Why does red eye appear when taking pictures?

A person's pupil is like the aperture of a camera, and its size can be adjusted. When people are in the dark, their pupils will automatically enlarge to see the scenery clearly. When taking a photo with a flash, the pupil can't be contracted due to the instantaneous strong light, and the light is projected onto the retina through the pupil. The blood vessels on the retina are very rich, and the photographed eyes are red, which is often called "red eye". There is a "red-eye prevention" mode in the camera flash mode, and the icon is eyes. When taking pictures in this mode, the flash will flash a few times before taking pictures to stimulate pupil contraction, so that the red eye will not appear in the photos.

■ Practical skills of digital photography ■

★★★★★★ How to eliminate red eye ★★★★

Nowadays, many cameras say that red-eye phenomenon can be completely eliminated, and many digital cameras have red-eye elimination mode in their flashlights. Can this annoying phenomenon really be avoided? ※? Unfortunately, in fact, no camera can do this now. What they can do is to mitigate the impact of this phenomenon to some extent. Of course, some cameras do well, but the beauty phenomenon does exist.

The word "red eye" is actually aimed at people. When the flash illuminates the human eye, the pupil will enlarge to let more light pass through, and the retinal blood vessels in the photo will turn red. ※ If the color of the subject's glasses is dark, the degree of beauty phenomenon depends on the depth of pigment, so sometimes the beauty phenomenon is not particularly obvious, such as the human eye in the legend on the right.

Red-eye phenomenon in animals is different. According to experts, red-eye phenomenon is widespread in domestic animals. Pets' eyes usually have a reflective layer at the bottom of the retina, which can enhance their visual ability at night. Obviously, these reflective layers of the eye mask are different from the animal's own coat color, so the phenomenon will be different under the stimulation of flash. The black dog on the right has a green radiation layer, so it is not a red eye phenomenon but a green eye phenomenon. ※

The mixed-race dog in the left picture has a blue radiation eye mask, so it will have a slight blue eye phenomenon under the flash. Some animals, such as some dogs and rabbits, don't have the radiation layer of blindfolds, so they will have the same red eye phenomenon as humans under the flash. ※.

The phenomenon of human red eye is usually that when shooting in a dark environment, the pupil will expand to let more light pass through, so the blood vessels in the retina in the photo will be dyed red, but for animals, this phenomenon will occur even when shooting in sufficient light. ※.