Red-eye effect

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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).

Contents 1 Causes of red-eye 2 Preventing red-eye 3 White eye 4 References

[edit] Causes of red-eye

The light of the flash occurs too fast for the iris of the eye to close the pupil. Light is focused onto the blood-rich retina at the back of the eye and the image of the illuminated retina is transmitted to the camera resulting in the red appearance of the eye on the photo. (This principle is used in the ophthalmoscope, a device designed to examine the retina.)

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.

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 colour of the reflected light from species to species. Cats, for example, display blue, yellow, pink, or green eyes in flash photographs.

[edit] Preventing red-eye

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
   • digitally post-processing the image to increase its brightness.
   • Pushing the film development to increase the apparent film speed
5. 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.
6. Having the subject not look straight at the camera, instead look at the shoulder of the photographer.
7. Increase the lighting in the room so that the subject's pupils are more constricted.
8. 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.

Various graphics editing software packages have functions to help remove red eyes from digital photographs although it is usually a multi-step process. More recently a few software packages have made this function a fully automatic process which can be applied 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.^{[citation needed]}

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.

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

[edit] References

• Fixing red eye manually and automatically using Photoshop and Photoshop Elements
• Red eye and how to avoid it on the Photocritic blog