Canon III-A User Manual
Page 15
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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