When the pupils in someone’s eyes appear unnaturally red in the photograph, it is referred to as red eye.
The cause of Red Eye
Red eye occurs when you use a straight-on flash, such as a built-in flash. The light actually goes into the pupil of the subject’s eye and bounces off the retina and back into the camera lens after being tinted red by the blood in the eye. (Pretty gross, huh?)
Minimize Red Eye in your flash lit Photos
Most digital cameras have a red-eye reduction mode. It works by firing a pre flash to cause the subject’s pupils to get smaller in reaction to the initial bright light, so that there’s less chance of reflecting light from the back of the eyeball during the
actual flash. This technique works, but it causes a couple of undesirable problems:
The preflash causes the subject to blink, and the end of the blink, when the eyes are reopening, occurs just when the real flash occurs and the shutter clicks. So the subject looks drugged, because the eyelids are half open.
This kind of shot increases shutter lag by a factor of two to three, because the camera has to wait for the preflash to complete before the flash and the shutter can fire—so you almost always miss the critical moment.
Your best bet is to turn off red-eye reduction and fix the red eyes in your image editor. The exception, of course, is when you’re making shots that you want to transmit immediately to another source or place on the Web.