What causes red-eye in photos, and how can I prevent or remove it?

Question

Sometimes my flash photos show red-eye, but other shots of the same person do not. What causes red-eye, and does it depend on distance, lighting, or flash position? What are the best ways to avoid it when shooting, and how can I remove it afterward with free software?

user6745 · Accepted Answer

The colour comes from the blood in your eye.

When light rays, from a flash for example, enters your eye, it hits the blood vessels and is reflected back to the camera, appearing red.

We all know how horrible that looks.

Understanding the cause, we can avoid it. For example, red-eye occurs when light enters straight into the eye and bounces right back out, this means the light is traveling in the same direction that your lens is pointing, and at a very close axis too.

If you position the flash higher-up, the reflected ray will hit somewhere else because you have created an angle. As long as it does not hit the sensor of the camera it won't appear.

If the iris of the eye is small, this can somewhat prevent red-eye, compared to when the iris is widely opened.

That's also why on compact camera, the method used is to blind them with pre-flash before taking the photo. This causes the subject's eye to cut down the amount of light entering: shrinking the iris opening.

What causes red-eye in photos, and how can I prevent or remove it?

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