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Whenever I adjust my white balance during post processing I try to select a sample of 18% gray somewhere in my picture. In the real world, that color doesn't always exist in every frame that I capture, so I picked up a gray card. I have used one before and found that it is very helpful for color correction, especially with my non-calibrated monitor.

However, I have noticed that many gray cards come with at least a black card and a white card as well. Why is this? In what situation would you color correct with either the white or black cards provided?

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  • \$\begingroup\$ To adjust your white balance, you don't need to find 18% grey in the scene, you just need to find any grey. But I agree, finding a pure grey in a natural scene (particularly when a colour cast comes into consideration) is not straightforward. \$\endgroup\$
    – osullic
    May 11, 2016 at 9:03

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A little history will help you understand the purpose of the gray card:

In the mid 1930's, Messrs Jones and Condit at the Kodak Laboratory determined that statistically, a typical sunlit scene integrated to a reflectance value of about 18%. About this time, the Western Electric Company brought to market the first light meter. Kodak Labs published a recommendation; place a Kodak film box in the scene. Seems the box reflected 18% of the ambient light. Now measure the reflected light from the box top and use this reading to set your exposure.

In 1941, Ansel Adams, a prominent landscape photographer ,and his friend, Fred Archer, a photo magazine editor, jointly published the Zone System which provided photographers with a method to precisely fine-tune exposure. Their zone system revolves around the use of an 18% placard (battleship gray). This card replaces the Kodak box top. The 18% gray target became the de facto standard. Today film and paper speed as well as the digital chip are calibrated, and film and digital ISO is established using the 18% gray card.

Kodak then made and marketed a gray card as an aid to help photographers make exposure determination. Because early light meters sported a selenium cell, they failed to properly read the gray card in dim light. Under these conditions a white card was often substituted. When the white card was used, the ISO was adjusted by dividing it by 5. Such a reading is about the same as one taken from a gray card.

The gray card kit often contained a black placard as well as white. These placards could be placed in the scene and photographed. The resulting negative or slide thus contained references used to make measurements to aid in control development and exposure. In color photography the gray card became the de facto standard for a neutral color balance for color film, color slides and color prints.

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    \$\begingroup\$ The white side of the grey card set is a 90% white. Many spot meters had an IRE scale with a 90% peak white that we used in the days of B&W television. The dark side of the card was for a shadow value that represented 0.3 density over base plus fog. \$\endgroup\$
    – Stan
    May 11, 2016 at 7:42
  • \$\begingroup\$ The white card will be better for white balance, likely a more accurate neutral color. 18% is pretty dark for white balance. White balance cards are white or very light controlled gray. And the 18% card is much older than Kodak or Adams. Old time ink press printers using the first halftones used it to judge their ink flow levels, to compare an 18% dot pattern. But of course, there is absolutely nothing that says a photo scene will be 18%. Some are, some are not. It depends on what you aim at. Todays meters use 12.5%. Kodak always said to open 1/2 stop from the gray card, which is 12%. \$\endgroup\$
    – WayneF
    May 11, 2016 at 20:08
  • \$\begingroup\$ The 18% gray card reads 0.75 density units on a reflection densitometer. An image of the gray card on film is said to be an exact match when it reads 0.75 densities using a transmission densitometer. A print made from this film of the gray card image is said to be an exact match when it reads 0.75 density units on a reflection densitometer. Thus 0.75 is the only tone (shade) that remains the same original – film image – print. Thus 18% = 0.75 density the key density (shade) of the photographic scale. \$\endgroup\$
    – Alan Marcus
    May 11, 2016 at 20:30
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18% Gray cards are not primarily intended for white balance adjustment. They are designed for exposure adjustment, as are the black and white cards that sometimes accompany them. A gray card intended primarily for white balance calibration will usually be around 80% gray, and not 18% gray.

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    \$\begingroup\$ Does this answer the question? Maybe you should expand on why you might need a white card, grey card and black card for metering. \$\endgroup\$
    – osullic
    May 10, 2016 at 19:14
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    \$\begingroup\$ The question does not ask "Why do you need a gray, black, and white card for metering." It asks, based on an incorrect assumption that an 18% gray card is for white balance adjustment, "Why do gray cards come with black and white cards and how do I use them for color correction?" The answer is , "You don't, because they are intended for exposure calibration, not color calibration." \$\endgroup\$
    – Michael C
    May 10, 2016 at 23:50
  • \$\begingroup\$ Yes, but the OP is unsure of what the black and white cards (and indeed the grey card) are for. Your answer would be better if you actually expanded on that. Get to it ;) \$\endgroup\$
    – osullic
    May 11, 2016 at 9:00
  • \$\begingroup\$ Or don't. Just my opinion. \$\endgroup\$
    – osullic
    May 11, 2016 at 9:07
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Some specific aplication for the white part.

If you are in a dark room, probably is easier to take a photo of the white point instead of the gray. But do not over expose it.

The same with the black. You are taking a nice High key photo overexposing some part of the photo, and you want to take a photo on that light situation, you could use the black one.

The white would be used more offen to white balance than the black.

But besides a white balance they can be used as a Tonal Range target. if the three are visible at the same time. You take a shoot with the exposition on the gray. If you see the histogram on your photo, and the 3 tones are in range you actually used the gray as an exposimeter, besides the white balance.

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  • \$\begingroup\$ The reflection densities are as follows: White side = 90% reflectance. The grey is 18% reflectance. The dark grey is 3% – 5%. The grey can be used to convert foot-candles to foot-lamberts. \$\endgroup\$
    – Stan
    May 11, 2016 at 7:44
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I think that I was able to piece together the answer to my question based on the responses I received from Michael Clark and Alan Marcus. If my understanding is correct, the white and black cards are sold with gray cards because the primary purpose of these cards is to assist with metering, not white balance.

As of now I am sampling in the gray card to accurately get my color temperature and tint for color correction and white balance. However, the intended use is to use the gray card for metering purposes. After the picture is taken, the other two cards can be sampled in (for example with the levels tool in Photoshop) to set the proper black and white points in the photo.

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    \$\begingroup\$ Yes. You got the idea :0) \$\endgroup\$
    – Rafael
    May 10, 2016 at 21:45
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    \$\begingroup\$ But, If your photo has a whiter point than the white on the card or black and deeper shadows use thoose. \$\endgroup\$
    – Rafael
    May 10, 2016 at 21:57
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    \$\begingroup\$ The light side is not dead-white. The light side is a 90% reflectance card. When using it, take some care to avoid setting it to specular white. A 90% white should hold some highlight detail. \$\endgroup\$
    – Stan
    May 11, 2016 at 7:50
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    \$\begingroup\$ The grey is 2-1/2 stops between the white and the black. That should help to illustrate the relative position of the reflection tone densities. You could meter on the white and reduce your exposure by 2-1/2 stops. Road asphalt is about a stop different from the grey card. Your hand palm is about a stop different from the grey card. In emergencies, you can meter off what's handy if you figure out the "offset" from the standard objects such as a grey card. : ) \$\endgroup\$
    – Stan
    May 12, 2016 at 18:16

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