the lens (perpendicular to the lens axis). For example,
when the subject is moving straight towards or straight
away from the camera, it may appear as though it is
hardly moving and a fast shutter speed is not required to
produce a sharp image; however, when that same subject
moves at the same speed across the field of view of the
camera, the speed of the subject appears much faster. A
faster shutter speed is required to stop the action in this
case.
The camera-to-subject distance also affects the
amount of image movement at the film plane; for
example, a car moving across your field of view at 55
mph from a distance of 700 yards appears to be moving
slowly. The same car moving at 55 mph and only 15 feet
away appears to be moving very fast; therefore, the
closer a moving object is to the camera, the faster the
shutter speed must be to capture a sharp image. When
the subject is moving diagonally across your angle of
view, movement is more apparent than when moving
straight away or toward the camera, but less apparent
than when moving straight across the field of view.
Remember, long-focal-length lenses exaggerate the
effects of camera and subject movement, and short-
focal-length lenses reduce the effect.
Experience and common sense are your best guides
for determining shutter speed that will minimize image
movement, but the following can be used as a guide to
help make these determinations:
Double the shutter speed when the subject speed
is doubled.
Halve the speed when the speed of the subject is
halved.
Double the shutter speed when the camera-
to-subject distance is halved.
Halve the shutter speed when the camera-
to-subject distance is doubled.
Double the shutter speed when the focal length
is doubled.
Halve the shutter speed when the focal length is
halved.
When in doubt, use the next higher shutter speed.
There are mathematical formulas used to determine
appropriate shutter speeds for subjects moving at all
speeds when photographed with various lenses, but the
use of these formulas is not practical. Table 4-2 shows
stop motion relationships when a 50mm lens is used.
This table is not intended to be memorized but is only
intended to provide a better understanding of the
relationship of subject motion, distance, and direction.
COMBINING APERTURE AND
SHUTTER SPEED
So far three camera controls have been discussed
separately: focus, aperture, and shutter. Focus is the
most straightforward because it is used to produce a
sharp image of the subject. Aperture and shutter each
affect the image in two distinct ways. They both control
the amount of light that makes the exposure, and they
both affect image sharpness. The aperture alters depth
of field, and the shutter controls the image movement or
blur.
The light-sensitive material must receive the correct
amount of light to produce a quality photograph. Under
most lighting conditions, it does not matter whether you
use a wide aperture with a fast shutter speed or a small
aperture with a slow shutter speed. When the
combination is correct, both provide the same amount
of exposure.
Aperture and shutter speeds each have a doubling
and halving effect on exposure. This doubling and
halving relationship of aperture and shutter allows you
to combine different f/stops and shutter speeds to alter
the image, while, at the same time, admitting the same
amount of exposure to the light-sensitive material; for
example, you have determined that the correct camera
settings for your subject is 1/125 second, at f/16. Instead
of using this combination of shutter speed and f/stop,
you could double the shutter speed (to stop action) and
halve the f/stop. In this example your new camera setting
could be 1/250 second at f/11, 1/500 second at f/8, or
1/1000 second at f/5.6, and so on. Or when you need
more depth of field, 1/60 second at f/22 or 1/30 second
at f/32, and so on, can be used. These shutter speed and
f/stop combinations are called equivalent exposures.
Equivalent exposures are used to control depth of field
and to stop motion. Table 4-3 shows some equivalent
exposures of a typical situation.
same exposure; however, the amount of depth of field
and image blur are different in each image. The
combination of shutter speed and f/stop is used to best
capture the subject and effect you want to create.
You should use a light meter for most of the
photographs you take. The light meter provides you with
a number of f/stop and shutter speed combinations;
however; depending on the situation, the level of light
alone can determine the camera settings. For example,
4-16

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