Focusing for Infrared Photography Because infrared radiation is longer in wavelength than visible radiation, it will come to a focus farther from the film plane. Thus the visible focus must be adjusted for critical exposures.
Some cameras carry an infrared focusing mark on the lens focusing ring.
They may be focused visually and then be reset to the infrared focus mark. The depth of field usually offsets the slight difference between the visual and infrared focus. Exact film speeds for setting exposure meters cannot be given for infrared films because the ratio of infrared to visible energy is variable and meters do not respond to infrared radiation. Film speeds can be offered only as suggested starting points for trial exposures.