Red-eye typically occurs when a camera flash is used to light a dark scene. This lighting effect occurs when the intense bright light of the flash passes through the pupil and reflects off the retina at the back of the eye ball. Without getting too technical on how the eye works only the red wavelength is allowed back through the lens of the eye, this then travels back through the pupil to the camera lens and is recorded on the film, giving the person red-eye.
The red eye effect can also occur in animals such as dogs, cats, birds or cattle, though the effects can sometimes be different. Red-eye is common in all animals, though yellow-eye can sometimes be seen in dogs, cattle can sometimes have blue or frosted eyes.
The only accurate way we have found to remove the discolouration is by hand, as we have yet to find any software based application that can remove red-eye correctly. The vast majority of software products tend to leave the eye a lot darker than it should be, so we edit each photo by hand to ensure a more accurate colour is achieved.