What's the reason for red eye in photos? It only comes sometimes. Does it depend on the distance between subject and camera? Or light adjustment? I took 4 photos, some of them have red eye and some are don't. How can I avoid this? And what is a good, free tool to remove red eye?
The colour comes from the blood in your eye.
When light rays, from a flash for example, enters your eye, it hits the blood vessels and is reflected back to the camera, appearing red.
We all know how horrible that looks.
Understanding the cause, we can avoid it. For example, red-eye occurs when light enters straight into the eye and bounces right back out, this means the light is traveling in the same direction that your lens is pointing, and at a very close axis too.
If you position the flash higher-up, the reflected ray will hit somewhere else because you have created an angle. As long as it does not hit the sensor of the camera it won't appear.
If the iris of the eye is small, this can somewhat prevent red-eye, compared to when the iris is widely opened.
That's also why on compact camera, the method used is to blind them with pre-flash before taking the photo. This causes the subject's eye to cut down the amount of light entering: shrinking the iris opening.
Red eye is caused by strong light reflecting from the blood vessels on the back side of an eye, and those reflected rays reaching back out via iris. For this to happen, the iris needs to be wide open (typical in low-light situations) and the strong light needs to be coming from the same direction as camera (typically that's a direct flash, coincidentally also used in low-light situations). The iris contracts after experiencing the strong strike of light, so a subsequent shot would likely not show any red-eye - the smaller iris means there's less light getting in, and even less reflected light getting out.
That's exactly how red-eye reduction works - by conditioning the eyes of victim (um... subject) with pre-flashes so the eye would have contracted by the time of main flash. Another options include using off-camera flash, bounce flash or rely on ambient/constant lighting.
Red-eye removal is often done by turning the red areas into black-and-white. So any tool where you can select an area and change its color saturation will do. Most advanced image editors (e.g. IrfanView, Gimp) offer a specialized tool that will only affect the red areas, thus allowing you to be less careful when selecting the red eyes.
Red eye is caused when light, almost always from flash, bounces off the retina of the eye and reflects back to the camera. Because the retina has lots of blood in it, the light appears red.
In case any one is uncertain of the anatomy of the eye, here is a (very rough) diagram:
And that should say "vitreous humour" rather than "vitrious humour"
To not have red-eye, move the flash off the camera. Use what is typically called a "strobe flash". All of the manufacturers make them.
You didn't say what camera you are using. Whatever you use, the popup flash is too close to the lens/sensor axis, so you get the red-eye that others have described. Thus, the solution is to not use the popup flash. There is a huge amount of information on this and related topics on the Strobist site: http://strobist.blogspot.com/