When to Turn off the Red Eye Reduction Feature on Your Digital Camera
Red eye reduction works by emitting a pre-flash before the main one to contract the subjects' pupils. This reduces the red eye problem, but does not eliminate it.
Turning off Red Eye Reduction
Because of the nature of red eye reduction, you may want to turn it off sometimes. For instance, if you're taking photos in a very loud area like a club, the pre-flash might confuse your subject into thinking that was the main flash, and by the time the main one goes off, you have a nice photo of them walking away.
The same situation can apply to photographing children who have a hard time sitting still and following your instructions. It might take you a couple of minutes to get the kid to calm down and stay still in the pose you like. Then the pre-flash goes off, he thinks you're done, and you need to start over.
Turn off the red eye reduction pre-flash when you're in situations when it could cause confusion to the subject.