|
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.
|