Why do my eyes look eerily red in some photos?

Am sure this has happened to all of us: When posing for a photo, we smile heartily. That is, initially. But the photographer does not click. He keeps twiddling with the controls, awaiting the right moment, which would ALWAYS arrive just after we stop smiling and literally start saying cheese, in a meek attempt to sham a smile.

After all this rigmarole, we rush over to the photographer to look at the picture. And our eyes look as red as #FF0000.

This phenomenon is called "red-eye" (now, you didn't guess that, did you?).

Why does it happen?

Visualize a room having a window with curtains. The wall opposite to the window, is red in colour. The curtain can be opened or closed to control the amount of light passing through the window.

The human eye has a very similar structure. The wall is the retina, which is red because it is rich in blood cells. (For the girls, the shade of the colour is what you would call "blood red")

In front of Retina the Wall (akin to Rahul the Wall) is the window with the curtain - the iris. It is designed to let in an optimum amount of light. So in very bright conditions, the iris will close as much as possible, allowing little light to enter the eye. In dark conditions, the iris will open as wide as possible to gather all the light it can grab. (So the iris is similar to us in being discontent with what it gets by default)

When we take photos in low-light conditions, the iris of our eyes would naturally be wide open. When the photographer clicks, a blinding flash appears..

And everyone dies..

No, I mean, the flash is fired from the camera. The white light from the flash falls on the eyes of the people posing for the photograph. (On other parts of the anatomy as well. But we are interested in the eyes only. For now). This light is of very high intensity and is fired at very short notice. This short duration is not sufficient for the iris to close completely. So by the time the camera stores the photo, the iris is still wider open than it should be, meaning the retinal wall is still largely visible. And remember, it is red in colour. This is what is reflected as red-eye in photographs.

How do we avoid red-eye effect then?
After a picture is taken, one can remove red-eye by software means. There are so many tools that do this.
But a way of eliminating red-eye from occurrence itself is called "advanced strobing". Most new digicams have this feature. If you set "Red-eye reduction" to "on" and try taking a picture in low-light conditions with flash, the flash would be fired twice. The first time is just to trigger the iris into closing, and the second time is to take the actual picture. So by the time the picture is taken, the iris would have closed and blocked the (literally) bloody retinal wall.

So the next time someone wonders why red-eye occurs in photos, you would be able to tell them....
.... where to look for info right? (He he.. My Google analytics reports can do with more numbers)