Camera flashes do not cause your pupils to dilate. Your eyes are constantly red on the inside. The bright light from the camera flash only highlights the colour. The inside of your eye is a hollow ball filled with clear fluid. The pupil, the hole in the front of the eye, allows light to enter the empty region inside the eye. The light goes through this area and strikes the retina, the inner rear surface of the eye. The retina is densely packed with cells that detect light, convert it to electrical signals, and send the information to the brain, where the light pattern is seen as a visual image.
The retina is densely packed with cells that detect light, convert it to electrical signals, and send the information to the brain, where the light pattern is seen as a visual image. The human retina’s about 100 million light-sensitive cells provide incredible visual resolution, but they also require a lot of blood to keep them working. The crimson colour of the retina comes from this blood.
When you stare through a hole in someone’s pupil, you’re actually looking at their retina. The red hue of the retina appears black under normal lighting circumstances because the chamber of the eye works like a dark cave that absorbs most of the light. The redness of the retina becomes more visible when the light is bright enough. A camera’s flash of light is bright enough and oriented enough to highlight the redness of the eye. The outcome is red-eye in pictures, which is all too common. However, the camera’s flash does nothing more than provide more light. In reality, the redness of the human retina can be seen without the use of a flash or camera.
Place a friend in front of a standard light bulb so that he is facing it. Place yourself between the bulb and your friend, such that your shadow falls just below his eyes. Now look at his pupils, which will be brilliant red. Other lights (including sunlight) must be turned off or blocked, and your friend’s pupils must be widely dilated for this to function. Turning off all lights for a minute or having him close and cover his eyes for a minute can cause his pupils to dilate. When you open his eyes with the light on, there will be a brief period before his pupils contract, which is when you can see the red eye the finest. If you don’t notice a very dark, crisp shadow of your head directly below his eyes, then the lighting isn’t set up correctly.
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