HOW COME? What causes red eyes in photos

When you take a photo, why do people's eyes often look red instead of their real colors? asks a reader.When you look at your recent party pictures, are the birthday boy's eyes glowing crimson? Check out some photos of your friend in the sunny outdoors. If all you see are baby blues, you can safely rule out demonic possession.

There's actually a recipe for red eyes, which means there are ways to avoid the creepy photo effect: Eyes look red in a dim room, when a person is looking directly at the camera, as a flash pops.

Like blushing, the red-eye effect can be blamed on blood vessels. When a camera's flash goes off, bright light streams through the pupils, the eye's adjustable portholes. As the beam passes on through each eye's lens, the light is focused on the retina, in the back. Beyond the retina, the light passes into the choroid, a layer of connective tissue and blood vessels just under the eyes' outermost layer (sclera).

Here's where the red starts. White light contains a rainbow of hidden colors -- red, orange, yellow, green, blue, indigo, violet. When the burst of white light hits the choroid's crisscrossing blood vessels, the vessels absorb light from the blue end of the spectrum -- and reflect back red.

So while white light entered the eyes from the camera flash, crimson light shoots back out -- and into the camera lens. Result: those creepy red eyes.

Why don't we see the red-eye effect in photos taken outdoors, on a sunny day? In bright light, pupils are at their most constricted. So even when a flash goes off, much less light makes it to the choroid. And our eyes show their true colors.

But in the much dimmer indoors, our pupils are open wide, to admit every stray scrap of light. A burst of light will trigger a pupil-constricting reflex, but muscles in the iris must bunch up to squeeze the pupil shut.

When a flash goes off, the wide-open pupil doesn't have enough time to close. Bright, focused light shoots straight through, to the layer of blood vessels in back. (Since pale-colored irises have less melanin pigment to block light, light blue eyes look reddest in flash photos.)

But there's no need to don sunglasses for your next birthday party. Turn up the room lights, and try taking pictures without flash. Bounce an adjustable flash off a white wall or ceiling, instead of aiming it at the guests. Or use a camera designed to reduce the red effect, by firing off several fake flashes before actually snapping the picture. The idea: Pupils have extra time to shrink down, keeping light from bouncing off pesky blood vessels.