If the light in scene is excessively bright or dark, exposure deviation might be produced.
Gray card can balance all mixed lights to be 18% neutral gray. With a photometer you’ll get precise exposure value by means of recording the light that was reflected from the grey card. (it can measure the exposure value through the exposure photometer, the TTL SLR camera and the digital camera etc.)
A 18% grey card not only have the use of measuring the camera exposure value, also be able to adjust color balance and revert the color truly and perfectly by controlling the gray balance in digital photography
