How Infrared Films work

Black-and-white infrared film is sensitive to almost all visible light and this includes ultraviolet and infrared. Hence, a deep red filter is employed to absorb almost all but the infrared portion of the spectrum. If a total infrared image is required, you will need to use an infrared filter made for the purpose. These opaque filters pass infrared only and no visible light. Because they are very dense, viewing the scene to be photographed through an SLR viewfinder is not possible with the IR filter in place.


Infrared radiation is present in a variety of light sources. All the common ones photographers encounter—tungsten, strobe, and sunlight—emit varying amounts of infrared. Be aware that infrared behaves differently than visible light. It is most pronounced at sunrise and sunset and shortly thereafter. Some subjects reflect infrared more strongly than others. Human skin and green chlorophyll-bearing live plants are good infrared emitters. Shooting these subjects with IR film produces surreal shimmering white skin tones or vegetation. On the other hand, clear sky and other low IR-sensitive portions of a scene will be rendered very dark or even dead black.
 

There are a few hitches using black-and-white infrared film. To begin with, there is no set ISO number for any IR film. This is because IR radiation does not correspond to that of visible light, so ordinary light metering is virtually useless. Also, IR radiation varies wildly depending on reflecting surface, light source, and any number of other factors. In sunlight, seasoned IR users will start off at 1/125 second at f-11 for distant scenes and 1/30 second at f-11 for close-up subjects less than 100 feet from the camera. Even with these guidelines, most users bracket up to four stops up and down.


The second hitch involves focus shift. Because IR wavelengths are longer than visible light, focus must be adjusted when using IR film. A diminishing number of camera lenses include a red dot or line indicating the IR compensating point to use. In practice, if shooting at f-8 or smaller f-stops, the slight difference between visible focus points and IR can be discounted.


Infrared film's high contrast and pronounced grain is a curse for some and a blessing for others. In any event, do not expect fine-grain, mural-quality negatives. What you can expect is a negative with good tonal qualities and distinctive rendition of high IR radiating subjects. One thing is sure. Results are difficult to predict accurately. One last caution about handling IR film, all IR emulsions are very sensitive to visible light and must be loaded and unloaded in complete darkness. A darkroom is highly recommended for these operations.
 

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