How to Use a Gray Card
Exposure and color balance information can easily be provided to the lab by shooting a KODAK Gray Card Plus before every major lighting setup. The gray portion has 18% reflectance and neutral color, while the black and white patches provide reference for 3% and 90% reflectance. The KODAK Gray Card Plus has the property of reflecting red, green, and blue in equal amounts, which greatly facilitates postproduction and color grading.
Exposure
When shooting the gray card, it is recommended that you meter the card with a spot meter. Point the meter at the center of the card and use the reading directly. It is a good practice to have the meter reading for the gray card match the shooting stop, which is typically determinded by using an incident meter. In most cases - if the gray card is in the key light - the two readings will be very close. Tilting the card toward the key light may be necessary for them to match exactly. It takes practice, but this method yields accurate and repeatable results.
Color Balance
By shooting the KODAK Gray Card Plus at the beginning of a major lighting scene, you're defining your white light - or neutral light - source. If you're shooting a tungsten-balanced film in tungsten studio lighting, shooting the gray card is very straightforward. When shooting in daylight - which changes color throughout the day - shooting the gray card depends upon the light you want to define as the white light source. If you'd like the sun to appear warmer, illuminate the gray card with the sky plus clouds, blocking the sunlight. If you'd like the sun to appear as the neutral source, shoot the gray card in the sunlight. A daylight-balanced film is very easy to color grade in a scene shot in a mixture of sun and skylight.
If it's overcast, you won't have much of a choice unless you're introducing your own lighting. At sunrise or sunset, use the sky opposite the sun to illuminate the gray card. This will naturally keep the light of the sunrise or sunset very warm (reddish).
When shooting a flashback or nostalgic scene, you could light the scene with tinted gels - perhaps an orange-yellow color. To preserve this look, shoot the gray card with the same light source, but remove the gels. This defines the white light source and give post personnel a good reference for your intentions for the scene. If you keep the color gels on the lights and then shoot the gray card, the subsequent color grading will neutralize the unusual color of the scene. You can also shoot a slate with a comment on it such as "warm look OK".
If you plan to color balance the scenes yourself using editing or post software, having the gray card as reference saves you time in achieving a baseline color balance. Keep in mind that once the scene is color balanced, there are still tweaks in color saturation and contrast that can be done to enhance the look you are creating.
Visit KODAK Gray Card / R-27 for the User Guides and more information.