Science Forums 2

[jab2]

Originally Posted by freeztar

Why do dilated pupils cause "red eye"?

Due to the many blood vessels in the retina, it will display a red colour when illuminated. If the iris is wide enough and the flash/lens axis is of very small angle and strong enough, one can actually see this red colour displayed as "red eyes" in a picture. This effect is not normally visible as normal illumination (sun, roof lighting etc) is never from a source that shines horizontally and would thus not reach the retina in the back of the eye. The one cure for “red eyes” is to force the iris to contract by giving a pre-flash, thus forcing an reflectance angle that is smaller than that between the flash and lens axis as described by Alexander. This however have the disadvantage that people react to the pre-flash and might have "lost" their “pose” by the time the exposure is made. A much better solution is to move the flash off-camera, (only possible with high end cameras) thus increasing the flash/lens angle outside the cone of reflection of the retina through the iris. This have an added advantage that the resulting photo will have much more depth and natural looks as the off-camera flash will cast slight oblique shadows due to the greater flash/lens angle. This is particularly true for faces. In normal on-camera flash one get a very flat looking photo as the flash do not cast visible shadows due to narrow angle.