The gray of photography

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

2. Say 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 the white card makes the picture defining white balance have little noise, which is very beneficial to clarity. 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 of them are R=G=B g = bg = B. That is to say, 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 card, also called "shaking black card", is 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 a black card when the door is slow, so that the upper sky is exposed less, so that the light ratio can be balanced. In order to make the boundary line of the black card inconspicuous, shake the black card by hand. So black cards are also called "shaking black cards".

The three concepts of black field, white field and gray field are usually mentioned when adjusting the color scale and curve. They are not commonly used concepts, but their existence is meaningful and there must be something to use. Color correction is its main function.

We have mentioned the RGB mode (see here) and know the composition of colors. Then it is very simple to understand these three concepts.

The red boxes in the figure are black field, gray field and white field in turn.

Black field: Use the black field pipette to define the color of a point on the image (for example, R=50, G=50, B=50, which is gray). The black field will use this color as the dividing line to replace all colors equal to or smaller than this color (for example, the color values in the figure are R≤50, G≤50 and B≤50, which are all darker than the defined gray level).

White field: The concept is the same as black field, but the relationship between colors is defined as greater than.

Grey field: The concepts of white field and black field are defined. Based on one point, dark colors are darker and bright colors are brighter.