The eye's pupil and a camera's shutter function similarly by permitting light to pass through them. In a camera, the shutter allows light to pass through a lens and onto film where the image can be recorded. The eye's pupil opens to the translucent jelly filling the eye. Light permeates the jelly and reaches the retina via the pupil. The brain interprets nerve signals received from the retina as sight. The flood of light delivered by a camera flash produces temporary changes in the retina's visual function.
Impaired Color Perception
The retina contains two types of specialized cells that respond to light. These cells, or photoreceptors, are called rods and cones. Numbering more than 120 million, the rods function best under low light conditions and facilitate night and peripheral vision. The cones, numbering approximately six million, perceive and recognize colors. Cones need bright light to function. A camera flash saturates the cones in the retina, leaving them temporarily unable to perceive color accurately.
The perception of colored lights can result from the saturation of the cones by a camera flash. The phenomenon intensifies when looking directly into the flash. The cones are temporarily overwhelmed by the significant influx of light and require a few moments to recover. Although the effects of a camera flash are short-lived and harmless, looking directly into the sun or other bright objects for sustained periods can cause permanent damage.
Appearance of Red Eye
Light from a camera flash enters the pupil and bounces off an area located at the back of the retina, called the choroid. The choroid contains of a rich network of blood vessels. Because the pupil does not have time to contract before the picture is taken, a red reflection of the back of the eye, or "red eye," is seen in the picture. Not everyone is equally susceptible to red eye. Individuals who are not directly in line with the flash will not exhibit red eye. Also, a layer of melanin exists inside the eye called the pigment epithelium. Red eye may not be seen if the photographer's subject has a dense pigment epithelium, as it absorbs some of the flash's light. Natural differences are observed among individuals in how wide the pupil appears in varied levels of light. People with smaller pupils are less likely to exhibit red eye than those with wider pupils.
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