Confused about gray cards

I did some research about 18% gray cards. Now I have some questions I couldn't find answers to.

They can be used to set the exposure. Do you have to fill the whole frame with the card to set the exposure? Or can you set it to spot metering and partially fill the frame with the card?

As for setting the white balance, some people say use the 18% gray card and some said use a white card. So which is the correct card to use to set the white balance?

If you have a spot meter, use it. Just make sure that the card is large enough in the scene - it needs to fill the spot.

If you don't have a spot meter, switch to a zoom and fill the frame with the card. Note the exposure. Swap back to the preferred lens and dial in that exposure setting.


Either a gray card or white card can be used to set white balance. It doesn't even have to be an 18% gray card, or a 100% white card. The idea is that the card has to be a perfect shade of gray - no color tint at all. (White is a shade of gray, just a really bright shade!)

It does not matter how you do it as long as the gray card fills the metering area. If useing eval...it senseses the entire view, so you have to fill the entire view with the card, if you are spot metering, you only have to fill the area the camera uses as a spot.

The color can be off for metering reasons, NOT THE SHADE, but the color , it does not have to be neutral. But for white ballence it can be just about any shade of gray from white to all but black, as long as it is a NEUTRAL gray. However, gray cards are most commonly found in 18% or 20% so it can do double duty for metering and color.

It can in fact even be black...

Like Scottes said "Absence of Color" is all you really need.

Mark1 wrote in post #8454671
It does not matter how you do it as long as the gray card fills the metering area. If useing eval...it senseses the entire view, so you have to fill the entire view with the card, if you are spot metering, you only have to fill the area the camera uses as a spot.

The color can be off for metering reasons, NOT THE SHADE, but the color , it does not have to be neutral. But for white ballence it can be just about any shade of gray from white to all but black, as long as it is a NEUTRAL gray. However, gray cards are most commonly found in 18% or 20% so it can do double duty for metering and color.

Few (none?) white balance products are 18% gray!
Few (none?) 18% gray cards are truly color neutral (same value for each color of the R-G-B triplet)

Wilt wrote in post #8454797
Few (none?) white balance products are 18% gray!
Few (none?) 18% gray cards are truly color neutral (same value for each color of the R-G-B triplet)

+1. I thought 18% grey by definition was not neutral, so can it be used for WB?

Wilt wrote in post #8454797
Few (none?) white balance products are 18% gray!
Few (none?) 18% gray cards are truly color neutral (same value for each color of the R-G-B triplet)

From the back of my Kodak Gray Cards packaging:

"The KODAK Gray Card is a consistent neutral test target..."
"The KODAK Gray Card is an invaluable aid for ... optimal color rendering..."

While that doesn't pin-point color neutrality, it certainly does point that way...

AxxisPhoto wrote in post #8454818
+1. I thought 18% grey by definition was not neutral, so can it be used for WB?

By definition? Gray is gray, not blue-gray or reddish-gray. I don't get this.

18% has nothinng to do with being neutral. 18% is halfway from black to white. But the particular color of gray used to make the gray needs to be neutral to be used as a WB. It can be 18% with a green cast, red cast...etc..etc or if the company takes the time they are neutral.

Wilt wrote in post #8454797
Few (none?) white balance products are 18% gray!
Few (none?) 18% gray cards are truly color neutral (same value for each color of the R-G-B triplet)

Very true, but they still get marketed that way.

Don't use a gray card at all for Evaluative. Evaluative is looking for a scene to match with its database. Don't confuse the computer.

There are lots of companies marketing products for white balancing all quibbling about this and that. Marketing, marketing, marketing.

The fact is, the Kodak card is good enough for any light your eye accepts as white. When you get into weird stuff that even your eyes tell you is a crime against nature (like sodium vapor), or then you need a card with more perfect response.

But in those cases, your best option is to shoot raw and take a picture of the card to click-balance in during conversion.

Wilt wrote in post #8454797
Few (none?) 18% gray cards are truly color neutral (same value for each color of the R-G-B triplet)

Hmmm. Couldn't this be tested to confirm? A shot of a gray card taken under a single light source should show whether the same values come up for red, green and blue. Right?

Scottes wrote in post #8455560
Hmmm. Couldn't this be tested to confirm? A shot of a gray card taken under a single light source should show whether the same values come up for red, green and blue. Right?

I doubt you'll find that happening - unless the white balance setting in the camera and the color temperature and tint of the light source are precisely matched AND the card is truly covered with a neutral color.

Yep, you're right. Blonde moment.

You can just take a picture of the gray card and set it as a custom WB right? Or am I thinking of something else?...

jbone wrote in post #8455785
You can just take a picture of the gray card and set it as a custom WB right? Or am I thinking of something else?...

You can do that, providing that the gray card is truly a neutral gray and that it's in the same lighting as the intended subject. There are times, especially when the lighting is coming largely (but not completely) from an on-camera flash unit, that it will be hard to get the card in the same lighting as the subject and still have the card fill the viewfinder enough to be recognized as a custom white balance reference.

I like to put a gray card IN a scene and make a test shot. I shoot in RAW mode. When converting the RAW file to an image file, I click on the gray card with an "eyedropper" tool and that sets the color temperature and tint for the image. Then, I take those numbers and apply them to the rest of the images from the series.