Gray card

A gray card is used in photography to calibrate the exposure. There is usually a strong cardboard or a plate of plastic, which is inked on one side neutral gray and white on the other. The gray side reflects about 18 percent and about 90 percent of the white light falling on it. The two sides are coated with metameric colors, so you will have the same reflectance and on illumination sources with different color temperatures (daylight, fluorescent, incandescent ). (For metering the palm may alternatively be used as a gray card. It should be noted that the measurement is a one stop higher value, such as zone 6 instead of zone 5 shows, as a "real" gray card measured. )

Gray, with the exact value of 17.68 per cent of reflection corresponds to the logarithmic mean of the imaged contrast range of 1.50 log. D density, which are exactly 0.75 log. D.

Nearly all light meter are calibrated so that they provide settings that apply to a scene with average brightness distribution. From the distribution of brightness in the scene an integral value is always determined.

However, if a subject is not average (eg white rabbit in the snow ) or " a nearly black object against a black background ", the values must be corrected, because this will cause incorrect measurements and the resulting photo does not match the lighting conditions of the scene. In the case of the white rabbit in the snow recording would underexposed because the exposure meter reduces the exposure to a value for a scene of medium brightness ( The result would be a gray hare against gray snow).

Using a gray card, this error can be compensated for by placing them as close to the object and the card with the light meter full format anmisst (object measurement). This measurement should be done diffuse. This is achieved by unsharp position. Alternatively, the exposure by light measurement would ( exposure meter with attached ( diffuser ) cap in the direction of the camera or " away from the subject " measured ) provide almost the same exposure value. It would remain at the " white rabbit in the snow ."

Another tool for very precise metering is a spot light meter with measuring angles - depending on the design - between 1 ° and 10 °.

White balance and color tint

In digital photography, the gray card can also be used for white balance. Every lighting situation taken with a gray card may very well be refinished regarding manual white balance. The map is the image processing software as a reference for determining the color temperature of the light.

Color gray card

In the color gray card two density fields and six color blocks with defined minimally different color densities (0.05 D) are additionally applied. This affects the card as a traffic light. This makes the color cast judgment on color - defective vision, printing, computer -aided image processing or eg fatigue in the laboratory. If on the one hand, two fields are not so easy to recognize, as opposite to the contrary box to see more or vice versa.

It must always be the same good to see all the fields. If not, there is a color cast, or the receiving film has an awareness gap and is unsuitable.

Similarities:

To adjust the screen monitor calibration is performed with a so-called "target".