What causes Red Eye in Photographs?

06 December 2009

Question

Can I first start by thanking you all for such a great show, I have all my family hooked and we can hardly wait each week for the next addition of the podcast to be released.
I have a question about "redeye" when taking a picture using a camera. I've recently started the task of correctly sorting through all by digital pictures on the computer. I have noticed that in some of my pictures people have red eyes.
I've read somewhere before that this is due to light reflecting back out of the eye, but in the case of my photographs, it only seems to be certain members of my family who have red eyes. I myself NEVER have red eyes on photos, but my wife often does.
This is especially strange in family photographs when I am the only one who doesn't get the red eye effect.
Just wondering if you could explain what redeye is, are there any tips to avoid redeye when taking picture, and why some people are more prone to others to having redeye.
Thanks, Love the show.

Answer

Red eye occurs because when you take a flash photograph - the camera produces a big burst of light to illuminate the subject and that burst of light goes in through the open pupil of the subject, and bounces off the back of the eye. There's a layer called the choroid which has a very rich blood supply inside the eyeball and that reflects red light, the colour of the blood, back out of the front of the eye and into the camera. This happens so quickly that there's not time for the pupil to contract before the camera takes the picture. So you see this red reflection of the interior of the person's eyeball. Red eye reduction works by the camera shining a brightish light or doing a few 'spoof' flashes at the subjects first. This constricts the pupil down in the people you're taking a picture of, and then it takes the proper picture. This means that there's a very small aperture in the front of the eye as the pupil has got very tiny, therefore much less light gets in and it's much harder for light to reflect back out again. And that's how red eye reduction occurs.Why are some people more prone than others? Perhaps, the pictures that you've been taken in are not pictures involving flash photography, so you don't seem to have the effect. Perhaps also, you're not directly in line with the camera flash. If the flash didn't actually illuminate the eyeball sufficiently on its interior, not enough light will come back out of the camera. One other possibility is that you have very, very dense pigment epithelium. This is a layer of melanin (the same stuff that gives you a sun tan) inside the eye and that's what soaks up some of the extra light. People also naturally differ in how wide their pupil is in any particular light. People with wider pupils are more likely to end up with red eye in photos, whereas those with smaller pupils are less likely.

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