Red-eye effect

With Red -eye or Red-eye effect is called the appearance of red pupils in images of creatures that were photographed with flash.

Cause

The effect occurs when the flash is almost the same axis mounted with the lens, so there is close to the optical axis. It is caused by the reflection of the flash by the strong blood red eye's retina. If the person looks directly into the lens, the effect is particularly prominent, but generally occurs whenever the light reflected from the retina flashlight falls back in a straight line into the lens.

Avoidance

The effect can be influenced by means of various methods of flash. As a workaround, move the flash as far as possible from the optical axis out. This allows, for example, the use of a side-mounted next to the camera flash stick or techniques such as the slave flash in which the flash units can be placed anywhere in the room; disadvantageous here are enlarged Shadow. Alternatively, the flash can also be " diverted " so that it does not fall directly on the eye of the subject of the photo. This is achieved for example by turning the flash towards the ceiling. Another way perform more light the object or the person, the use of other light sources.

The problem does not normally occur in professional flash photography or when using a studio flash, as are usually placed far away from the optical axis of the light sources here. In the amateur photography, the effect is, however, more frequently, for example when using the built-in mini flash a compact camera or when plugging a Compact flash to the hot shoe of a camera.

Modern cameras and flash units often try to reduce the red -eye effect by a pre-flash. By this, the pupil opening of the eye is reduced in size and reduces the reflection of flash light on the retina. However, the reduced pupil is in the interest of achieving a pleasing portraits undesirable, since it is known from behavioral biology and psychology of perception that images of people with small pupils are perceived as less likeable and friendly. A pre-flash can effectively prevent red eyes only if the person is not looking directly into the lens or the flash.

Eliminate the red-eye effect

In the image processing can be recolored relatively straightforward in both the photo lab by darkening and by aid of digital image processing of the red-eye effect.

The figure demonstrates four phases of editing with Photoshop:

Note

Not primarily occurs in a newborn baby or toddler the red-eye effect one or both sides on, but is instead to see a bright white pupil, which may indicate the presence of retinoblastoma ( a malignant eye tumor ) or a horror be stars of the newborn. Only quick action remains a chance of obtaining the vision in the affected eye.

Animals

In a number of animal species, there occurs a similar effect. In animals that need to see well in the dark, the retina of the eye is covered with a reflective layer, so that the reflected light on the way back has a second chance to trigger a nerve impulse in the visual cells. The remaining light passes through the pupil again and leads to highly luminous eyes. Particularly well known is the phenomenon of cats, so light reflectors are also called cat eyes.