What causes red eye in photographs is a very common question. What is Red Eye
There are probably millions of family albums with red eye in the photographs. Red eye is a term commonly referring to a bright red pupil in photographs. The common reason given for this is that the light reflected off blood vessels in the back of the eye. More accurately, the light is reflect so that you are seeing the light illuminating the back of the retina.
Causes of Red Eye
Red eye occurs when light enters the eye at almost level with the eye. Because light reflects at an angle equal and opposite to its entry (in layman's terms, it makes a "V" shape) if bright light (such as from a flash) enters the eye at almost no angle it will be reflected back, or visible to, the camera. This most often happens when the pupil of the eye is dilated and gives the light more room to enter and exit the eye.
Red Eye is Most Likely to Occur in These Situations
Built-in camera flash
Camera lens is level with the eye
Preventing Red Eye
Preventing red eye is not as difficult as you might think. To prevent red eye you just have to alter the angle the light enters/exits the eye or change the harshness of the light.
Ways to Prevent Red Eye
Use a flash with a tilt head
Use a reflector to add light
Use a diffuser on your flash
Shoot from a slightly higher or lower position than your subject's eyes