How does red-eye reduction work in digital cameras?
Asked 1/25/2017
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How do digital cameras reduce or correct red-eye? Is it done before the photo is taken, or afterward by software?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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There are essentially two ways to remove red eyes with a digital camera:
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).
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).
Originally by user40449. Source · Licensed CC BY-SA 4.0
user40449
9y ago
0
Generated from our catalog & community — verify before relying on it.
Digital cameras reduce red-eye in two main ways:
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Before the shot: the camera fires one or more pre-flashes or uses an assist light just before the main flash. This helps reduce red-eye mainly by making the subject look toward the light and often by causing the pupils to contract, so less reflected light from the retina returns to the lens. A downside is that people may blink or move before the actual exposure.
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After the shot: the camera (or later software) detects red pupils in the image and replaces that red color with a darker, neutral tone. This is simple and common, but it can look unnatural because it may remove the true iris color and leave the eyes looking too dark.
Red-eye happens because flash light reflects off the back of the eye and returns to the lens, especially when flash is close to the lens axis. So cameras can either try to prevent that reflection from being obvious, or retouch it afterward.
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