How is the "red eye" produced in photography?

Text | Red-eye phenomenon often appears when Leng Xue in the mountain turns on the flash to shoot portraits in low light. Although it only happened in the small place of eyes, it was so striking that people couldn't recognize it. The eyes are the windows to the soul. The phenomenon of "red eye" destroys the overall image of the characters. I read a photo album recently and finally found the answer. It turns out that the pupil of the human eye changes with the change of light in the external environment. When the light is strong, the pupil automatically becomes smaller, and when the light is dark, the pupil automatically becomes larger, so that the object can be seen clearly. These truths are generally understood by ordinary people. At the same time, human eyes also have physiological mechanisms of light adaptation and dark adaptation. Being in the dark for a long time, the pupil adapts to the dark environment. If you suddenly enter a bright environment, your eyes will be unbearable and there will be an adaptation process. This is why people who come out of the mine in the mine disaster have to wear blindfolds. Without an adaptation process, eyes will be seriously injured and even blind. On the contrary, from light to dark, there is an adaptation process. Knowing the above principle, you can probably know the appearance of "red eye". Before shooting in the dark, the subject's eyes have adapted to the light in the dark environment, and his pupils are also wide open. When suddenly encountering high-brightness flash, the pupil will contract immediately, resulting in instantaneous congestion around the pupil. Because the intense flash captures the moment of eyeball congestion, the phenomenon of "red eye" is formed. Because congestion occurs in an instant, most people can't see the congestion process of human eyes, and only the camera with synchronous flash can lock this moment. In order to prevent the occurrence of "red eye", there are generally two methods: one is to turn on the camera's red eye prevention function, that is, when the shutter is pressed, the flash flashes first to let the pupil have an adaptation process, and then flashes formally, thus eliminating the phenomenon of "red eye". Second, when shooting, the subject's eyes should not look directly at the position of the flash, which can also effectively avoid the phenomenon of "red eye".