Canon AT 1 - Manual (Page 103)

Infrared Index Mark
The red dot infrared index mark engraved
on the lens barrel is a focusing correction
index mark for infrared film. Because infrared
light rays have longer wavelengths, they focus
on a plane slightly behind that of ordinary
visible light rays. Therefore, it is necessary to
slightlly modify the normal method of focus­
ing the lens. After focusing the same as usual,
note the tiny red dot engraved on the lens
barrel just to the right of the distance index
and turn the focusing ring slightly to align the
focused distance with this red dot. For
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