About photography black card and gray card, white card, and black field, gray field and white field in ps.

1. Gray card, generally refers to 18% reflectivity gray card. Because the camera exposure is based on 18% reflectivity, we can get the correct exposure by using gray card to measure light. Another purpose of the grey card is to define the white balance. Because of the above correct exposure value, the white balance information carried by the gray card is the most correct.

2. Let's talk about white cards first. White cards are basically used to define white balance. Because white cards are easy to obtain, they can often be replaced by a piece of white paper. The correct exposure of white cards makes the picture defining white balance have little noise, which is very beneficial to definition. However, the correct exposure of white cards is more difficult than that of gray cards, so it is generally recommended to use gray cards to define white balance.

why can both gray cards and white cards be used to define white balance?

because both are R=G=B g = b. That is, as long as the color cards with R=G=B can define white balance, only in this way can the white balance information they carry be separated.

3. Black card. Black cards, also called "shaking black cards", are a way of exposure. For scenes with large light ratio, such as scenes with bright sky, we can cover the upper part of the lens with black cards under the condition of slow door, so that the upper sky is less exposed, so that the light ratio can be balanced. In order to make the boundary line of the black card not obvious, shake the black card by hand. So the black card is also called "shaking the black card".

the three concepts of black field, white field and gray field are usually mentioned when adjusting the color scale and curve, which are not commonly used concepts, but their existence is meaningful, and there must be some places where they can be used.

The RGB mode has been mentioned earlier (see here), and the composition of colors is known. Then it is very simple to understand these three concepts.

The red boxes in the figure are in turn. White field.

black field: define the color of a point on the image with a black field pipette (for example, R=5,G=5,B=5, which is gray), and the black field will take this color as the dividing line, which will be equal to or less than all colors of this color (as in the previous example, the color values in the figure are R≤5,G≤5,B≤5, and. Just define the relationship between colors as greater than.

gray field: connect the concepts of white field and black field, and the definition will come out. Based on one point, dark colors are darker and bright colors are brighter.