Why does D90' s camera shoot red eyes?

"Red eye" is a normal expression of animals with iris, which can be removed by digital post-processing. Iris can reflect light, just like a mirror. As long as the external light enters the subject's eyes and is reflected by the camera, it will record "red eye", which is more obvious when the pupil of the human eye is dilated at night and the camera is used for internal flash. The solution is simple. According to the principle mentioned above, just avoid any light that can be absorbed by human eyes and reflected into the camera, such as choosing the angle of side light to shoot, or using an external large-angle flash. However, this will require higher technology and equipment for photographers. The principle of "anti-red-eye" function of the camera's built-in flash is to narrow the pupil of the object by stroboscopic, thus reducing the reflection, but this function is not effective for everyone and under any circumstances. So when "red eye" is inevitable, it can only be eliminated in the later stage. Add illustrations. The left picture shows a scene with a high probability of "red eye", while the right picture pays attention to the scene light and the angle of supplementary light, and basically there will be no "red eye" problem. The red arrow is the flashing direction, and the blue line is the reflection.

Hope to adopt