What are exposure compensation and white plus black subtraction?

Exposure compensation "white plus black minus" is a formula of photographic exposure, which means that the exposure should be increased when shooting the whole bright (white) scene and decreased when shooting the whole dark (black) scene. Many novice photographers will be confused by this formula. They'll think the scene is already bright. Isn't exposure overexposed? Similarly, shooting dark scenes also requires underexposure. Isn't it underexposed? There is such a question because I don't understand the automatic exposure principle of the camera. The automatic exposure of the camera is calculated according to the photometric results of the camera. The metering system of the camera has certain principles. The theory of this principle is 18% reflectivity, but you only need to remember one conclusion: the camera will take everything in gray. Although this conclusion is not rigorous enough, it is practical enough. Because everything the camera shoots is gray, if it shoots snow, the result of automatic exposure must be "gray snow" (underexposure), so it is necessary to add exposure; If you shoot the blackboard, the result must be "grey board" (overexposure), so reduce the exposure. This is the truth of "white plus black minus". Now let's explain the principle of photometry and the reflectivity of 18%. We can see objects, and cameras can shoot objects because objects can reflect light. The metering system of the camera is to collect the light reflected by the object to calculate the exposure. However, different objects have different reflective abilities (reflectivity) to light, and the camera doesn't know what the specific reflectivity of the object is. Therefore, manufacturers must formulate a unified standard when manufacturing cameras. Scientists believe that the average reflectivity of natural objects is 18%, so the camera takes 18% as the standard. The result of adopting 18% uniformly is to treat all objects as gray. In general scenes, the reflectivity of most objects is close to 18% (or the average value is 18%), so the automatic exposure result of the camera is relatively accurate. However, if you shoot objects such as snow scenes or blackboards whose reflectivity is much greater than or less than 18%, the results will be completely inaccurate. At this time, manual exposure compensation is needed.