Canon A-1 - Manual (Page 134)

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Infrared Index Mark
Since infrared light rays have longer wave­
lengths which focus on a plane slightly
behind that of ordinary visible light rays, it
is necessary to slightly adjust the focus of
the lens when using black and white infrared
film. The infrared index mark engraved on
the lens barrel is used for this purpose. After
focusing the same as usu
2 il, note the tiny red
dot engraved on the lens barrel just to the
right of the distance index, and turn the
focusing ring to align the focused distance
with this red dot. For instance, if the focus
is at 5m on the distance scale, turn the
focusing ring to align the 5m mark with the
red dot. After that focusing correction, you
can release the shutter.
When using infrared black and white film,
visible light rays must be kept out by means
of a deep red filter (Rl) over the lens. When
using infrared color film, there is no need to
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