Infrared

Learning goal

By photographing near infrared light, which the eye can't see, you will be able to explain how frames form in which green leaves look snow white and the blue sky pitch black. You will compare why the IR-cut filter in front of an ordinary camera's sensor stops most of this light, the difference between an infrared filter attached in front of the lens and a converted camera, and the effect of each on exposure, the viewfinder and focusing. You will explain through the reflection and scattering of light why leaves, the sky and water behave differently in infrared, and recognize focus shift and the bright spot that appears with some lenses. With a custom white balance and channel swapping you will build a black-and-white or false-color result; you will know how infrared changes skin in a portrait and how it brings different skin tones closer together, and you won't present such a frame as a person's real skin color.

The question

In the print the older club member took out, the trees on the bank of the stream are snow white, as if it had snowed. But the branches are full and the leaves lush; this isn't a winter landscape. The sky is almost black, with a few bright clouds hanging in it; the stream's water is a dark strip resembling ink. The meadow, too, is light like the trees, shining. They say they shot the photo in summer, at midday, on a day when the leaves were at their greenest.

Your friend holds the print up to the light and looks at it for a long time. They ask whether it is an editing trick; the older member smiles and explains that they attached a dark, almost black glass to their camera's lens. Looking through that glass, nothing could be seen, but the camera saw something. Why do the leaves look white? What does the camera see while the eye doesn't?

Context

In the white balance lesson you saw how to take a custom white balance from a neutral surface in the scene (Lesson 11.2), and in the strong black and white lesson how to manage gray tones by blending color channels with the channel mixer (Lesson 21.10). These two tools are the two basic tools of infrared photography, but here they are used for another purpose: turning a light the eye never sees into a visible image.

All the lessons so far worked with the light the eye sees. Yet the sensor is also sensitive to light a little outside the range the eye perceives. Camera manufacturers usually block this light on purpose, because if it mixes in, it spoils the colors. Infrared photography removes or gets around this block and shows the same landscape with a completely different tonality.

Simple explanation

What light does the camera see?

Light is a wave, and its wavelength decides its color. The light the eye sees lies roughly between 380–400 and 700–780 nanometers; the limits vary a little depending on the source. Violet sits at the short end, red at the long end. Near infrared is long-wavelength light just beyond visible red, which the eye doesn't perceive but the sensor can record. Sunlight carries plenty of energy in this region too.

An IR-cut filter is a filter placed in front of the sensor in ordinary cameras that stops most near infrared light. Without this filter, infrared light would mix into the color channels and colors would shift in photos. An infrared filter attached in front of the lens does the opposite: it cuts most of the visible light and passes only what lies beyond a certain wavelength. These filters are usually named after the wavelength at which they start to pass light, and looked at with the eye, they are almost black.

When such a filter is attached to an ordinary camera, the light reaching the sensor is the small share of infrared that can leak through between the two filters. The result is very long exposures: several seconds or longer, even on a sunny day. The camera stays on a tripod, and moving leaves and clouds blur. A converted camera is a camera whose IR-cut filter in front of the sensor has been removed and replaced with a filter that passes infrared. Such a camera shoots handheld in daytime with close-to-normal shutter speeds, and shows the infrared scene directly in live view. Conversion is a process that isn't easy to undo, and it mostly takes the camera out of ordinary color shooting.

Why are leaves white and the sky black?

A leaf is green in visible light because the chlorophyll in it absorbs red and blue and reflects part of the green. In near infrared, on the other hand, chlorophyll hardly absorbs this light, and the leaf's inner tissue, the many boundaries between cells and air spaces, scatters the light back strongly. That is why healthy, living leaves and grass look bright in infrared; in a black-and-white print they come out white as snow. Dried or diseased leaves lose this brightness.

A clear sky is blue because the molecules in the air scatter short-wavelength light much more than long-wavelength light. Because near infrared is among the longest wavelengths, it scatters very little in the air; the light the sky sends in this region is small, and the sky looks dark. Clouds, on the other hand, are made of water droplets and scatter all wavelengths in a similar way; they stay white in infrared too. That is why bright clouds against a dark sky build such a strong contrast.

Water absorbs near infrared strongly. A still stream or lake swallows this light instead of reflecting it and looks like a dark, ink-like surface. When the brightness of the leaves and the darkness of the sky and water come together, the same landscape takes on a tonality completely different from how it looks in visible light.

What needs attention while shooting and processing?

Infrared focus shift is lenses focusing infrared light at a slightly different distance than visible light, and a frame that looks sharp in visible light coming out slightly soft in infrared. Some older lenses have a separate mark on their distance scale for this difference. When shooting with a filter in front of the lens, focus is set before the filter is attached, and the aperture is closed down a little so that depth of field increases enough to cover this difference. On a converted camera, focusing in live view mostly removes this problem. Some lenses produce a bright, round spot in the middle of the frame in infrared; this comes from the lens's internal reflections, and you need to test a lens before using it in infrared.

A raw infrared frame often carries a dark red or purple color. When a custom white balance is taken from green grass or leaves, this color is largely neutralized. If the frame is converted to black and white, the classic infrared look is achieved. A channel swap is exchanging the places of the red and blue channels during editing; this turns the dark sky blue, the leaves look white or golden, and a false-color infrared image emerges. These colors aren't a scene's real colors, but an interpretation of an invisible light.

What happens to skin?

Near infrared light penetrates a few millimeters into skin and scatters back from there. That is why in an infrared portrait skin takes on a smooth, waxy look; spots, freckles and pimples largely disappear, while the veins under the skin sometimes show clearly. Eye color can look different too. The pigment that largely decides skin color absorbs visible light strongly but absorbs near infrared much less. As a result, dark and light skin tones come out much closer to each other in infrared than in visible light. This is a strong reason not to present an infrared portrait as a record showing a person's skin color: the image shows not the skin, but the skin's behavior in another light.

Makeup and tattoos also look different in infrared; some inks disappear, some darken. Before photographing someone in infrared, explaining what they will see and showing them the resulting frame keeps an unusual image from catching them unprepared.

Visual evidence

At midday on the bank of the stream you set up the camera on the tripod and set the composition and focus without the filter. When you attach the almost black infrared filter in front of the lens, the viewfinder goes dark. The exposure rises to several seconds on a sunny midday, and the edges of the leaves swaying in the wind soften slightly.

The raw frame is dark red. When you take the custom white balance from the grass, the color is neutralized. Converting to black and white, the trees and grass are snow white, the sky pitch black, the clouds bright, the stream a strip dark as ink. When you process the same frame with a channel swap, the sky turns blue and the leaves take on a pale golden color. In the first frame there is a faint, bright spot in the middle; when you shoot the same scene with another lens, the spot disappears.

Your friend asks for an infrared portrait of themselves. In the frame their dark skin is light gray and smooth; the warm brown of visible light is gone. The face of the light-skinned friend beside them is almost the same gray tone. When you show the frame, your friend understands at once that this isn't their own skin color, but an interpretation of another light.

Common mistakes

Summary

Infrared photography records near infrared light, which the eye doesn't see: because ordinary cameras' IR-cut filter stops most of this light, very long exposures are needed with a filter in front of the lens, while a converted camera shoots handheld and with live view. Living leaves look white because their inner structure scatters this light strongly, the sky dark because it scatters this light little, and water like ink because it absorbs it; focus shift and the bright spot with some lenses are checked, and a black-and-white or false-color result is built with a custom white balance and channel swap. Skin smooths out in infrared and different skin tones come closer together; such a frame isn't presented as a record showing a person's skin color.

Mini assignment

In this assignment you will shoot a scene with leaves, sky and water, with an infrared filter or a converted camera; if you don't have a filter, you can shoot the same scene in visible light and plan how it will look in infrared.

1. Draw a sketch predicting which areas of the scene will come out bright and which dark in infrared.
2. Set up the camera on the tripod, set the composition and focus without the filter, attach the filter and find the exposure with test frames.
3. Take the custom white balance from the grass; process the frame once in black and white and once with a channel swap, and compare the two results with your predictions.
4. If you shoot a portrait, put the same person's frames in visible light and in infrared side by side; note how the skin changed and how you will explain the frame to the person.

Check yourself

• Can you explain why leaves look white and the sky and water dark?
• Can you say what the difference is between a lens filter and a converted camera, and the effect of each on exposure?
• Can you describe how an infrared portrait changes skin and why it isn't a record of skin?

Glossary

Mini quiz

1. Why do living leaves look white in an infrared photo?

• A) Because the leaf's inner tissue scatters near infrared back strongly
• B) Because chlorophyll strongly absorbs infrared
• C) Because the filter turns green into white
• D) Because the long exposure lightens the leaves

2. Why does the exposure get very long when a lens-mounted infrared filter is attached to an ordinary camera?

• A) Because the filter slows the sensor down
• B) Because infrared light reaches the sensor more slowly
• C) Because the camera meters the exposure wrongly
• D) Because the cut filter in front of the sensor already stops most of the infrared

3. An infrared frame that looks sharp in visible light comes out soft. What is the most likely cause?

• A) The shutter speed is short
• B) Infrared focus shift
• C) Channel swap
• D) Custom white balance

4. In an infrared portrait, the faces of a dark-skinned and a light-skinned person come out almost the same gray tone. Which is true about this frame?

• A) The camera's white balance is broken
• B) The two people's skin color is actually the same
• C) The image shows skin in another light; it shouldn't be presented as a record of skin color
• D) A channel swap brings this difference back

Answers

1. A — Chlorophyll hardly absorbs near infrared; the leaf's inner structure scatters it back strongly.
2. D — The filter in front of an ordinary sensor cuts infrared; when the lens filter also cuts visible light, very little light is left for the sensor.
3. B — The lens focuses infrared at a slightly different distance; focus is set before the filter and the aperture is closed down a little.
4. C — The pigment that decides skin color absorbs infrared little; the tones come closer together and the image isn't a record of skin.

Summary card

• Near infrared is light the eye can't see and the sensor can.
• Lens filter = very long exposure; converted camera = handheld shooting.
• Living leaves are white, the sky and water dark.
• Set focus before the filter; test the lens for a bright spot.
• An infrared portrait shows skin in another light; don't present it as a record of skin.

Bridge to the next lesson

The same evening, the older club member asked your friend for a favor: they were going to shoot a catalog of rings and necklaces for a jewelry shop and needed an assistant. In the test shoot everything looked sharp on the camera's small screen; when they got home and opened them on the computer, they noticed that some of the stones were soft and the silver had shifted slightly toward yellow. The shop owner had also wanted to see which frame would be chosen during the shoot. Why would you want to see the photo on a computer while you shoot? The next lesson tells about connecting the camera to a computer with a cable or a wireless connection and checking each frame instantly on a large screen.