The How and Why of Using a Gray Card to Achieve Proper Exposure
It seems hard to believe that the key to getting consistent proper exposure in photography is nothing more complicated than a piece of gray cardboard about the size of a writing tablet and available for a couple of bucks at any photography store. Anyone who is in interested in learning photography should certainly have one of these in their camera pack.
Camera light meters are calibrated to average the light reflecting off a scene to 18% of the color, which is considered a neutral color. Focusing on this card will enable you to take pictures that more accurately reflect the actual colors in the scene.
Imagine you were trying to photograph a wedding with a very dark couple from Nigeria and they the groom was wearing black and the bride white. It would be nearly impossible to get accurate readings because if you focused on the black the camera would look to lighten the scene and if you focused on the white the camera would try to darken the scene. In comes the gray card.
Set your camera to manual and get your exposure off the gray card. Make sure you are using the spot meter setting on your camera and make sure that the light hitting your gray card is the same light that will be hitting your photographed subjects.
I took the liberty of borrowing a diagram from an old Pop Photography magazine that shows how to place the card.
Since you can't always have a gray card available, always look to focus on the neutral colors of red, green and blue that you see in the scene, preferably with your spot meter
Digital Photography Success!