Infrared (900nm) light is refracted less than visible light (400-700nm) hence the mark.

["Thomas H. Hogan" <flzhgn@xxxxxxxxxxxxx>]

There is probably not enough difference between 700nm and 
        750 nm. Probably not enough to merit putting a mark on the 
        lens to get the tiny amount of increased focal length
required.          
        The infrared index mark is intended to show the increased 
        focal length required to bring light of the longer 900nm
        wavelength (to which only a few films like Kodak High Speed
        infrared are sensitive) into focus.  

        Some lenses like mirror lenses even focus infrared light
        to nearly the same degree as visible light and do not
        require an infrared focus mark.
 
        I have a little 35mm to 70mm lens that does not have an
        infrared focus index mark (red mark) on it.  I have used it
        with many infrared exposures both with the #87 wratten filter
        (800nm ?) and a number 25 (600 to 700 ?) using both Tech Pan
        and Kodak High Speed infrared).  It does not seem to require
        any adjustment for wavelength.  Perhaps the chromatic aberration
        correction technology has improved.

        Does anyone know if the knew hybrid aspheric lenses need 
        correction for infrared?

        Hank Hogan <http://www.pic.net/~flzhgn/>

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Topic No. 6