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28.3 General Flash Photography 28 FLASH P HOTOGRAPHY
Infrared Index Mark
Infrared light has longer wavelengths which focus on a plane slightly behind that of
ordinary visible light. It is necessary, therefore, to slightly adjust the focus of the
lens when using blackand-white infrared film. The infrared index mark engraved on
the lens is used for this purpose. After focusing in the usual way, note the small
red dot engraved on the lens barrel to the right of the distance index, and turn the
focusing ring to align the focused distance with this dot. For instance,if the focus
is at 1 Om on the distance scale,turn the focusing ring to align the10m mark with
the red dot. After making this corre ction, you can release the shutter. When using
black-and-white infrared film, visible light must be kept out by means of a deep red
filter, such as an R1 or Ko dak Wratten 87. The position of the infrared index mark
has b ee n computed for the use of infrared film with peak sensitivity at 800nm, such
as Kodak IR 135.
For further details, refer to the film manufacturer’s instructions.
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