Canon AT 1 - Manual (Page 82)

and background of the subject will be deeper
at the greater distance.
Depth-of-Field Scale on the Lens
A depth-of-field scale is engraved on the
lens barrel, shown as a series of f/numbers on
each side of the distance index mark opposite
the distance scale. Focusing and depth-of-field
are so closely interrelated that the depth-of-
field scale is engraved together with the
distance scale.
You can tell the extent of depth-of-field
from the distance scale. For example, if you
use the camera with a standard 50mm lens
that is focused on a subject at medium
distance, say 3m with the aperture set at f/8,
the depth-of-field extends from 2.4m to 4.5m.
This tells you that with the 50mm lens
focused at 3m and the subject between 2.4m
and 4.5m the film image will be reasonably
sharp.