In animal, eyes often have a white, yellow or green glint. Red eye effect occurs when light from the flash bounces off the eye of a person or pet and reflects back to the camera lens.  Red eye is often occurs when using a built-in digital camera flash. Because a built-in digital camera flash is very close to the camera lens.

Different photo editing programs embedded a red eye correction filter, but this causes your photograph object to appear abnormal. These filters also do not work on the green eye effect.  The best way is to avoid the demon-eye effect when you taking photo.

Almost digital camera in the market embedded with a red-eye reduction feature. You can turn off or turn on this feature. This feature works by the camera fires a brief, pre-flash light at your subject before you snap the picture. The pre-flash light causes the object’s pupil to close when the main flash fires by take the shutter, less light is reflected from the retinas. This reduces the chance of red-eye but though that it's called red-eye reduction. This method not red-eye prevention mode, so you may still some red-eye

By avoid to direct flash from the camera so you do not directly take main flash on object’s eyes. Using a slave flash unit that is designed to work in a built-in flash or bouncing main flash off a nearby wall or other object it is reduce the chances of making demon-eye effect.