EXPOSURE-CONTROl FUNDAMENTALS
The
two
camera exposure-control settings
are lens operting (aperture)
and
shutter
speed.
The
size
of
the
aperture
determines
the
amount
or
volume
of
light reaching
the
film from a
given subject and lighting.
The
shutter
speed
determines
the
length
of
time this light acts
upon
the
film. Apertures are expressed in
f-numbers, which are larger
for
small openings
and vice versa (e.g.,
f/16
represents a small
opening,
f/2
a large
one
I.
Shutter
speeds are
expressed in seconds
or
fractions
thereof,
which
are generally
the
reciprocals
of
the
numbers
shown
on
·shutter-speed scales (e.g.
60
=
1/60
sec., and 2 =
1/2
sec.l. At usual apertures, each
f-number setting (e.g., f/S) lets in twice as much
light as
the
next
numerically larger
one
(f/11)
and half as much as
the
next
smaller (f/5.6).
Similarly, each
shutter
speed (e.g.,
1/60
sec.!
allows light
to
strike
the
film twice as long as
the
next
higher . speed (1/125)
and
half as long
as
the
next
lower
one
(1/30).
The
interval
between
two
standard f-numbers (say,
f/4
and
f/5.6)
or
shutter
speeds (say,
1/15
and
1/30)
is
one
"stop."
Total
exposure
on
the
film
is
determined
by
the
combination
of
aperture
and
speed.
Other
things being equal, using
the
next
smaller f-number (i.e., giving one
stop
more
exposure) will balance using
the
next
higher
shutter
speed (i.e., giving
one
stop
less expo-
sure),
and
so
on.
A great range
of
combinations
(e
.g.,
f/5.6
at
1/30,
f/4
at
1/60,
f/2.S
at
1/125,
f/2
at
1/250,
etc.!
will
thus
yield
the
same
total
exposure.
The
specific
combination
you
choose
under given I.ighting conditions will
depend
upon
the
degree
to
which
you
want
the
greater
depth
of
field (see
p.36)
of
smaller apertures
or
the
greater movement-blur preventing ability
of
faster speeds (p.
2Sl.
21
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