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When adjusting white balance, there are two ways you can do it: you can do it intuitively, adjusting the white balance however you see fit; or, you can use a tool called a gray card. We will get into the details of the easy process of actually adjusting the white balance in the Processing Photographs on the Computer section, but let's take a moment to discuss what a gray card does and how to use it during a photoshoot.
A gray card is special piece of plastic, paper, or foam that is a neutral gray color. Because this neutral gray is a widely-accepted standard, when you take a photo of it in any given light, cameras and photo-processing software can determine the color temperature of the light because they know exactly what color the gray card is. All you have to do is tell the software that yes, that card is that special neutral gray color.
What the software will do then, is set the white balance to the exact color temperature of the light. This will make the gray card appear perfectly neutral gray, and it will make, for example, a white piece of paper look white, like in the white balanced image shown earlier in this section. So if you take, say, a hundred photographs in the same light (for example, outside on a sunny day), all you need to do is take a single photo in that same light with the gray card. The card doesn't even need to be in focus, you can just hold it out in front of the lens, making sure the light is hitting the front of the card, and then take a quick photo.
When you process the photo on the computer later, you can use this photo to get your white balance setting and apply it to all a hundred photos you took during the photoshoot.
There are a couple things to be aware of. First, a gray card will not work if the photo of it is too overexposed or underexposed. That's because when you overexpose or underexpose something too much, the color goes away in the image, and if the color is gone from the gray card, it obviously can't do its job. So while it's okay for your gray card shot to be out of focus, you do need to make sure the gray card is exposed more or less properly.
Secondly, when you neutralize the white balance with a gray card, you are effectively neutralizing the color temperature of the light. That means if you took a photo of someone with a warm, orange light shining on them, and then you took the exact same photo but switched the light bulb out for a pure white light, after balancing the photos with a gray card, they will appear exactly the same. It would be as if the warm orange light was white! Think about that: if you had some wonderful, moody photographs of a beach at sunset, and you neutralized the color temperature with a gray card, it wouldn't look like a sunset at all. All the warm oranges and reds