The red-eye effect is seen in many animals as well.
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.
Cause
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.citation needed
Certain medical and recreational drugs cause the pupil to dilate. Red-eye reduction techniques will be less effective when photographing people using such drugs.
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
A flash photo of a raccoon, showing the red-eye effect.
The red-eye effect can be prevented in a number of ways.
- 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.
- 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.
- Taking pictures without flash by increasing the ambient lighting, opening the lens aperture, using a faster film or detector, or reducing the shutter speed.
- Using ambient light and then
- digitally post-processing the image to increase its brightness, or
- Pushing the film development to increase the apparent film speed
- 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.
- Having the subject not look straight at the camera, instead look at the photographer's shoulder.
- Increase the lighting in the room so that the subject's pupils are more constricted.
- 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 addition, people do not look natural with small pupils, and direct lighting from close to the camera lens is considered to produce unflattering photographs.
Removal
Various graphics editing software packages have functions to remove red eyes from digital photographs. Some have automated this process, and can apply it to many photos at once. Red-eye removal is also built into many popular consumer programs like Microsoft Windows Vista Photo Gallery, Apple's iPhoto and The Gimp. The downside to fully automatic software-based red-eye removal is that it is only accurate to approximately 75%.[1]
There are several dedicated red-eye removal solutions available:
References
- ^ Raimondo Schettini and Francesca Gasparini and Fadi Chazli: "A modular procedure for automatic red eye correction in digital photos","Color Imaging IX: Processing, Hardcopy, and Applications" 5293, 139-147. SPIE, 2003
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