrated measurement of the exact amount of light.

Did you know the 18% gray card was not meant for white balance calibration? Or have you never used something to calibrate light or white balance? Please leave your experience in a comment below.

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39 Comments

Mike Shwarts's picture

Those of us taking photos before the digital age knew that. Well....some of us knew that. While working at a camera store, we had a customer who photographed his fly fishing flies. Exposures were off and we explained how he should use a gray card. However he would meter with the card, remove the card and meter again. Then he complain his exposures were all over the map. I know he complained, because I was one of the two guys running our lab. :D

Nando Harmsen's picture

haha
Great story.
Thanks for sharing

Kirk Darling's picture

Well, first, some gray cards (including the age-old Kodak gray card, which was intended for light balancing from the start), are indeed intended for color balancing from the start. It was intended to be placed into the scene for a densitometer reading from the color negative (which would provide the same information as the eyedropper in Photoshop. A good gray card will use coloring formulated to reflect a clean neutral gray even from many light sources that are not full-spectrum specifically so that they can provide accurate color balance under the greatest variety of light sources.

Second, camera light meters are not calibrated for 18% gray. They're calibrated against an actual light source which has no reflectance of any value, as it is an emitter. And the resulting value isn't necessarily a gray value with 18% reflectance. In a great many cameras and meters, the shade produced at the center of their measurement is 14% gray. Back in the film days, it was perennially in hot dispute whether 14% or 18% was really the right shade of gray card to use.

That's true of Canon cameras, for instance. A "correct" exposure reading of an 18% gray card by a Canon camera will spike the histogram not in the center but about half a stop to the left. And the camera isn't wrong, that's what it's designed to do.

Nando Harmsen's picture

Thanks for the explaination.
Nowadays there is little reason to use a gray card, I think.
But nevertheless, I think some can stil use it like the way I descibed it. But do you say it differs from other camera' brands?
It would be nice to test it.

Jeff Marcus's picture

It’s a bit pointless to debate the percentage of average. Find a subject that has an average reflectance (gray card, red brick wall, medium blue sky, healthy green grass, etc.) and zero out the spot meter on that tone.

Nando Harmsen's picture

I don't find a spot meter very convenient to use. Move an inch to the left of right, and you measure on a completely different area.
On top of that, I think most of us cannot see beyond colors to determine if it is medium gray.

Jeff Marcus's picture

Nando, I’m really surprised to read that from you. I think 30 minutes of reading your histogram while shooting different colors would provide all the training one needs to understand how your internal light meter reads colors. Also, I firmly believe that in tricky lighting conditions where there’s extreme contrasts, getting the exposure correct on the subject is achieved with spot metering. How many photographers have had the gray snow or that white blob of a moon because the internal meter incorrectly read the scene. Spot metering is the only accurate meter to specifically measure the subject correctly. I’ve never experienced an issue with being an inch off. It measures approximately 3% of scene. That’s a small but easily attained target. Match your exposure to the meter and you have a perfect exposure every time.

Nando Harmsen's picture

It would need a bit of research to have an idea about the luminosity of different colors. But I menat, a lot of photographers cannot see through the color itself.
Concerning spotmeter, it is possible to measure on a certain spot, but you need time to do this accurately. I don't want to take that time, or I don't have that time available. I always use an average metering and I can read the light situation. I know when to compensate the measurement of the light meter. It works much faster this way. Well, I find it much easier and faster. Others perhaps not.

Michael Purcell's picture

I've used a gray card for both metering and color reference since the days of C-22 developer. either a reference exposure or included within the frame but outside the composition, put the color analyzer on the card image and adjust the filter pack, shift to density and set the time. 9 out of ten prints are on the nose. For digital images I find the gray has a slight red component and corrects to a cyan shift.

Kirk Darling's picture

Which is how I still use it to balance a RAW image.

mike d's picture

My experience is with Nikon DSLRs. The manual states for setting a preset custom white balance, to use either a neutral gray OR white target. I've tested both over the years with different models and always found the gray card to be the more accurate/pleasing color balance. I also find a gray target easier to tweak in Lightroom....it is easy to find white and black in an image, not as easy to find neutral gray.

hey ab's picture

In college if I didn't have the gray card shot within the rest of my dngs I'd get an F. The instructor was terrifying.

Nando Harmsen's picture

White is not for exposure, but for white balance.

Amilcar de Oliveira's picture

Waaaay back in the 1970's, Popular Photography's writer, Cora Wright Kennedy, said that you could match a 18% card reading by measuring the open palm of your hand and opening 1 f/stop from this reading. She mentioned that whatever the color of the photographer, the palm's reflectance was practically the same and concluded the article saying that there is equality in open hands.

Salvador Avila's picture

Well , if you want to get the perfect color , ... you better use a color Checker

Ted Mercede's picture

Well, not sure if anything is "perfect", but it sure goes a long way tobgetting there..

Nando Harmsen's picture

I know people who use the color checker to get everything perfect, and then they add their precious Ligthroom preset that changes everyhting
lol

Timothy Gasper's picture

I always use an 8x10 grey card when I want to do very precise metering. The meter in cameras measure reflected light. More acurate readings come from incident meters, light falling on the subject. Long ago there were debates as to whether the actual grey scale value was 18% or 12%. I didn't and still won't fall into that debate. Ever since I've used the 18% card, I have always had great results so I don't want to experiment. Another trick I've learned was that if you didn't have a grey card with you, you could meter an asphalt road nearby, if there was one. I did a few times and it was pretty damn good. Anyway, thank you for the article. Oh, I still always use my Gossen meter or Pentax Spot V meter.

george andrews's picture

Maybe a dumb question, but why isn't the histogram touching the right side if the sheet were truly white?

Kirk Darling's picture

The reason might be that the sheet should not be reproduced as purely white in this instance of a correct exposure.

That's the inherent fallacy of the "3-tone" exposure cards. A white object in shadow should not be reproduced as white, but some shade of light gray. A black object in very bright light should not be reproduced as black, but as some tone of dark gray. So rarely, should the histogram of a black, white, and gray 3-tone card (which presents a single surface facing one direction) produce spikes perfectly at each extreme side and center of a histogram.

Nando Harmsen's picture

Not a dumb question at all.
It is difficult to have perfect white. Just as perfect black is very difficult to achieve

Danny Colombo's picture

Back in the Cretaceous Period, I used a plain Nikon F, often metering off the card with a Gossen Luna Pro. I learned it as a ‘middle gray’ card. While not particularly convenient, I learned photography much quicker than today.

Nando Harmsen's picture

I agree. Back then it was more simple. Then we had just one single film sensitivity, and a variable aperture and exposure. Today everything is variable,... and we think there is more than there is in reality
(did I say that in the correct way? hmmm)

Danny Colombo's picture

Understood. I try to say it in a way so as not to offend. I’ve seen articles speculating as to whether it’s easier or harder to teach photography in the digital age. I say both. I volunteered for teaching local scout troops.
Instructing use of a camera itself was way easier than today. The benefit today is chimping exposures for immediate feedback and near endless shots. Previously the cost of film plus the wait involved before seeing results taught some discipline. I miss black and white darkroom work a bit, it was magic. However, using a computer with a modern printer is better for color. I can recall hours in the school lab trying to get Cibachrome right.

Nando Harmsen's picture

There are always pros and cons to think of, when comparing these things. I love digital photography for a lot of reasons. I also believe it would be better if todays photography was a bit like the old photography because you had to learn to see and understand before taking that shot.
I think we agree on these things completely

Alnoor Meralli's picture

I still have my Kodak Grey Card. I still use it to verify auto exposures in-camera for ambient light metering and for speedlight output measurement. This was developed in the era of film; you had your choice of daylight balanced film or tungsten film. Nothing in the instructions suggest color neutrality. It is meant for mid-tone exposure, at the time, used with a reflective light meter. For color temperature, I rely on my WhiBal card which is meant for that.

Kirk Darling's picture

"Color neutrality" as we determine it today wasn't necessary back then. The Kodak gray card was, indeed, heavily used to determine color balance in color printing, either by eye or by densitometer.

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