Why is there red eye when taking pictures at night?

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 ※ Now many cameras say that red eye can be completely eliminated, and many digital cameras have red eye elimination modes 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 phenomenon of beauty 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. ※ In the picture on the left, the shaggy dog has a blue radiation blindfold, so there will be 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. ※. There are many different ways to realize the red-eye elimination system, but all of them are based on the same idea: reducing the magnification of the pupil to reduce the light shining on the retina, thus reducing the red-eye phenomenon. ※. There are two common ways to eliminate red eye. One method is to emit bright light in the same direction as the lens. Another method is to start the flash before exposure, or shorten the flash duration. In fact, red eye can not be completely eliminated by the above two technologies, and there are other factors that are important to eliminate the phenomenon of weakened red eye, which are beyond the camera itself. Luck is one of the factors. Why do you say that? If the subject is facing the camera, you will be very surprised at the result. There is no red eye phenomenon at all, and it doesn't even need to be removed. ◇ The "green-eyed" black dog just seen in the picture above is fine now, and there is no red eye in the picture. This is because the dog's eyes are facing the camera when shooting, which you can understand in practice. If the red-eye elimination function is turned on, but the glasses of the subject are staring at other places at the moment of shooting, then the red-eye elimination function is completely invalid. From the photo on the left, we can see that the subject looks up when the flash is turned on. Although the red-eye elimination function is turned on, the result is still disappointing. The red-eye elimination system has no effect on the pupil. The result is this ■ Tips ◎ There are some things we should know when taking photos that need a flash. Whether the red-eye elimination function is turned on or not, the following techniques can help us effectively reduce the red-eye phenomenon ◎ and keep ourselves in a position with light sources. When you shoot, the subject's pupil is illuminated by ambient light. So it will not be amplified by strong light ◎ It is best not to shoot with a flash in a particularly dim place. After turning on the red-eye elimination system, try to ensure that the subject is aimed at the lens ◎ Try to shoot in a place with sufficient light, so that the pupil will remain in a natural state ◎ Use an advanced flash with adjustable angle. When shooting, the flash should not be parallel to the direction of the lens, but should move upward at an angle of 30 degrees with the lens. When flashing like this, it actually produces ambient light, which can also effectively prevent the pupil from being stimulated and enlarged. It is very difficult to create a suitable light source or judge what kind of situation has the best shooting light. Of course, both professionals and novices are experiencing the differences in these experiences and skills, and their experiences are constantly accumulating ■ The ultimate solution※ What should I do if I have taken a photo with red eye phenomenon? Haha, don't worry, use the ultimate solution now and adjust the saturation with software to achieve the purpose of eliminating red eye. This is our Photoshop solution 1, make sure not to touch the surroundings. Needless to say, the selection method in PS. 2. Use the corresponding function (menu command) to reduce the saturation of the selected area, thus eliminating red eye. The following is the absolute skill in this operation: if the subject is black or brown eyes, adjust the saturation of the three main colors (red, green and blue) to a value close to 0, thus eliminating red eye; If the subject has blue or green eyes, adjust the red saturation of the three main colors to 0, so that red eye can be eliminated. 3. Finally, choose the area around the pupil of the eyeball to add black, so it looks natural.