Red-eye effect

The red-eye effect in photography is the common appearance of red eyes on photographs taken with a photographic flash when the flash is too close to the lens (as with most compact cameras). This is not to be confused with an actual red coloration of the iris.

Causation
The light of the flash occurs too fast for the iris of the eye to close the pupil. Light passes through the blood-rich area alongside the iris (called the choroid) and then strikes the retina. Some of the light is reflected back out through the iris. The camera records this reflected light which has now passed twice through the reddish choroid. This principle is used in fundoscopy, an examination of the retina with an opthalmoscope, wherein a positive reddish-orange reflection is a normal finding.

The effect is generally more pronounced in people with grey or blue eyes and in children. This is because pale irises have less melanin in them and so allow more light to pass through to the retina. Children, despite superficial appearances, do not have larger pupils but their pupils are more reactive to light and are able to open to the fullest extent in low light conditions.

Certain medical and recreational drugs cause the pupil to dilate. Red-eye reduction techniques will be less effective when photographing people using such drugs. The alkaloid belladonna was so named because a large pupil was supposed to increase the beauty of the human face.

If photos or videos are shot with infrared-sensitive equipment, the eyes also usually look unnaturally bright. The reason is the same: the blood-rich retina reflecting light into the camera.

In many species the tapetum lucidum, a light-reflecting layer behind the retina which improves night vision, intensifies this effect. This leads to variations in the color of the reflected light from species to species. Cats, for example, display blue, yellow, pink, or green eyes in flash photographs.

White eye
Retinoblastoma is a cancer of the eye which often causes the appearance of a "white eye" effect instead of the expected red eye.

Prevention
The red-eye effect can be prevented in a number of ways.
 * 1)  Using bounce flash in which the flash head is aimed at a nearby pale coloured surface such as a ceiling or wall or at a specialist photographic reflector. This both changes the direction of the flash and ensures that only diffused flash light enters the eye.
 * 2)  Placing the flash away from the camera's optical axis ensures that the light from the flash hits the eye at an oblique angle. The light enters the eye in a direction away from the optical axis of the camera and is refocused by the eye lens back along the same axis. Because of this the retina will not be visible to the camera and the eyes will appear natural.
 * 3)  Taking pictures without flash  by increasing the ambient lighting, opening the lens aperture, using a faster film or detector, or reducing the shutter speed.
 * 4)  Using ambient light and then
 * 5) *digitally post-processing the image to increase its brightness.
 * 6) *Pushing the film development to increase the apparent film speed
 * 7)  Utilising red-eye reduction capabilities built into many modern cameras. These precede the flash with a series of short, low-power flashes triggering the iris to contract.
 * 8)  Having the subject not look straight at the camera, instead look at the shoulder of the photographer.
 * 9)  Increase the lighting in the room so that the subject's pupils are more constricted.
 * 10)  Some computer digital image editors have the ability to lessen the red eye by adding a hint of blue to it.

If direct flash must be used, a good rule of thumb is to separate the flash from the lens by 1/20 of the distance of the camera to the subject. For example, if the subject is 2 metres (6 feet) away, the flash head should be at least 10 cm (4 inches) away from the lens.

Professional photographers prefer to use ambient light or indirect flash as the red-eye reduction system does not always prevent red eyes, for example if people look away during the pre-flash. In any case, people with small pupils do not look natural on photographs. Many people also find the pre-flashes annoying. By coincidence, lighting which produces red-eye effect is also believed to produce very unflattering photographs; hard and flat lighting is considered something to avoid.

Removal
Various graphics editing software packages have functions to help remove red eyes from digital photographs, although it is usually a multi-step process. A few specialized software packages have partially or fully automated this process, which some software can apply to many photos at once. The downside to fully automatic software-based red-eye removal is that it is accurate only about 75% of the time.

Manual
You can use Adobe Photoshop to remove red-eye defect using the following steps:
 * Choose Ellipse tool (М) and mark red-eye region
 * Choose (Ctrl+U) "Hue/Saturation" tool
 * Adjust pupil color

Semi-automatic

 * bitmap editors: Microsoft Picture It! Foto, Corel PHOTO-PAINT, Microsoft Office Picture Manager, iPhoto and others have special red-eye removal tool.


 * specialized software: Remove Red Eye, Red Eye Pilot, ArtEyes etc.

Automatic

 * specialized software: StopRedEye!