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AstroChuck's avatar

What causes red eye in flash photography?

Asked by AstroChuck (37439points) September 21st, 2009 from iPhone

This has got to be a pretty easy question. I know more than one of you out there have the answer.

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13 Answers

DarkScribe's avatar

Blood. It is the retinal circulatory blood in the eye when the camera flash is directly in line with the iris. It can eliminated by offsetting the flash. The “pre-flash” method isn’t good as it causes the person’s pupils to contract, giving a “beady-eye” look. Much better to have the flash bounced or off the camera. Large pupils are considered more attractive than small ones.

Dr_C's avatar

It’s caused by light reflecting off the retina of your subjects’ eyes. The retina is highly vascularized.. so it’s not directly a reflection of blood and is not a reflecion of the iris either.. it is light going through the iris and relfecting off the surface of the retina which happens to be red.

When it’s daylight red eye is seldom a problem. This is because in bright light the pupils in our eyes close down. This works like the aperture on a camera lens. By closing the pupils less light enters your eye and we are able to cope with bright conditions.

The plus side of this concerning flash photography is that, because the pupil is so small, the red retina can hardly be found by the flash. So, no red eye.

However . . . when it’s dark, our pupils automatically open up to let in more light. And here’s where the problem begins. Because our pupils are wide open there’s a larger target area for the flash to hit. And it does. It hits the red retina, illuminating it, and this is picked up by the camera.

But I told you there is a cure for this unfortunate side effect of flash photography. The “reduce” or “anti” red eye function which is almost certainly on your digital camera.

These functions work in a couple of different ways, but the aim is the same – to close down people’s pupils so that there’s a smaller target for the flash to hit. Smaller target = less red eye.

How to combat the effect‘in-camera’
One method – the camera has a secondary light or lamp that comes on prior to the photo being taken. This lamp stays on for about two seconds – enough time for those pupils to close a bit.

Once the lamp has been on for a couple of seconds, the flash fires and the photo is taken. And because the subject’s pupils would have closed a little, hopefully there won’t be any red eye!

Things to be careful about with this method? Well, let your subject know that there will be a light on for a second or so before the flash fires. Because they know what’s coming, they can be sure to smile at the right moment!

Another popular method of reducing the effect from camera flash is the strobe effect.

With this method the camera fires a burst of about four or five “pre-flashes” before firing the main flash. So how does this help? The effect is the same as above. The pre-flashes make your subject’s pupils close down before the main flash fires.

A smaller pupil means less chance of red eye.

Things to be careful about with this method of? Again, as before, let your subject know that there will a few flashes before the main flash. If not, they’ll be all smiles at the first flash, and will then be looking away by the time the main flash fires. Or they’ll look completely stunned because of all the flashes – like a rabbit caught in a car’s headlights!

The high-tech ‘in-camera’ method
There is a final method of reducing the effect in-camera, and it’s a very recent invention.

Instead of trying to avoid red eyes with pre-flashes and lamps the camera just takes the photo as normal, including all the red eye effect.

Then, before the camera writes the digital photo to the memory card it quickly analyses the picture you have just taken. There is software built into the camera that looks specifically for red eye.

The software knows how to identify a face. It then looks for eyes. And finally it looks for red in the eyes. If it finds it, it fixes it automatically.

This software fix is very similar to the sort you find in image editing software.

Reducing the effect – the best method
The best method of avoiding red eye isn’t something that’s available to users of compact digital cameras – moving the flash away from the lens.

You see, part of the problem with flash photography is that the flash and the lens are too close. When the flash fires the light comes straight back into the lens. To avoid this we can move the flash away from the lens.

When the flash is moved away from the lens, the light from the flash is reflected back from your subject’s eyes above the lens. This results in no red eye at all.

The only cameras that will allow you to do this are those fitted with a flash ‘hot shoe’. These are almost exclusively found on digital SLRs.

If you own a digital SLR – invest in a flashgun that can slot into this hot shoe, and all your devil-eye worries will be forgotten!

Another benefit of these flashguns is that they are more powerful than any on-camera flash, and the light they emit is more diffused. This creates a much less harsh feel to flash photography.

AstroChuck's avatar

Thanks, guys. I probably coulda found this out via Google but I’m feeling exceptionally lazy tonight.

Dr_C's avatar

too much?

AstroChuck's avatar

Nope. Just right.

XOIIO's avatar

O

M

G

!

astrochuck didn’t know something!! My world I’d shaken

IchtheosaurusRex's avatar

The spectrum of light emitted by flash photography reveals the demons that live in all of us. Be afraid. Be very afraid.

Sorry, Chuck, but you’re always quick with the snark, so I’m just giving some back.

AstroChuck's avatar

Land snark.

sandystrachan's avatar

Teh light from teh flash , reflecting off the blood vessels in the back of your aye ( eye )

XOIIO's avatar

(means is)

laaaa's avatar

i dont know how its caused, however i do know how to avoid it. look directly above the lens.

XOIIO's avatar

@laaa really?

And welcome to Fluther!

laaaa's avatar

thank you!

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