In low-light photographs of people with flash, red eyes are often visible. This effect occurs because the flash light is reflected off the retina.
Eye device and flash
In many animals, including dogs, cats, deer, the retina is covered with a special lining layer. It is called a mirror because it has reflective properties.
If you shine a light in the eyes of animals at night, you will see how they glow with a greenish or white light. This is because the pupil of the eye expands in the dark. Since the pupil transmits light, it hits the back wall of the eyeball and is reflected from the retina. Thanks to this arrangement, animals can see well at dusk. The mirror collects light, and the animal's eye, like a headlight, illuminates objects in half-darkness.
In humans, unlike animals, there is no lining on the back of the eyes. If you shine a light in the eyes of a person in the dark, then no reflection will be seen.
But the light from the camera flash is bright enough to get the reflection. What you see in the pictures are the blood vessels that feed the eyeball and the retina. The choroid of the human eye is very thick. Because the blood vessels are red, beams of red light are reflected off the retina and sent back through the pupil. During the picture, everything happens so quickly that the pupil does not have time to shrink, it lets in a lot of light, and the inner side of the eyes is visible in the picture.
Human eyes differ in structure. Some people have slightly more dilated pupils than others. Then their eyes in the pictures will be more red. There are people with a high content of melanin in the skin. In the eyeball, the layer of this pigment will absorb some of the light, thus avoiding the red-eye effect.
How to avoid red eye
Many cameras have a special anti-red-eye device. The flash is turned on twice. The first time - just before the shot. At this time, the pupil narrows from bright light. The flash fires a second time during the picture.
To avoid red-eye in the picture, you can turn on all the lights in the room. This will also constrict the pupil.
When the pupil is narrow, it lets in much less light into the eye. Thus, light is not reflected from the inside of the eyeball.
Before taking a picture, some photographers first turn on the flash several times to narrow the pupils of the subjects.
Another way to get rid of red eyes in photos is to move the flash as far away from the camera lens as possible. In small cameras, the flash is only a couple of centimeters from the lens. Reflection from the retina is directed directly at the lens. This makes the red-eye effect more noticeable.
If the flash is removable, you can move it as far away from the lens as possible to avoid an ugly shot. If it is possible to mount the flash so that it can bounce off the walls and ceiling, then this technique will work much better.