red eye - How does red-eye reduction work on digital cameras?

How does the auto red-eye correction work in digital cameras?

Answer

There are essentially two ways to remove red eyes with a digital camera:

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  While taking the picture, contract the subject eye's pupil. This can be done for any camera, not just digital: the flash blinks shortly before taking the actual picture. The main drawback of this technique is that the subject may move or close eyes completely due to the pre-flash(es).

  
  


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  While processing the picture (either on a computer or built-in a camera, see e.g. this article for an example. In this case, the processing algorithm just detects red eyes and replaces the red color with a neutral one like black. The main drawback of this approach is that it may lose the actual color of eyes (for example, if you expect a nice blue iris with a small pupil, you'll get a very narrow iris with a very wide black pupil).

  
  

Best is to attack the effect at the source: red eye effect is caused by a source of light close to the camera and pointing directly to the subject. See e.g. How to Avoid Red-Eye in Photos? for techniques to avoid this (bouncing the flash is simple and efficient if you have the right flash for that).