Red-eye occurs when light from the built-in flash of a camera bounces off the subject's retinas and reflects back to the camera lens. Along the way, the light takes on the tint of the blood vessels in the eye, which causes the eyes to appear to be glowing red in the picture. Below are some tactical tips to help you to avoid red-eye in your digital pictures.
When inside, turn on as many lights as possible. With the additional light, The persons eyes will constrict a little, so less flash will be reflected back to the lens. Most cameras also adjust flash output based on the ambient room lighting, so the brighter the room, the weaker the flash needs to be.
If you're shooting indoors , position the person next to a window. The light coming in through the window will have the same effect as turning on additional room lights.
Change the camera settings to red-eye reduction mode. With red-eye reduction mode on, the camera fires a short, preflash in advance of the main flash. The idea is the same as turning on lots of lights, the eyes constrict in response to the preflash so that when the main flash fires, less light is reflected from the eyes. Remember, that it's called red-eye reduction and not red-eye prevention mode for a reason: That short preflash can do only so much, so you may still wind up with some red-eye areas. Be sure to tell people to expect two bursts of light. or, they will think that the preflash is the real flash and assume that the picture was taken. Some cameras actually fire three lights for each shot. The third , which the camera sends out when you depress the shutter button halfway, helps the camera's autofocusing mechanism pinpoint the subject-to-camera distance.
Consider posing people so that they're not looking directly into the camera. A profile shot can be as captivating as a regular image. You can also ask people to look to one side or slightly up or down. Since the flash light won't be heading straight for the eyes, red-eye reflections will be minimized.