Why do people's eyes reflect red in dim light?

If you have ever used a flash to take pictures, you must have encountered the phenomenon of "red eye". Take the increasingly popular digital camera as an example. When you take a picture with a flash and send it to the computer, you find that the subject's eyes are all red and look like the devil in the game. What is the reason?

In fact, "red eye" is caused by flash reflection of retinal blood vessels. In low light, the pupils are good.

It is very large, allowing light to enter the retina and then reflect back to the camera lens, which also leads to "red eye". Different cameras have different chances of "red eye" because of different designs. Generally speaking, the closer the lens is to the flash, the greater the possibility of "red eye", and the photographer usually uses a long focal length to increase the chance of "red eye". Nowadays, many digital cameras also consider the problem of "red eye" in their design. At present, the new product will provide users with the function of reducing and preventing "red eye". In addition, now digital camera manufacturers also make the distance between the lens and the flash very far, thus preventing "red eye" to a great extent.

Generally speaking, the simplest solution is to turn on the red-eye reduction flash on the camera, which can reduce the pupil of the subject in the first flash and take normal photos in the second flash. In addition, there is a passive way to solve the "red eye". You can let the photographer stare at the brighter light source or turn on the light more first, which can narrow the photographer's pupil and avoid red eye.