The
shutter of a camera has only one function – to control the amount of the time that
the film is exposed to the scene. This is necessary for two reasons: (1) if the
object is moving or the scene is changing in some other way, then the shutter
can be used to limit the exposure to a time interval that is short enough to “freeze”
the motion, and (2) the exposure time controls the amount of light that strikes
the film to be sure that the film is properly exposed. See the next topic (Topic
35. Film speeds and exposure calculations) for more details about this.
Early
cameras used leaf shutters that were located in front of the lens or between
two elements of a multi-element lens. As the shutter opened, a circular hole
was formed that increased in diameter until the shutter was fully opened and
then decreased in diameter until the shutter was fully closed again. Since the
shutter was not in the image plane, the opening and closing of the shutter
affected the entire image in the same way. That is, the opening of the shutter
gradually allowed more and more off-axis rays to reach the film plane, but all
of the film plane was exposed to the incident light at all shutter positions. The
shutter function was often combined with the lens opening adjustment into a
single device whose opening speed set the exposure time and whose maximum
diameter set the f/number. Note that the shutter has to move faster and faster
at shorter exposure times, and this requirement set a mechanical limit on the
shortest exposure times that could be obtained.
Although
shutters of this type have some theoretical advantages, they are difficult to
use in cameras with removable lenses. Since
the shutter is an integral part of the lens, both the shutter and its trigger
mechanism must be built into all of the different lenses that will be used. They
are also difficult to use in single lens reflex cameras, since those cameras
use a through-the-lens viewfinder to compose the image. The viewfinder
obviously couldn’t work if the shutter was closed. All single lens reflex cameras and most other cameras with
removable lenses therefore use a focal-plane shutter.
A
focal plane shutter is located just in front of the film. It consists of two
pieces of opaque material called the “curtain.” When the shutter is to open,
the first piece of material slides horizontally across the film plane,
gradually exposing the film from one side to the other (or sometimes from top
to bottom). After a preset time, the second piece of material starts to move
across the film plane gradually closing the opening again. At relatively slow shutter speeds (typically
about 1/60 s and longer), the second piece of the curtain does not start moving
until after the first piece has reached the other end of the film. The entire
image area is therefore exposed for some period of time before the second
curtain starts to close the opening. At higher shutter speeds, the second
curtain starts moving to close the opening before the first curtain has
finished opening it. The result is a moving “slot” which exposes different
parts of the film at slightly different times. In most cameras, the curtains
move at the same speed for all exposure times, and the width of the slot is
varied to set the required exposure time. The shortest exposure time is then
set by the speed of the curtains and the minimum width of the slot. Most
cameras of this type can support exposure time down to about 1/1000 s.
Focal
plane shutters cannot be used at all speeds with flash bulbs or electronic flash
units. This is because the duration of the flash is very short – typically about
0.001 s. If the shutter is operating in its “moving slot” mode, then only a
small piece of the film will receive the light of the flash unit – the piece of
film that is being exposed when the flash is fired. In order for the entire
image to receive the light from the flash unit, the shutter must be set to a
slow enough speed so that it is running in its fully open mode. That is, the mode
in which the second curtain does not start closing the opening before the first
curtain has finished opening it. The flash fires in this instant after the
first curtain is fully open and before the second one starts to close. The
maximum speed of the shutter in which this is true varies from one design to
another but is typically about 1/60 s, and this speed is usually indicated on
the shutter speed selector in some way.
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