If you're in an indoor or darkened space, your pupils grow larger to let in more light. The pupils can't shrink fast enough to compensate for a camera's flash, so when that light reflects off the back of the eye, it causes red eye. Many newer cameras have flashes that flicker before the picture is actually snapped, giving the subject's pupils a chance to dilate and greatly reduce the effects of red eye. But chances are that you still have some older photos lying around that you'd like to repair. The Red Eye Removal tool offers an effective way to remove red eye, simply by changing pixels from one color to another.
To remove red eye from a photograph