28.3 General Flash Photography | 28 FLASH PHOTOGRAPHY |
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 correction, 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 Kodak Wratten 87. The position of the infrared index mark has been 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.