When turned on the camera shoots
When turned on the camera shoots a series of pre-flash strobes followed by one more strobe when actually taking the photo. The pre-flash strobes cause the pupils to reduce in size and by the time the photo is taken the pupils are small enough for the eye redness to substantially reduce.
The red-eye reduction feature does what it is supposed to do: reduce the red-eye effect but almost never is it completely prevented. There are many limitations to this feature for example pupils reaction time to light can vary. In addition this feature can have a side-effect that results in photos having people?s eyes closed. The reason is that the pre-flash strobes blind the people and cause them to close their eyes.
Other ways to prevent red-eye
Understanding what causes red-eye helps being more creative in preventing it. Following are some ways to prevent red-eye other than using the built-in camera red-eye reduction feature:
Increasing the light where photos are taken (for example by turning on the lights in a room before taking photos of people) causes people?s pupils to reduce in size and eye redness to reduce.
Point the flash away from the eyes.
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