Our eyes are sensitive to light from approximately 400nm - 700nm, while the infrared region lies above 700nm. A filter is necessary to block visible light and allow infrared light to pass. Filters are graded based on their cutoff point. For example, a 720nm IR filter will block light below 720nm (visible light) and allow light above 720nm (infrared light) to pass. It's not a sharp cutoff and some traces of visible light will seep through.
The higher the cutoff wavelength, the stronger the filter. For example, a 720nm filter will pass small amounts of visible light while an 830nm filter is a stronger filter that would only be appropriate for black and white images. However, I've obtained superior results using filters from 650nm to 720nm and converting to B&W during post-processing. In addition, lower wavelengths can be used for some colorful effects.
There are three ways to convert a camera for infrared photography:
- Add a filter, such as the Hoya R72 filter, in front of the lens. This will block visible light and let IR light to pass. Unfortunately cameras have a filter that passes visible light and blocks IR light. With one filter passing IR, and one filter blocking IR, exposure times tend to be long. Bring a tripod.
- Remove the IR-blocking filter from the camera. Add a filter in front of the lens that only passes visible light and shoot conventional images. Add a filter that only passes IR and shoot IR. In addition to selecting visible or IR light, IR filters with different cutoff frequencies can be used. Several filters may be required for cameras with interchangeable lenses.
- Remove the IR-blocking filter from the camera and replace it with a filter that blocks visible light and passes IR. In this case the choice of an IR filter is fixed and the camera cannot be used for conventional photography. Since the filter is builtin to the camera no additional filters are required when changing lenses.
For options 2 and 3 there are several firms that will modify a camera for infrared photography: Life Pixel, Spencers, and a nice person on ebay. As usual, cameras with large sensors yield better results than those with small sensors. This is especially the case for IR.
For prosumer or mirrorless cameras, changing white balance in the camera will change the in-camera preview. Green grass appears white or shades of gray in IR images so a white-balance on grass is appropriate and the preview will approximate the final results. Cameras that only have an optical viewfinder, where the image is viewed via a mirror, are unable to preview an IR image. For this reason SLR cameras should be avoided and prosumer or mirrorless cameras are recommended.