Finally, I understand why exposure value is determined by the gray scale of 18 degrees.

We all know that if an object is absolutely black, its reflectivity is 0; If it is absolutely white, its reflectivity is 100%. In fact, all objects are between these two extremes. If a gray tone reflects 18% of the light, it is 18% gray. If the bright, dark and intermediate tones of all objects in nature are mixed, 18% ash will be produced. So the metering system in the camera is designed to reproduce the brightness of this 18% gray. Whether you aim the metering system at an object of any hue or not, it always "thinks" that the object is in a medium gray hue, and provides exposure data of gray hue when copying. That is to say, if you put the camera on the snow, the exposure data you get is to restore the snow to gray, and when you measure light at a black object, the exposure data you get is to restore the object to gray. At this time, exposure compensation is needed.

If the photometric object happens to be a medium gray tone with a reflectivity of 18%, including a medium gray tone with a comprehensive brightness of 18% for various scenes within the photometric range, the exposure combination recommended according to the photometric reading can produce accurate exposure, that is, various brightness levels of the scene can be expressed to the maximum extent. However, it is impossible for most subjects to have a reflectivity of 18%. At this time, Kodak 18% gray board can be placed under the same lighting conditions as the subject, and then the metering system in the camera can be aimed at the gray board to get accurate exposure data. Determine the exposure value, light ratio, color balance and density, macro and exposure of remake photography, and set white balance for digital cameras and camcorders. 18% gray board has undergone professional chromaticity correction, which is a necessity for people with strict exposure requirements. With the exposure meter, the effect is better, and it is an ideal accessory for portraits, flowers, remakes and other photography.