We had this topic on our site once before, but it seems to be a common question. Red-eye is something we all get once in a while. The problem is caused by the camera flash and the room light level.
The red you are seeing is the reflection from the inside of the back (retina) of the eye. In effect you have photographed the inside of the eye. Why cats and some dogs will have a green, blue, or even yellow reflection is something that I can't answer, but it is caused by the same problems. Because the iris of the eye is open too wide and the flash is too close to the lens of the camera, the light from the flash goes right to the retina. This is almost always the case when taking pictures indoor with a camera with a built in flash. The following should help you some. See fig.# 1.
Many of the newer small cameras with bult in flash have what is called "red-eye" reduction. What these cameras do is fire several small bursts of flash before the main flash fires, or they shine a small bright light from the front of the camera. In either case the result is the same. The iris or pupil of the eye contracts before the actual photgraph is taken thereby reducing the chance of the flash bouncing off the inside back of the eye. Or the early addition of light decreases the size of the red spot rendering it less noticeable. If your camera doesn't have the "red-eye"reduction feature, you can reduce the chances of red eye by turning on the room lights. This will cause the pupils of the eye to contract and have the effect described above.
If you have a camera with a detachable flash, then you can solve the problem. Buy or make a camera bracket that will mount and hold the flash several inches from the lens. Then the angle of the light will cause any flash entering the eye to be trapped and not reflected directly back to the camera. You could also go one step further and turn on the room lights, when possible, to force the pupils to contract a bit more. This should give you more pleasing looking eyes.
