Red-Eye

Red-eye is the red glow that fills people’s pupils in a flash photo. It happens when the flash sits so close to the lens that light going into the eye comes straight back out into the camera, carrying the color of the blood at the back of the eye.

It matters because it ruins otherwise good pictures of people, and because it is almost entirely a matter of geometry. Once you see the triangle between the flash, the eye and the lens, you can predict when it will happen, prevent it without any special mode, and know why the usual advice sometimes fails. The cat above shows the animal version of the same geometry: a phone flash a centimeter or so from the lens, a dark room, and eyes that light up.

Where the red comes from

The pupil is a hole. In a normal photo it looks black because almost no light comes back out of the eye toward the camera. A flash changes that. Its light passes through the pupil, crosses the eye, and reaches the back wall. The retina, the light-sensing layer, is nearly transparent. Behind it lies the choroid, a layer packed with blood vessels that feeds the retina. That is what reflects the red.

The eye’s own lens then does something unusual with that returning light. Because the eye is built to focus incoming light onto one spot at the back, the light that scatters back out is sent mostly toward where it came from, in a narrow cone. Road signs and bike reflectors work the same way, which is why they glow in headlights. If the camera lens sits inside that returning cone, the pupils record red. If it sits outside, they stay dark.

Two things set how strong the glow is. The first is pupil size. A wide pupil lets in more light, lets more back out, and makes the returning cone wider, so dim rooms, where pupils open up, are where red-eye lives. The second is pigment. Melanin in the layer just behind the retina absorbs light before it reaches the choroid. People with light-colored eyes often have less of it, and young children have large pupils and less pigment behind the retina, which is why they show red-eye most strongly. Red-eye says nothing about exposure. A perfectly exposed photo can have it, and a badly exposed one can be free of it.

Wondering why a flash right beside the lens also makes faces look flat? Direction of light explains what light from the camera’s position does to shape, and what moving it to the side gives you.

The angle at the eye decides everything

Picture three points: the flash, the subject’s eye and the lens. The angle at the eye, between the line to the flash and the line to the lens, is the number that matters. When it is wide, the returning cone of red light misses the lens. When it shrinks to a few degrees, the lens falls inside the cone and the pupils turn red.

That angle depends on only two things: how far the flash is from the lens, and how far the subject is from the camera. Bigger gap, wider angle. Greater distance, narrower angle. This is why stepping back makes red-eye more likely, not less. The pupils get smaller in the frame, so the red is less noticeable, but the geometry gets worse with every step.

The angle is easy to work out yourself: divide the flash-to-lens gap by the subject distance and take the arctangent. For angles under about 10° you can skip the arctangent: the gap divided by the distance, times 57, gives the angle in degrees within a fraction of a degree. The table below does it for four common setups. The gaps are typical rather than exact, so measure your own.

Two rules of thumb fall out of the table. First, a phone flash is about the worst geometry there is: at any normal distance the angle is under 2°. Many phones detect red pupils and correct them automatically, which hides the problem rather than avoiding it. Second, for a flash pointed straight at the subject, keep the subject no farther away than about 10 to 20 times the gap between flash and lens. For a 10 cm pop-up flash that means within roughly 1 to 2 meters; for a 25 cm bracket, roughly 2.5 to 5 meters.

Preventing red-eye when you take the picture

Every reliable fix does one of two things: it widens the angle at the eye, or it shrinks the pupil. In rough order of how well they work:

1. Bounce the flash. Turn a flash head toward the ceiling or a wall and the light reaches the eye from a broad surface far from the lens. The angle becomes large and red-eye disappears in practice. The light also becomes softer and more flattering. Bounce flash covers how to aim it and how much power it costs.
2. Take the flash off the camera. A flash on a stand to one side, or held at arm’s length, gives an angle many times wider than any flash on the camera can. Off-camera flash explains the triggers and placement.
3. Raise the gap. A flash in the hot shoe sits higher than a pop-up, and a bracket raises it further. This helps, but only within the distances in the table. A direct speedlight aimed at someone across a dim hall can still give red-eye.
4. Move closer. Halving the distance roughly doubles the angle. For a pop-up flash, working at 1 meter instead of 3 takes the angle from about 1.9° to 5.7°.
5. Brighten the room. Turn on every light you can. Pupils shrink in brighter surroundings, the returning cone narrows, and less red comes back. It also helps the flash blend with the ambient light.
6. Have the subject look slightly away. If the eyes point a little to one side of the lens, the returning cone points away from it. A glance at a spot beside the camera works; you lose direct eye contact, which is not always what you want.
7. Turn the flash off. In many dim scenes a wider aperture or higher ISO gives a natural-looking photo with no red-eye at all.

Ready to point the flash somewhere other than straight ahead? Bounce flash shows how a ceiling or wall turns a small, harsh light into a broad, soft one, and why red-eye goes with it.

What red-eye reduction mode does, and what it costs

Red-eye reduction mode attacks the pupil, not the angle. Before the real exposure the camera shines a lamp at the subject or fires a short burst of weak flashes, typically for about a second. The pupils respond to the light and shrink, so less light gets in and the returning red is weaker.

It works partly, not completely, and it has real costs. Pupils take a fraction of a second to start closing and about a second to close most of the way, so the camera has to wait, and so do you. During that wait people blink, think the photo is done and look away, or freeze into a stiff expression. For candid moments it often misses the shot. It also does nothing about the geometry, so at long range with a phone or pop-up flash you will still see some red.

A common mix-up: with automatic flash exposure (TTL), the flash fires a quick measuring pulse just before the main flash. That pulse is so close in time to the exposure that it looks like one flash to the eye. It lasts far too short a time to shrink anyone’s pupils, so it is not red-eye reduction, even though it is also a pre-flash.

Curious what that measuring pulse is doing? TTL flash metering explains how the camera reads it to decide how much light the main flash should give.

Eyeshine in animals is a different effect

Cats, dogs, deer and many other animals have a mirror-like layer behind the retina called the tapetum lucidum. It bounces light back through the retina to help them see in the dark, and it returns far more light than a human eye does. In a flash photo or in headlights it shows as a green, yellow, blue or white glow, like the cat at the top of this page. Animals without much tapetum pigment can show red instead.

The same geometry applies, so bounced or off-camera flash and a wider angle help just as they do for people. The fix afterward is different, though. Red-eye tools look for red and often do nothing to a green or yellow glow, so eyeshine usually needs a dedicated pet-eye setting in your editor or careful hand work.

Photographing animals after dark? Wildlife photography covers working with low light and animals that will not hold still, where eyeshine is part of the picture.

When only one eye is red, or one looks white

If one pupil is red and the other is dark, the subject was usually turned or looking slightly to one side. The two eyes sit a few centimeters apart and point in slightly different directions, so one can have the lens inside its returning cone while the other does not.

Light that lands on the optic nerve, where the nerve leaves the back of the eye, reflects pale rather than red, so an eye looking off to the side can show a dull white or yellow glow. That is usually just the angle. If the same child’s pupil shows white or yellow again and again in photos where the other eye shows red, it is worth mentioning to an eye doctor. This is one of the rare cases where a snapshot is a useful health clue.

Fixing red-eye in editing

Almost every photo editor, on a computer or a phone, has a red-eye tool. Most work by finding the red inside the circle you mark and pulling out its color while darkening it. To make the result look natural:

• Zoom in to 100 percent or more and size the tool to the pupil, not the whole eye, so the iris keeps its color.
• Keep the catchlight. The small bright reflection of the light source is what makes eyes look alive. If the tool darkens it, paint it back or redo the correction with a smaller circle.
• Aim for very dark grey, not pure black. A flat black disk looks like a hole. Lower the darken amount if your tool has one.
• Check the edge. A pink or orange ring where the pupil meets the iris means the selection was too small; a dark ring eating into the iris means it was too big.
• Use the pet setting for animals, or select the glow by hand and reduce its brightness and saturation, since the red-eye tool may not touch it.

Editing fixes the color but cannot restore detail that the glow covered, so prevention gives a better eye than correction. For broader cleanup of portraits, retouching covers the rest of the face.

Want the eyes to sparkle instead of glow? Catchlights are the good kind of reflection in an eye, and that page shows how to place and keep them.

Common mistakes with red-eye

• Stepping back to avoid it. Distance narrows the angle at the eye, so the red gets more likely. It only looks less bad because the faces are smaller.
• Assuming any external flash cures it. A flash in the hot shoe pointed straight ahead only widens the gap a little. Across a large dim room it can still cause red-eye. Bounce it or take it off the camera.
• Blaming the exposure. Changing aperture, shutter speed or ISO does not change the geometry. What matters is the angle at the eye, the size of the pupils and the eyes themselves.
• Thinking the automatic flash pre-flash prevents it. The TTL measuring pulse is far too short to shrink pupils. Only the dedicated red-eye reduction mode, with its lamp or long burst, does that.
• Painting pupils solid black. It looks dead. Darken the red to a deep grey and keep the catchlight.
• Thinking phones never get it. Phones have the worst geometry of any camera. Their software usually corrects the result, but not always, and not for pets.
• Treating pet eyeshine as red-eye. It comes from a different layer, is a different color, and red-eye tools often miss it.

Try this: predict red-eye, then test it

You need a willing adult, a room you can make dim, and a camera or phone with a flash. It takes about ten minutes.

1. Measure the gap between your flash and the center of your lens, and predict the angle at 1 meter and at 3 meters, either from the table above or as the gap divided by the distance, times 57.
2. Dim the room until it is comfortable to talk but too dark to read. Wait a minute so your subject’s pupils open.
3. Ask the subject to look straight into the lens. Take one flash photo at 1 meter and one at 3 meters. If your phone has a red-eye or auto-correct setting, turn it off if you can.
4. At 3 meters, take another with the subject looking at a spot about 30 centimeters to one side of the camera.
5. Turn on all the room lights and repeat the 3 meter shot looking at the lens.
6. If your flash can tilt, aim it at a white ceiling and repeat once more.

Zoom in on the pupils of each photo. You should see the red get stronger from 1 meter to 3 meters, weaken or vanish when the subject looks away, weaken when the room is bright, and disappear with bounce. If your results match your predictions from the table, you understand the triangle. If the 1 meter shot is red too, your gap is small and your subject’s pupils are wide: that is the phone case, where only bounce, more light or a glance away will save it.

Red-eye questions

Why do some people get red-eye and others don’t in the same photo?

Each person’s eyes sit at a slightly different angle to the camera, and pupil size and the pigment behind the retina vary from person to person. Children and people with light-colored eyes tend to show it most. Someone looking slightly away from the lens may show none at all.

Does red-eye mean my flash was too strong?

No. Red-eye is caused by where the flash is, not how much light it gives. A weaker flash makes the whole face darker along with the pupils, but the red is still there at the same angle.

Why do I get red-eye at parties but rarely with flash in daylight?

In daylight the pupils are small, so little flash light gets in and the returning cone is narrow. In a dim room the pupils open wide, and people are often a few meters away, where the angle at the eye is small. Both push toward red.

Can I get red-eye without a flash?

Rarely. It needs a bright light close to the lens axis and wide pupils. A ring light around the lens, or a bright lamp right behind the camera in a dark room, can produce it, but ordinary room light or daylight comes from too many directions to make a strong red glow.

Is it better to fix red-eye in editing or prevent it?

Prevent it when you can. Bounce or off-camera flash avoids the problem and gives better light as well. Editing works well for snapshots you cannot retake, but a corrected pupil is never quite as natural as one that was dark to begin with.

Want to check what you know about flash more broadly? The flash photography quiz covers speedlights, TTL and sync speed, and flash photography is the place to start if any of it is new.