Red-eye Query

I took a photograph last week of a couple standing side-by-side, one had red-eye, the other didn't, despite the fact I had red-eye reduction set on my 450D. It was easy to remove in PP, any ideas why this should happen?

Red eye is caused by light reflecting off the back of the retina and back through the blood vessels in the eye.
A rough expalation is that the person who didn't have the red eye was on a different plane to the flash and the sensor.

Someone else can probably explain it better

The only explanation I can think of is that the preflash (which is supposed to reduce redeye by closing the iris of the eye was missed by the red eyed person (might have been blinking at the time).

The only way to truly eliminate any chance of red eye is to move the flash away from the lens - it is the narrow angle between flash and lens which causes the light to be reflected back. A hot shoe flash or off camera flash will virtually eliminate any chance of red eye due to the increased angle.

Get the flash away from the camera.

Thanks guys.