A gray card is used for two purposes:
There are a lot of types of light and using the gray card can help you solve some of the problems that will arise as your camera sees different light than you do.
How to use a gray card to set a custom White Balance
If you have a digital SLR, the manual will probably give you pictures and walk you through the steps. Essentially what you’re doing is instead of taking one of the presets (Florescent, Outdoors, Cloudy, etc.) you have a custom preset that is set using the card. This is useful because just about every shot could use a custom white balance as few shots are ever the same. The gray card can also help for accurately metering a scene.
How to Meter a Scene Using a Gray Card
You can use an 18% gray card for getting a great exposure. The reason you need to use the gray card is because your camera tends to try and correct exposure, but it doesn’t always see things perfectly. The gray cards puts things into perspective. The 18% gray is the exact neutral gray that camera’s are made to sense as the “right” exposure.
You get the perfect exposure using a gray card by:
- Staging your Shot
- Place Grey Card
- Remove Grey Card
As you focus the camera will think it’s taking a perfect exposure because it see’s the gray card. You remove the card to reveal a staged shot with the same lighting and the exposure will come out perfect.
How to Use a Grey Card : Photography Tips (video)