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EXPOSURE MEASUREMENT-continued
Exposure Control
The
amount of
light reaching the film plane is deter-
mined by
a combination of
the lens
aperture and
the
shutter speed.
Since the two
are
interrelated, different
combinations will
give the same exposure.
A 1-step
change
in
the shutter speed, or a
1-stop
change
in the
aperture setting,
will
either
halve
or
double
the exposure.
For
example, a shutter speed of
1/125
second
passes
twice as
much light
as a setting of
1/250
second, and
only
half
as
much light
as a speed of
1/60
second; for an
aperture setting of f/11, twice as
much light
as f/16, and
half
as
much
as f/8, is passed.
This
feature characterizes
the oJ)eration throughout the available range of shutter
speeds and aperture settings.
With
this in mind, it's easy
to
see that
if
a correct exposure for a scene is
1/125
at
fill, then
1/60
at f/16 or
1/250
at f/8
will be
equally
acceptable.
The best
comb
ination for your needs will depend
on
the
results desired. Use
fast
shutter speeds
to freeze
motion,
or
use
slow speeds
to produce
deliberate and creative
blur. Small
apertures give greater
depth
of field,
while
large
apertures restrict sharp focus
to
the
main
subject.
The
creative selection of
both
speeds and apertures
will
greatly enhance your
photography.
24
Metering Range
If the center
"correct exposure"
LED
fails to illuminate,
even after all
possible lens-aperture/shutter-speed
com-
binations have been
tried,
then
the available
light is
too
bright
or too
dim
for the
meter's range.
To correct this
situation, several
measures
may
be taken, as follows:
Switch
to a
new
film (either
higher
or
lower
ASA) that
more
closely
matches
the available
light; mount
a
neutral
density filter
on the
lens
to
decrease
the light
reaching
the
film
plane;
or
use
artificial
lighting
(i.e.,
an electronic
flash
unit)
to
increase
subject illumination.
Remember,
too, that
the
lens in use
can greatly influence suitability
for bright
or
dim
shooting.
For
example, a 50mm f/1.4
lens (with ASA 100
film) couples from EV
-
2 (f/1.4 at
8
seconds)
to EV 17 (f/8
at
1/2000
second) for excel
-
lent low-light
performance; on the other hand, a 200mm
f/4 lens proves more usable
at
bright-light levels,
coupl-
ing (with ASA 100 film)
from
EV 1 (f/4
at
8
seconds) to
EV 20 (f/22 at 1/2000). Thus, choose the lens carefully
to match the
existing
lighting conditions.
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