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12. DISTANCE SCALE ON
FOCUSING RING-
When the
2
images are
brought
together
by turning
the len focusing
lever to
the
proper
posi-
cion. you
can
then read the
distance scale
in feec as
indicated on
focusing ring
on
fla nge (
Illust. P)
The letter R
indicates
th
e
position at which focusing should be set when
using an infra red film
.
In other words, if you happen to bring the
lens
into focu
s
and th
e
d istance scal
e
indicates that you are 10 feet
away,
you should th
e
n move the focusing lever so as to set the 10 foot
mark opposite
R.
The lens is then in
focus
when using infra red film
.
To
repeat,
the lens is focused in th
e
usual manne
r
and the helical
mount is then turned until the
index
R
points
to that position on
t
he
distance scale which was indicated in the first place by
the
main
index.
13. DEPTH OF FIELD SCALE-
The
range of
sharpness
of
any
lens
is called the
depth of
field and
depends on the
relative
aper-
ture setting
and the
distance
to
the
subject on which
the
lens is
focused
.
T
he
depth of field
actually
indicates
maximum sharpness obtainable
in
a certain plane at a
distance
from the
subject
photographed.
This
plane covers
a
certain distance in
front
of the
subject and
a
certain
distance behind
the subject.
Within these
areas
objects
are
in
focus.
T
he smaller
the
aperture
being
used,
the
greater
the
depth of field
.
In
other
words, wi
th
a small aperture
things become
sharp
far beyond
the
subject
on which
the
lens is focused, whereas
the larger
the
aperture
the smaller
the depth of field
and
things become diffused
and
out of
foc
us
beyond the
subject.
In order to determine just what is
in sharp focus within
the
depth
of
field range
at a
given
lens
aperture
and
distance,
a
special
scale is
16
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