For years now, film and digital camera makers have been inflicting red-eye reduction settings on their customers. Its a series of bright, strobing flashes that’s not only annoying to the people you’re photographing, but doesn’t even work very well.
What causes red eye? In a dimly lit environment, the subject’s pupil dilates, revealing more of the retina. The back of the retina has blood vessels over it, hence the red colour that is caused when the flash bounces of the back of the eye.
On systems where the flash is close to the camera lens (as it almost always is), the light from the flash shines through the dilated pupil, bounces off the retina, and reflects as a red circle directly back into the lens. (the same thing happens to animals, too, except that the colour is sometimes green or orange instead of red.)
The fix is to move the flash head away from the camera lens or use bounce flash. That way, the reflection from the retina doesn’t bounce directly back at the camera. But on a camera that fits in your pocket, its not practical to achieve much separation of flash and lens.
Since camera manufacturers couldn’t move the flash away, they went to Plan B: firing the flash just before the shutter snaps, in theory contracting the subjects’ pupils, thereby revealing less retina.
Unfortunately, it doesn’t work very well, and you may wind up with red eye anyway. The other problem is that some people see the pre-flash as the picture being taken and then move while the shutter is open. Not a good result.
Basically you have three ways out of red eye. You can turn up the lights to help contract the subjects pupils. You could also use a camera that accepts an external, detachable flash. And if none of that works, remember that you can edit out red eye on your computer, using for example, the red eye reduction tools offered in many photo editing programs.