Have you ever wondered how some photographers are able to consistently produce photos with such accurate color and exposure? It's often because they use gray cards and color checkers. Knowing how to use these tools helps you take some of the guesswork out of capturing photos with great color and correct exposures every time.
Because the color of light changes depending on the light source, what you might decide is neutral in your photograph, isn't neutral at all. This is where a gray card comes in very handy. A gray card is designed to reflect the color spectrum neutrally in all sorts of lighting conditions, providing a standard from which to measure for later color corrections or to set a custom white balance.
By taking a test shot that includes the gray card, you guarantee that you have a neutral item to adjust colors against later if you need to. Make sure that the card is placed in the same light that the subject is for the first photo, and then remove the gray card and continue shooting.
When taking a photo of a gray card, de-focus your lens a little; this ensures that you capture a more even color.
Because many software programs enable you to address color correction issues by choosing something that should be white or neutral in an image, having the gray card in the first of a series of photos allows you to select the gray card as the neutral point. Your software resets red, green and blue to be the same value, ...