Grey cards, such as the one illustrated below, are a simple but very useful standard reference for obtaining consistent images in traditional film or digital photography. They are used in conjunction with reflective light meters to establish a standard middle grey against which this type of light meters are notionally calibrated. Grey cards are merely flat, usually rectangular, cards of suitable dimensions with a uniform, neutral mid-grey colour. They are typically made of materials such as plastic, paper, or even foam. Most importantly, they have a flat 18% reflectance across the visible spectrum. Flat spectral reflectance ensures that such cards not only appear neutral in colour but also remain neutral under any source of illumination.
As with many issues, the situation is much more complicated when looked at in detail. It seems that the 18% grey standard has its roots in the print word where it was adopted because it was seen to be mid-way between white and black. However, when the manufacturers of light meters are consulted, a somewhat different story emerges. Some use a 14% grey standard and others some other similar value. Light-meter manufacturers usually refer to ANSI standards and importantly differentiate between luminance and reflectance. Luminance refers to the light energy as measured falling on a surface, whereas reflectance refers to the measured energy reflected from the surface. There is a significant difference. The only snag is that the ANSI are not freely available to photographers.
In practice, some manufacturers use a luminance value equivalent to a reflectance value of 12%. However, a printed surface which is half way between white and black reflects about 18% of the light energy falling on it.The two figures represent a half-stop difference in exposure, with the 18% grey reflecting less light energy.
Users of 35mm slide film should photograph a grey card under controlled conditions and examine the slide with a densitometer matched to the 12% or 18% value of the card. However, most photographers just photograph a person, holding a grey card and a Macbeth color checker card, in a uniformly lit scene. Take a meter reading exclusively from the grey card whilst the card is angled between the lens axis and the light source so that the exposure meter measures the reflected light. Then take a series of frames at one-third-stop increments on either side of the metered value. If the optimum exposure appears to be obtained using the metered value, everything is fine. However, if the best result is produced by the frame overexposed by +0.3 stops, set the exposure compensation value on the camera to +0.3 stops.
Digital photographers should photograph a grey card under even lighting at the metered value, and at one-third-stop increments with the card angled a little towards the light source. Then examine the histograms for each exposure - on the camera rather than within Photoshop. Choose the exposure setting that generates a centered value and set the relevant compensation value on the camera.