Digital Grey Card

I shoot a Nikon D40

1. Place Card gray lighting and position your subjects for photo.

2. Objective camera to the gray card and let the camera meter to tell you what combination of shutter speed, aperture and ISO is appropriate.

3. Manually set the camera settings.

4. Remove the gray card and start shooting ..

The gray card will "tell" you what the atmosphere "temperature" of the photo is, that is what temperature in Kelvin, you must use to get the white balance.

The reason you want to use a card like this is that different light sources have different color values. Want to know what color is that is supposed to be "gray" with the specific light you are using. For example, tungsten lights are "red" and fluorescent lights are "green. However, a white shirt must be white in all the light you use.

This article tells the whole story:

http://www.cambridgeincolour.com/tutoria ...

Good luck!