What causes red eyes in photographs
Why do you have red eyes when looking into the camera while taking pictures with the flash?
Because the retina - because of its strong blood supply - is red and the light emitted by the flash is reflected by the retina and beamed back towards the camera.
In principle, the following applies: the more acute the angle of the flash-eye-lens, the higher the probability that the light reflected in a frontal shot will hit the lens exactly.
In addition, the angular size of the emerging cone of light depends on the size of the pupil at the time of exposure. The larger the pupil (and this is extremely dilated in the dark), the greater this angle. In the worst case scenario, you are photographing with a very acute angle of the flash-eye lens and the subject's pupils are wide open. In this case, the so-called »red-eye effect« almost inevitably occurs. As soon as it hits the strongly perfused red retina, the flash takes this color "with it on the way back" to the camera lens.
What can be done to avoid this effect?
One method is to change the angle ratio. The closer the photographer gets to his subject, the greater the angle of the flash-eye-lens, and the greater the chances that the light cone will not hit the camera optics. However, the recording situation, especially with snapshots, cannot always be influenced in the desired way.
The second option would be to increase the distance between the flash and the camera optics, for example to position the flash significantly higher and thus increase the angle. But here, too, it is true that in most cases this will not be feasible. Most compact cameras, for example, only have a built-in flash so that the distance to the camera lens cannot be changed.
The simplest solution to the problem is therefore to reduce the size of the pupil of the person portrayed. This can be done, for example, by increasing the brightness of the room. If this is not desired or not possible (for example at night in the great outdoors), many cameras have an "anti-red-eye flash mode" that does something similar. The camera flashes one or more times at the subject shortly before the actual picture is taken. The pupil, "startled" by so much light, contracts, the light cone reflex described above narrows in the best case to such an extent that it misses the camera optics - and we look into beautiful green-brown eyes on the finished image.
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