Science Questions

What causes Red Eye in Photographs?

Sun, 6th Dec 2009

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Question

Rob Waite asked:

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.   Thereís 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 thereís not time for the pupil to contract before the camera takes the picture.  So you see this red reflection of the interior of the personís eyeball.  

Red eye reduction works by the camera shining a brightish light or doing a few Ďspoofí 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 thereís a very small aperture in the front of the eye as the pupil has got very tiny, therefore much less light gets in and itís much harder for light to reflect back out again.  And thatís 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 didnít 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 thatís 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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Rob Waite asked the Naked Scientists: Hi Guys, † 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, † Rob (in Leeds) What do you think? laird rob, Sun, 6th Dec 2009

One possible factor is that the light won't be focussed and reflected back properly if you are short or long sighted to a significant extent.
Also, if you (for whatever reason) don't look straight at the camera then you don't get red-eye. Bored chemist, Mon, 7th Dec 2009

I believe the red colouration is caused by the blood vessels in the retina.

BTW, if your eyes don't show red in flash photographs, you might want to check that you do actually cast a shadow during daylight. I believe the two effects are related.

(OK - The second bit is baloney, but I think the first part is true.) Geezer, Tue, 8th Dec 2009

Hello, just to let you know, my eye sight has always been really good. I've recently had my eyes tested and no problems in "sightedness" have been found.

Come to think of... another question - How does the "redeye reduction" feature work on cameras?

It's all very strange. laird rob, Tue, 8th Dec 2009


A number of low powered flashes or a really bright flash will occur before the main flash, causing the iris to contract. Chemistry4me, Tue, 8th Dec 2009


A number of low powered flashes or a really bright flash will occur before the main flash, causing the iris to contract.


That's the traditional way.

Some modern digital cameras use an image-processing technique to automatically/algorithmically 'paint out' redeye. Unfortunately the method is not foolproof, and can sometimes result in blacked out teeth and other such artifacts! techmind, Thu, 10th Dec 2009

Since the red eye effect is due to the light reflecting back from a persons eye, i think a person being short sighted has more chance of showing a red eye. The logic being, that a short sighted person without his/her glasses/contact lenses would have a dilated pupil while trying to focus on the camera. This will result more light getting out of the eye and hence more prone to this effect. Mani, Wed, 9th Jan 2013

I have a question regarding red eye in photographs. I was recently in the Amazon and was told by our guide that when we shine the light on different animals the nocturnal ones will reflect back red eyes and the diurnal animals reflect back green. Is this true? If so, why green? And then shouldn't humans reflect green? Tanya, Fri, 7th Jun 2013



Eyeshine any colour other than red is tapetum lucidum used to improve night vision.
However a red reflection can still be a nocturnal animal. RD, Fri, 7th Jun 2013

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