LESSON 65
Infrared
White leaves, black skies, invisible light.
IN THIS LESSON
- What infrared photography records
- The look
- A short history
- Digital infrared
- In practice: focus and exposure
What infrared photography records
In infrared photography the film or sensor responds to infrared light just beyond the red end of what we can see. Photography uses the near infrared, roughly 700 to 900 nanometres, which is different from the far infrared used for thermal imaging; an infrared photograph does not show heat. Because film and sensors also respond to visible light, an infrared-passing filter goes over the lens. It lets infrared through and blocks most or all visible light, so it looks black or deep red. Lesson 19 explains how a normal sensor sees colour.
The look
Leaves and grass reflect infrared strongly, much as snow reflects visible light, so foliage turns white or pale. This is called the Wood effect, after the pioneer Robert W. Wood, not after timber. Skies turn very dark, because infrared scatters less in the air, and haze is cut through, so distant views look clear. Clouds stand out strongly and water reflecting a dark sky looks dark too. In portraits, infrared penetrates a few millimetres into skin, giving a milky look, while eyes often look black.
A short history
Ordinary silver emulsions do not respond beyond blue and some green light without sensitising dyes, so infrared photography only became possible in the early twentieth century. The first published infrared photographs appeared in 1910, in The Century Magazine in February and the Royal Photographic Society's journal in October, illustrating papers by Wood. Film became widely available to enthusiasts in the 1930s, and by 1937 there were 33 kinds from five makers, including Agfa, Kodak and Ilford. Kodak's false-colour infrared film was first developed in the 1940s for camouflage detection, and in the 1960s Jimi Hendrix, Donovan, Frank Zappa and the Grateful Dead all issued albums with infrared cover photographs.
Digital infrared
Most digital cameras have an infrared-blocking filter, often called a hot mirror, in front of the sensor. With an infrared filter on the lens, such a camera needs very long exposures, often around 30 seconds, with noise and blur. The other route is to have the camera converted: the blocking filter is replaced by one that passes only infrared, or by clear glass for full spectrum use. A converted camera then works handheld at normal speeds. Some lenses show a bright hot spot in the centre in infrared, because their coatings are designed for visible light. A common edit swaps the red and blue channels, which keeps white foliage while turning skies blue.
In practice: focus and exposure
This section is working practice built on the source. Infrared focuses at a slightly different distance from visible light. Many older manual lenses carry a small red infrared index mark: focus normally, then move the distance to that mark. Without one, use a small aperture and hyperfocal focusing, but not too small, because diffraction is stronger at infrared wavelengths. Test on your own lens, since marks suit only certain filter and film combinations. Meters are not reliable in infrared, so bracket and check the histogram. Set a custom white balance from green grass to get a usable preview. Bright sun on foliage gives the strongest effect.
Try it today
- 01Find out whether your camera can see infrared: point a TV remote at it and press a button.
- 02Fit an infrared filter, such as a 720 nm type, and mount the camera on a tripod.
- 03Compose and focus without the filter, then fit it.
- 04Shoot a sunny scene with trees and sky at several long exposures.
- 05Set white balance from the grass and try swapping red and blue channels in the edit.