A camera works in a similar way. Instead of a pupil, it has a shutter that allows light to pass through the lens and onto a piece of film. The film records the image just like your retina does.
Back to the eye: Normally, when you look at someone, some of the light passing through their eyes bounces around inside the eyes and reflects back to you. However, the amount of this light is so small that the pupils just look black. A camera flash, however, sends a tremendous amount of light into the eye in a very short time. If the camera lens is close enough, some of this light ricochets back off the retina and into the lens. And because the back of the retina is filled with blood vessels, the pupils look red.
This problem has been perplexing photographers for over a century. One way to deal with it is to back off from the person you’re photographing: the farther away the camera lens, the smaller the amount of light that will reflect back into it.
However, this red reflex as it’s known has been very beneficial to eyecare practitioners. Back in the 19th century, the German physician Hermann von Helmholtz discovered that he could look inside the retina by holding a bright light near his own eye and shining it directly into a patient’s pupil. This allowed doctors of the time to see the inside of the eye non-invasively for the first time.