Canon III-A - Manual (Page 15)

DEPTH OF FOCUS SCALE See Illustration below. A little study will show how to plan
the exposure you want. The Distance Scale and the Depth of Focus Scale are used together.
Index Mark
Knurld
Ring
Focusing Ring
Distance Scale
Distance Index Mark Depth of Focus Scale
Depth of Focus means the range of objects
which will be in focus on your film. This range
varies with the distance of the object of sharp
focus and the Lens Aperture (see page 11).
It extends from in front of the object of sharp
focus to beyond it. The greater the Lens
Aperture, the less will be the Depth of Focus.
Depth of Focus also diminishes in proportion
to decrease in distance the object of sharp
focus is from the film plane.
The illustration shows the Lens Aperture at
f:1.8 and the Distance Scale at 25 feet. An
object at 25 ft is in sharp focus. The Depth of
Focus Scale shows that the nearest object also
in focus is approximately 21 feet and the
farthest object in focus is approximately 30
feet. If the Lens Aperture Scale is set at
f i l l the object in focus will extend from
12 feet to infinity (o>).
12