Red eye is an immediately recognisable problem in photography, which results in one or both of the subject's eyes being blotted out by a large red dot, usually over the iris. It is caused by the camera flash bouncing off the subject's retina. If it's not corrected before printing, it can ruin the photo. Red eye removal is usually a simple process which can be done in-camera or during post-processing using one of many software programs.
Flash power In order to avoid red eye when a flash is used, the pupils need to contract. The only way that this can be done is if the main flash is preceded by shorter, sharper bursts of flash so that by the time the main flash is triggered, the pupils have contracted, resulting in a lack of red eye. This function can be pre-set in many modern cameras. Practical factors What causes red eye? The answer would appear to be the concentration of a powerful light on the pupil in an otherwise unlit environment. This can be easily avoided by ensuring that there is more ambient light available when the flash fires, which will cause the pupils to constrict. However, if this isn't possible, another tactic is to ensure that the subject does not look directly at the camera.
Software There are several software programs that deal with red eye reduction if the photographer has been unable to avoid it in-camera. Simple ones such as Microsoft Windows Live Photo Gallery require only a few quick steps to try to fix red eye. However, red eye correction isn't always straightforward, particularly if the eyes are dark. Other more specialised programs such as PhotoShop offer more time-consuming methods to fixing a red eye photo. How it is done In most programs, the red eye effect is most effective when the problem eye is isolated by means of a crop rectangle. The eye is then desaturated, which means that the colour is drained from it in order to make the red eye less obvious. PhotoShop requires creating a new layer over the eye and using an eyedropper to sample the correct eye colour. Once this is done, the correct colour is painted over the red eye, making it almost impossible to detect.