When people take portraits at night, because the environment is too dark, in order to supplement the light, most people will choose to turn on flash photography. But after shooting, the figure is clear, but why does the pupil of the human eye turn into a horrible red? Remember that the wheel eye only exists in the second dimension. In fact, the pupil of a person, like the aperture of a camera, can be adjusted, but its size and response are shared by the sympathetic and parasympathetic nerves