Infrared Characteristics

Most 35mm, 120 (6cm)and sheet film cameras can be used for infrared photography in the range of spectrum from 700 to 925 nanometers. A few of the newer plastic body cameras may not be completely opaque to infrared.

You can easily test your equipment to see if it is completely suitable for infrared photography. Load a roll of infrared film into the camera and fire off several exposures at high shutter speed and small f-stop, with lens cap in place and indoors. If you open the fresh film container and load the film into the camera in total darkness you will have a completely unfogged roll making the test results easier to interpret. This will leave blank, unfogged film in the exposure chamber of the camera. Now place the camera in full 10 am to 2 pm April to September sunlight in several orientations for about 20 minutes in each (lens cap on if it is a rangefinder!). Process the film to normal or slightly greater than normal gamma. If the film is fogged, the equipment is leaking infrared radiation. Inexpensive, older model 35mm cameras usually make good infrared cameras if your plastic camera body leaks infrared.

The same test works for sheet film cameras and film holders. Sheet film holders are particularly troublesome with infrared photography and should be kept in the dark as much as possible. Some film holder bodies and dark slides may not be opaque to infrared. The dark slide, light trap may also leak infrared. Some view camera bellows may leak infrared. The safest solution is to keep direct sun off of the camera and film holders.

Since the range of infrared spectrum beyond 1350 nanometers is thermal radiation and imaging here involves recording heat patterns, infrared film tends to be especially sensitive to heat and age fogging. The further the film's sensitivity extends into the infrared region, the more apt it is to fog easily. Store infrared film in a cool place and shoot fresh film whenever possible. Process infrared film as soon after it is exposed as possible to minimize subsequent fogging.

Most photographic lenses are designed so that they have the same focal length for all colors of the visual spectrum. They are not usually designed to correct focus for infrared light. On many lenses there will be a red line or dot near the focus index. This is the correction factor for infrared. You focus visually, then move whatever is opposite the focus index to the infrared mark. This adjusts the lens away from the film by about 1/400th of the focal length (1/4 percent) and brings the infrared image into sharp focus at the film plane.

Most infrared films are sensitive to light from the visual spectrum in addition to infrared radiation. Various sharp cutting color filters can be used to limit imaging radiation to the infrared region. The following table gives mid and maximum usefully recordable wavelengths for several commonly available infrared films.