Canon AE-1 Instructions Manual page 61

Distance Scale
The distance scale
is for
distances
measured from the film plane. This scale
is
not generally used except for confirming the
depth-of-field, performing guide number
calculations in flash photography, or photo-
graphing with infrared film.
Read one-digit distances in the middle of
the number marked on the scale. Two-digit
distances should be read at the point in the
middle of the two digits.
Depth-of-Field Scale
You can determine the depth-of-field by
checking the depth-of-field scale and the
distance scale on the lens barrel. Both are
closelv interrelated.
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
slightly modify the normal method of focus.
ing the lens. After focusing the same a s 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
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