How to use
a Gray Card
The Gray Card is a process control benchmark for color management. Without one our camera, monitor and printer are only as good as our eye's ability to discern color, and our eyes are easily fooled. Absent any external reference our eyes will accept a wide range of variation in color as being normal. That's due in part because in real life we experience things under a wide range of light conditions.
For lack of a better tool the arbiter of when color is neutral for most photographers is the toolbar eyedropper tool in Photoshop. It should be set to 5 x 5 sample size for measurements which will average out any pixel-to-pixel variation. When a photo contains an image of a gray card which is known to be neutral, an eye dropper reading on the card will reveal from the RGB values how accurately the color balance in the camera was. When the color balance is dead on the R, G, and B readings in Photoshop will be identical.
So the first and most valuable use of a gray card is as a diagnostic tool. Regardless of whether the photo was taken in RAW or JPG, AdobeRGB or sRGB, and regardless of how the camera WB was set having a gray card image in a test shot will reveal, in absolute measurable terms, what the actual color balance was when the shot was taken.
The next most valuable use of a gray card is as a roadmap for automatic correction. In Adobe Camera Raw (ACR) or Levels there is a different type of eyedropper. In Levels the center eyedropper will make any area it is clicked on neutral gray, that is to say equal values of R, G and B. So if the color balance of a photo containing a gray card is off it can be corrected automatically by opening it in levels and clicking on the card image with the center eyedropper.
Since it is not practical to include a gray card in every shot Photoshop allows the settings in the test shot with the card to be saved and then applied to all the other shots taken under similar light. That is a world class butt saver is you've shot a whole session of portraits in the woods under the trees only to discover when you open the files the skin of the subject looks dull and gray due to green cast to the light. But if you started the session by shooting the subject holding a gray card you can correct that shot using the eye dropper in levels, save the settings, then apply the settings to all the other files. Do that one time and you will never again complain about the inconvenience, cost, etc. of a gray card.
Again all of the above can be done regardless of the camera settings when the photo was shot, but that brings us to the other useful thing about the card; setting custom white balance.
The ideal situation for color management is when the camera saves the files with neutral tones which contain equal RGB values. The camera has white balance settings for various standard lighting conditions such as daylight, tungsten, fluorescent, etc. but they may not exactly match the color of the light, especially if it is being changed by filtering through trees reflecting off colored surfaces. The camera also has the facility to set custom white balance from an object known to be neutral, such as a gray card. This is done by shooting the card and selecting that frame as the new standard for what is neutral. Once set that way if the card is included in a test shot is should have identical RGB values when measured in Photoshop.
In the example above of the portrait session in the woods, if the photographer started by shooting the card then set custom WB based on that shot there would be no need to color correct the files in post-processing because the camera color management would do the necessary correction when creating a JPG file, and store the white balance setting information in the header of a RAW file.
If shooting JPG is very important to get the color balance correct in the camera. It can be corrected in Levels after the fact, but other than minor tweaks can affect the image quality.
If shooting RAW the custom WB step is a convenience rather than a necessity IF a test shot containing a gray card was made. The custom WB setting from the camera does not change what is in the RAW file, only how Photoshop displays it. So for example if you forgot to change WB from tungsten when shooting outdoors and all the photos are blue when displayed they can be fixed without any effect on image quality by simply clicking the card image with the "make neutral" eyedropper, and then applying that same correction to all the files.
That pretty much covers the gamut of basic gray card uses. Now lets consider a more advanced variation.
Consider an interior shot which has daylight, tungsten, fluorescent lighting. If you had three gray cards and place one under each light source and shot the scene with one RAW exposure you could open the shot and save three copies, each "click" corrected for one of the light sources using the card which was placed beneath it. The three files would then be layered together in a single file with masks with the most dominant color on the bottom. The masks on the layers would then be opened to blend in color corrected sections illuminated by each light source. It may seem complicated, but is it actually very simple to do. The only technical skill needed is using the eraser tool to open the masks.
In light of all a gray card can do for you its pretty foolish not to own and use one don't you think? Even if you use the magic coaster (i.e. ExpoDisk) a gray card is still valuable as a diagnostic and correction tool.
As for the endless debate over which is best, the answer is that all the commercial gray card products will all do the basic job of providing a benchmark for color control. The world will not stop revolving on its axis if your RGB values are off neutral by few points. As I said at the beginning your eye is the weakest link in the color reproduction change and you will not see minor difference in color unless you do side-by-side comparison. Just keep in mind that to perform Custom WB the card must be large enough to fill the center circle of the viewfinder on Canon cameras.
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