For light meters and cameras, gray cards are economical, simple, and highly effective. Your light meter (whether external or in-camera) expects the average image to have the same amount of reflected light as this gray card (18% gray). If the image you want to photograph is significantly darker or lighter, the camera will measure incorrectly. For example, it would want to make a white wall gray, thus underexposing it badly. |
However, if you take a light reading by holding the gray card in front of the camera, the camera will give you the correct measurement for the actual amount of light falling on the image, without overly light or dark images confusing the reading.
It is important that you position the graycard in the same direction to the sun as your image, so that the light falls on it in the same way. The card is coated with gray pigment, not just printed, so it is very light resistant. However, after about 4 years of use (more if you are using it daily) you should think about getting a new one. Protect the gray card from direct sunlight when not in use.
Dimensions: 20x25 cm