Red-eye can be touched up after-the-fact with pens and software, but it doesn't look completely correct. It's best to avoid it altogether particularly when shooting film.
How it happens
Red-eye occurs when light (primarily from a flash) reflects off the retina of the subjects' eyes at or near the axis of the lens.
When subjects are in low light, their eye pupils are required to open wider (dilate) to allow more light into the eye. This is also when most people tend to use a flash. So, the light from the flash moves from the camera to the subject and reflects directly off the retina.
The retina is a wide area located at the back of the subject's eyes and continuous with the optic nerve. It consists of several layers and a web of blood-carrying veins. Therefore, the color reflected from humans is red or occasionally brown (depends on light angle). Color varies with different animals depending on the shape and the natural filtration qualities of their eyes.
How to eliminate red eye
Increase ambient light
Instead of making a portrait in a dark room, take the person to a better lit area. The subject's pupils constrict and red eye vanishes most of the time.
Red eye camera programs
Cameras equipped with red-eye reduction systems emit a series of pre-flashes or a beam of light to try to cause the subjects' eyes to constrict. This works sometimes, but not well enough or often enough. The process is annoying to both the photographer and the subjects. Use one of the methods below instead. Everyone will be happier with the process and the results.
Change light angle
To eliminate red eye, change the light angle. Since light is restricted as it passes through the pupil, it only reflects in the direction of the light. The remainder of the retina remains unlit and thusly appears normal.
The fastest way for most PJs to change the light angle is to bounce the light off a neutral-colored ceiling or wall. With tilt head flashes, the head is pointed in one of these directions and red-eye vanishes.
The additional benefit is the light becomes softer and more even. However, watch out for colored walls because they change the color of the light to the color of the wall surface.
If the flash is integrated with the camera body, try diffusing the light with a handkerchief, gauze or other neutral material. Simply tape a small piece over the flash. Although red eye can still occur, the light direction may be scrambled enough to avoid red eye.
If nothing else, a diffused image looks much better - even with slight red eye.
This is a worst-case attempt at a solution. Parallax is the difference between the view of object through a camera lens and a separate viewing eyepiece.
The same occurs with light (light parallax). Since the flash is separated from the lens, the closer the photographer is to the subject, the more parallax occurs between the flash angle and the lens axis. If the photographer is close enough and the camera can handle close focus, red eye can be reduced.
The opposite is almost certain. As the PJ moves further from the subject, red eye is more likely to happen because the parallax allows the axis lines to intersect and overlap.
Change PJ angle
Since PJs cover events and try to be unobtrusive, they normally move around and can decide when and from what angle to shoot. If PJs move to the side of subjects rather than directly facing them, the likelihood of red eye is reduced from an oblique angle.
Shooting from an oblique angle to the eye opening restricts the amount of light actually bouncing off the retina. Since red eye happens mostly when the flash is shot directly into a person's eye through a wide, circular pupil opening, it's less likely to occur when the circle becomes a tight oval (i.e. from the side).
Although it's not a guaranteed method to eliminate red eye, it helps.
Everything from here onward involves additional hardware. Most of these items are listed in the PJ Candy Store.Get the flash off camera
This is the preferred method. A through-the-lens (TTL) sync cord is about $80 and worth every penny. It allows PJs to control light volume and angle. I need to do a whole entry about how important it is to have the flash off camera. For now, we'll say this is best.
Night sports are notorious for red eye. PJs must use long lenses and powerful flashes in low light to capture fast action across the field. As the lens angle and the flash angle reach further to a subject, the angles are more likely to intersect and cause red eye.
If PJs are shooting players' faces at a considerable distance in low light, red eye is almost certain. Using a combination of suggestions above is helpful. For example, PJs might diffuse with softboxes at close range from an oblique angle.
As insurance, many PJs use flash brackets. A flash bracket is a metal or durable plastic frame equipped to mount both a camera and a flash. It increases the distance from the flash axis to the lens axis and increases light parallax angle.
Because PJs want the flash to hit the exact area in which the lens is viewing, the bracket locks the light and lens angles together to allow maximum light coverage. This increases the likelihood of red eye at greater distances, but reduces it at a normal distance even in low light (because light parallax is less).
Telescopic flash brackets are most useful for longer lenses. The further apart the PJ can mount the light from the lens, the higher the parallax angle is. Telescopic flash brackets extend mount sections to as much as a meter above the camera lens. As with tripods, the bracket becomes less stable the further it's extended. This does become more problematic when action goes out of bounds and lands on the PJ, but it's an issue each PJ must address.
When using most common flash brackets, see if the manufacturer makes camera-specific anti-twist plates to keep the flash and camera aligned.
Light the whole venue
PJs with Pocket Wizards and hand-held flashes or studio strobes (400WS minimum) commonly cross light gyms.
It can also be done with an outdoor field. This isn't a good option for most PJs. It takes a lot of work, time, weather cooperation and deadline flexibility. But, I've done it before. I'll probably do it again.
Undermount the flash
I haven't tried this because I prefer shadows to go down, many sports shooters mount their flash on a monopod below the camera with a Super Clamp or similar bracket. It achieves the same red-eye elimination as an extended flash bracket, but is more stable and mobile.
If choosing this option, make sure flash is set below ambient light (use as fill) so the shadows aren't as obvious. Low flash angle typically produces a horror-movie look and doesn't work well with face mask patterns.
Let's say you want someone to have red eye (for an illustration
Enough for now,