Have you ever wondered what causes eyes to appear red in photos? Usually this happens indoors or at night when the pupils are more dilated than usual. The light from camera flashes are generally located only a few inches from the camera’s lens. When the flash goes off, light goes straight into the pupil and gets reflected off the back of the eye, and straight in to the camera lens.
The effect is magnified with dilated pupils – that’s why the “red eye” mode of cameras involves a pre-flash before the main flash so that pupils become smaller before the real picture is taken and red eyes are minimized.