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The Basics of Infrared
Infrared (IR) light is the name given to the portion of the electromagnetic spectrum that has a lower frequency than visible light, yet higher than that of microwave and radio waves. Commonly associated with thermal processes, this portion of the spectrum can be broken down into four different groupings; far-infrared (FIR), mid-wavelength infrared (MWIR - associated with thermal imaging), short-wavelength infrared (SWIR), and near-infrared (NIR). MWIR and FIR are the frequencies given off by living things or machines, while NIR and SWIR are reflected light. It is this near-infrared region that infrared cameras, digital and film, detect.
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Infrared Film Photography
Film cameras have traditionally been much easier to use for infrared photography than digital cameras; the first infrared photograph was taken in 1910 by Robert Wood, and during the 20th century, the technology was enhanced and refined for use in aerial photography, astronomy, weapons systems, and other fields. Changing a film camera over to infrared is as easy as changing out the film, though such film may be a little harder to find these days (Kodak discontinued its Ektachrome color IR film in 2007). For a more detailed discussion of the history of infrared photography, go here.
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Converting Digital to Infrared
Digital camera conversion is more difficult, as infrared light is intentionally filtered out in order to prevent skewing of the image (after all, cameras normally take photos of visible light!). In order to take infrared photos, this filter (or hot mirror, since it reflects infrared light only) must be removed. In some cameras (notably, Sony’s NightShot-capable cameras), this component can be moved out of the way with the press of a button, though later models prevent the taking of infrared photos in daylight conditions or with manual shutter speed or aperture settings. Conversion of a digital camera to infrared can be a delicate process; as a result, an aftermarket camera-modification industry has sprung up around this area. One camera which allows the user to remove the filter is the Sigma SD14, and, as far as I'm aware, is the only production camera that allows this at this time. Its unique sensor design is also an advantage, as, with conventional Bayer imaging sensors, only ¼ of the photodetectors are actually sensitive to red light and interpolation is required to calculate the rest; in the Sigma SD14’s Foveon X3 sensor, the entire field of view receives color data.
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Seeing the World Differently
Not everything in infrared photography is the same as in visible light photography; since cameras are optimized for the visible spectrum, there are complications. One complication is that the refractive index of a material is dependent on its frequency, and refractive indices are what allow lenses to work. This means that what appears to be in focus in visible light won’t necessarily be when you are shooting infrared photographs (apochromatic lenses, lenses designed to have the same refractive index at multiple frequencies, are an exception to this rule). In the days of manual focus lenses, many had an index mark specifically to indicate the optimum infrared focus setting. Note that, if you’re taking a picture in pure IR and not visible + IR, the filter you use to block out the visible light will render your viewfinder useless (unless the camera focuses using the data from the sensor, that is)! For this reason, point-and-shoot cameras sometimes make better digital infrared cameras, since they do use their imaging sensor for autofocus purposes and display the result, as well.
If you’d like to give infrared photography a try, I’d recommend this website; it lists both helpful information and other resources you can investigate if you're truly interested in seeing the world in a different light.