Leaf shutters have an important advantage over
focal-plane shutters. Leaf shutters can be used with
electronic flash at all shutter speeds. This is not true with
focal-plane shutters. Focal-plane shutters can only be
used at slow shutter speeds, usually at 1/125 second and
below.
Focal-Plane Shutter
A focal-plane shutter is essentially two lightproof
cloths or thin metal curtains that move across the film
aperture in the same direction. This type of shutter is
housed entirely within the camera body and is mounted
on two rollers, one on each side of the film aperture
(fig. 4-16). As the curtain is moved from one roller to
the other by spring tension, the second curtain follows,
forming an opening that permits light to pass from the
lens to the film. After the opening has passed, the second
curtain stops and prevents additional light from reaching
the film. In the design of focal-plane shutters, the
curtains form a slit that travels across the film aperture
to expose the film. When a slow shutter speed is set, the
second curtain waits a relatively long time before it
follows the first curtain; in this case, the slit is quite
wide. When a fast shutter speed is set, the second curtain
quickly follows the first and only a narrow slit is formed.
Shutter Speed
A range of shutter speeds is available on
professional cameras. Common shutter settings are as
follows: T, B, 1 second, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60,
1/125, 1/250, 1/500, 1/1000, and 1/2000 second. The
fastest between-the-lens (leaf) shutter speed is 1/500
second. Some focal-plane shutters can be as fast as
1/12000 second. In addition to a given set range of
speeds, most shutters are made so they can be opened
for an indefinite period of time. At the setting marked
"T" (time), the shutter opens the first time the shutter
release button is depressed and remains open until the
shutter release button is depressed again. At the setting
marked "B" (bulb), the shutter remains open as long as
the shutter release button is depressed, but closes as soon
as it is released.
The interval that you want the shutter to remain open
is selected by moving a lever or shutter speed dial to that
particular setting on the shutter speed scale. Unlike
f/stops, the shutter speed you select must align exactly
with the index mark You cannot select a shutter speed
in between two indicated shutter speeds. On the shutter
4-13

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