Canon AT 1 - Manual (Page 79)

Depth-of-Field
When a certain subject is brought into
focus, there is only a limited range in the
foreground and background of the subject
which can be kept clearly in focus. This zone
of sharpness is the depth-of-field.
There are two methods of confirming the
extent of the depth of the field: by stopping
down the lens diaphragm or by reading the
depth-of-field scale on the lens.
Confirming the Depth-of-Field by
Stopping-Down the Lens Diaphragm
Press the stopped-down lever until it
locks. Once locked, the depth-of-field can be
checked by looking into the viewfinder. Thus,
the extent of the depth-of-field can be seen as
the zone of sharpness in the subject field
observed on the screen. When the stopped-
down lever's release button is pressed, full
aperture metering will be restored.