What is a grey card?
A grey card is made of cardboard or plastic and can be used to adjust the colour reproduction of a photo. This is necessary for limiting the influence of a light source on the colours and white balance and thus creates a correct colour image for the camera. The card is 18% light grey, bearing in mind that 0% is white and 100% is black. A good photo is calibrated to this value of 18%, so that a grey colour falls over the photo. By bringing the grey card into the visible light of the image, the camera can determine the correct settings.
Adjusting the white balance
When you take a photo with the grey card in the image of the camera, you can determine the correct white balance for the photo. It is important that the card is in the same lighting as the photo subject and that no shadows fall over the card. Focus completely on the grey card to set the white balance. The next step differs by camera, but in the settings you will often find an option to manually set the white balance. You must select the photo you've just taken with grey card as the reference photo. The next pictures will use the adjusted white balance.
Functional lighting
In addition to determining the white balance, the use of a grey card is also effective in determining the correct exposure. The principle works the same as when setting the white balance, but for exposure it is necessary that the value is set to 0. The exposure can be found in the camera settings. In manual mode, you can work towards 0 by changing the aperture or shutter speed. In automatic mode, the camera calculates the values itself.
