Gray card
A gray card is used in photography to calibrate the exposure. It is usually a sturdy cardboard box or a plastic sign that is colored neutral gray on one side and white on the other . The gray side reflects about 18 percent and the white about 90 percent of the light falling on it . Both sides are coated with metamerism-free colors so that the same degree of reflection is obtained even with lighting sources with different color temperatures (daylight, fluorescent lamps, incandescent lamps). (For exposure measurement, the palm of the hand can be used as a gray card. It must be taken into account that the measurement results in a value one f-stop higher, e.g. zone 6 instead of zone 5, than measured on a "real" gray card.)
Gray, with the exact value of 17.68 percent reflection , corresponds to the logarithmic mean of the reproducible contrast range of 1.50 log D density for photo paper (as a ratio of 1:32), i.e. exactly 0.75 log D.
- Clear: medium gray at 18%:
- See also zone system
- Descending from zone X with 100% brightness, descending is divided into 11 zones up to I ... and 0.
- In each case there is a factor in the intensity less than the difference in the respective lower zone.
- (or factor 50% for two levels)
Zone intensity X 100.00% IX 70.71% IIX 50.00% VII 35.36% VI 25.00% V 17.68% IV 12.50% III 8.84% II 6.25% I. 4.42% 0 3.13%
- Level V is exactly in the middle and therefore with 17.68% rounded 18%
Almost all light meters are calibrated to provide setting values that apply to a scene with an average brightness distribution. An integral value is always determined from the brightness distribution in the motif .
However, if a motif (photography) is not average (e.g. white rabbit in the snow or an almost black object in front of a black background), the values must be corrected because otherwise incorrect measurements will result and the resulting photo will not correspond to the light situation of the scene . In the case of the white rabbit in the snow, the exposure would be underexposed because the exposure meter reduces the exposure to a value for a scene of medium brightness. The result would be a “gray rabbit” in front of “gray snow”.
This error can be compensated for using a gray card by placing it as close as possible to the object and measuring the full-size map with the exposure meter (object measurement). This measurement should be done diffusely . This is achieved by defocusing . Alternatively, the exposure measurement by light measurement (hand-held exposure meter with attached (diffuser) cap in the direction of the camera or measured "away from the subject") would provide almost the same exposure value. It would stay with the “white rabbit in the snow”.
A further aid for very precise exposure measurement is a spot exposure meter with measuring angles - depending on the design - between 1 ° and 10 °.
White balance and color cast
In digital photography , the gray card can also be used for white balance if the gray card is coated with a metamerism- free color; d. In other words, it must always remain color-neutral even with different types or colors of light . Every light situation recorded with such a gray card can be reworked very well in terms of manual white balance. The image processing software uses the card as a reference for determining the color temperature of the light.
Color gray card
The color gray card also has two density fields and six color fields with defined minimally deviating color densities (0.05 D). This makes the card look like a traffic light. This makes it easier to assess color casts in the case of color ametropia, printing, computer-aided image processing or z. B. in the case of fatigue in the laboratory . If two fields are not so easy to see on one side, the opposite field can be seen more strongly on the opposite or vice versa.
It must always be possible to see all fields equally well. If not, there is a color cast or the receiving film has a sensitization gap and is unsuitable.
similarities
A monitor calibration with a so-called “target” is carried out to adjust screens .