Canon AE-1 - Manual (Page 102)

focused distance with this red dot.
For instance normally, when the focus is
adjusted at 5m on the distance scale, you turn
the focusing ring slightly so that the 5 on the
distance scale matches the red dot infrared
index mark.
When photographing with infrared black
and white film, visible light rays must be kept
out by means of a deep red filter (R1) over
the lens. The infrared index mark is engraved
in a position based upon the conclusion
arrived at through experiments that the film
most senstive to the SOOrry/ wavelength is
to be used with a red filter. For example, the
Kodak Film 1R135 and the Wratten Filter 87.
Please follow the directions of the
specific instructions of the film manufacturer
when performing infrared color photography.
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