Red-Eye in Flash Photos

What is red-eye, and why does it happen?
We�ve all seen it in snapshots we�ve either taken ourselves, or been shown by others: the pupils of your subject�s eyes are lit up in a devilish bright red color, unlike anything we�re used to seeing in real life. And of course, you never saw it in your camera�s viewfinder when you took the picture.

What is happening is that the pupil of a person�s eye, even though it appears to be black when we view it, is actually clear. Since the inside of the eyeball is normally never illuminated, we see it as a small black circle. But in some flash photos, if the flash is close to the lens of the camera, its illumination sends a burst of light right through to the rear of the inside of the subject�s eyeball. The retina of your subject�s eyes is rich in red blood vessels, and this produces the bright red color we see in the pictures.

If the flash is close enough to the lens, the light not only enters your subject�s inner eye, but can then reflect straight back out and actually be recorded by the camera. With the retina in your subject�s eye now fully illuminated, you get the tell-tale bright red glow that we all now know as "red-eye." So the first thing to know is that for all intents and purposes, red-eye in photos is a flash phenomenon. It�s very unlikely you�ll encounter it using sunlight, or available light only when shooting indoors.

Using the built-in flash:
Cameras like the Canon EOS Digital Rebel XT and XTi have a built-in flash, that pops upwards an inch or two for easy and convenient flash photography � at least, as long as your subjects are not too far away. And nearly every compact digital camera has a built-in flash as well. Since many of these cameras are so small in size, the designers are often forced to put the built-in flash nearly right next to the lens. Any time you use a built-in flash that�s close to the lens, you run the risk of occasionally getting red-eye in any pictures with people in the scene.

Obviously, you can�t physically move the built-in flash away from the camera lens (well, you could, but you wouldn�t be able to take pictures with it any more!). So manufacturers have resorted to another feature: Red-Eye Reduction.

Red-eye Reduction:
The idea of this feature is that by one means or another, the camera sends out light to your subject before the picture is taken. Since the subject�s pupils normally tend to dilate, or get wider, in dim light (and get smaller in bright light conditions), the idea is to reduce the level of red-eye by making the user�s pupils appear smaller before the picture is taken. With smaller pupils, there�s less area to be marred by red-eye, and even if it does show up in your pictures, it�ll tend to be less noticeable.

Some cameras do this by rapidly flashing the flash unit with "pulses" of flash illumination, for a second or two before the picture is taken. The hope is that your subject�s eyes will react to these rapid pops of light and the pupils will become smaller. A moment later, that actual picture is taken, with the flash firing again, but at (usually) greater power.

Canon�s cameras take a different approach, using a small, built-in lamp to shine continous white light directly at your subject for about two seconds, prior to the picture being taken. In fact, a scale appears on EOS digital SLRs with this feature in the viewfinder, to count down the two seconds that this red-eye reduction lamp is active, so that you know when to fully press the shutter button and take the picture. Again, the idea is that this added light before the picture is taken will cause the subject�s pupils to contract, and therefore reduce the level of red-eye if it appears.

It�s important to understand that neither of these approaches to in-camera red-eye control ever claim to completely eliminate the chance of getting visible red-eye. As the feature�s name says, it�s red-eye reduction, not red-eye elimination.

Using accessory speedlites:
Since an accessory flash unit (like Canon�s Speedlite 430EX, or 580EX) is larger and sits on top of the camera, the flash unit itself is raised several inches above the lens. The flash-emitting part of these units is nowhere near as close to the lens as in the case of a built-in flash. This distance between lens and flash means that right from the start, you�re less likely to get red-eye in your pictures. Let�s be clear: we�re not saying you won�t or can�t get red-eye in pictures taken with an accessory flash unit, only that because it starts out being a few inches farther from the lens, you can often avoid the probem in your shots of people.

more : canon

Canon : Tips & Techniques