system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we get the "red eyes" effect on photos?
gpt
It's the result of the light from the flash on the camera being reflected from the ~~irises~~ retina of the eyes directly back to the lens of the camera. For this to happen, the flash must be located close to the lens, and the problem can be fixed by placing the flash some distance apart from the lens. On small pocket cameras, the flash is by necessity very close to the lens, and therefore the world is full of photos with red eyes in them.

Edit: red-eye reduction in cameras with integrated flash units works by flashing the light several times before the shutter is released and photo taken. The preflashes make the pupils of the eyes contract, which makes for a smaller potential red area in the eyes of the subjects. The better way to get rid of the problem is to use a separate flash unit that is not fixed to the body of the camera and that you can point on the subject from some other direction rather than right into the eyes.