MadSci Network: General Biology

Re: Why do eyes sometimes appear red in photographs?

Date: Thu Jul 2 07:47:15 1998
Posted By: David Beck, MadSci Admin
Area of science: General Biology
ID: 895716492.Gb


Red Eye is caused by the flash on the camera rebounding off the back of your pupil. Your pupil does not absorb the red light, but absorbs the other wave lengths of light that are produced by the flash. Thus your eye appears read. This is the same thing as why grass is green, the plant absorbs all the other wave lengths of light except the one that looks green.

Now how to get rid of red eye. One way is to get a camera where the flash is farther away from the lens. This is simple physics. Imagine throwing a tennis ball straight at the wall, it comes back to you, the same with a flash, the closer it is to the lens of the camera the more likely you will have the red light reflecting back into the lens of the camera. But if you throw the tennis ball at an angle it will never comeback to you. That is why you see professional photographers using flashes that don't point directly at you, but point in some other direction.

Another way to reduce red eye, is to make the pupils smaller. This is done by making the room brighter. When the room is bright the pupil contracts and thus fewer waves of light reach the pupil and have the opportunity to reflect of the back of your eye.

Best Wishes, David Beck

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