What is the principle of 18% neutral gray in photography?

The principle of intermediate gray scale in photographic exposure-18% gray scale

Friends who play photography know that many traditional film cameras, especially digital cameras, have automatic aperture devices, which brings great convenience to our correct exposure. But automatic aperture is not everything, nor can it guarantee that every photo is exposed correctly. The photos taken with automatic aperture are either underexposed or overexposed, and the photos taken are dark and pale. Why is this happening? This has a lot to do with not knowing the principle of intermediate gray scale of exposure and not using the automatic exposure device correctly. Automatic aperture device is also called automatic exposure device. The basic principle is that firstly, the brightness of the shot scene is measured by the built-in metering system of the camera, and then the measurement result is transmitted to the chip, and then the chip is transmitted to the automatic aperture device to give the appropriate aperture and complete the exposure task.

The so-called intermediate gray scale principle means that no matter whether the built-in metering system measures a high-brightness subject (such as unpolluted snow) or a low-brightness subject (such as coal), if the subject is exposed according to the measurement results and developed normally, the subject will produce intermediate density on the negative. A photograph developed from a negative with an intermediate gray density gets an intermediate gray tone, that is, a negative with an intermediate gray density produces an intermediate gray tone photograph.

According to the principle of middle gray, according to which part of the scene is exposed, which part will produce the tone of middle gray. In our shooting practice, the brightness distribution of the scene is very complicated. For example, if we take a bust with blue sky, white clouds and green grass as the background and follow the light, we can get a correctly exposed picture with automatic aperture exposure. If we shoot in the backlight without compensation, the face will be black, because the built-in metering system measures the average brightness of the scene, not the brightness of the face. When shooting a snow scene, the brightness of the snow is very high. The height of exposure to snow will produce a moderate gray brightness on the negative. When the photo is developed, the gray tone in the middle is obtained, and the snow scene becomes a gray scene, losing the texture of snow. Shooting snow scenes should be exposed according to the shadow part of snow, that is, the texture of snow can be maintained by opening the aperture appropriately. Some photography textbooks say that because of the high brightness of snow, it is easy to overexpose when shooting with automatic aperture, and the aperture should be reduced appropriately. This violates the principle of intermediate gray scale of exposure and is misleading.