Shutter
Speed
Every video camera
boasts a high speed shutter, a
term borrowed from still
photography.(Still cameras
have shutters that open to
allow light to fall on the
film or CCD). Video cameras
don't have shutters. When the
video camera is switched on,
light falls on the CCD
continously, but the CCD
doesn't constantly record an
image. Recording occurs only
when the video camera supplies
the CCD with an electrical
charge, in other words when
you press record.
Fields
and frames- Like a
television screen, the CCD is
made up of a number of
horizontal lines. In some
countries like australia this
number is 625(PAL), in others
like USA is 525(NTSC). The
video camera only supplies a
charge to half of these lines
at a time. It alternates
between supplying a charge to
the odd and even number lines.
Each of this half images
recorded by the CCD is called
a field. Two fields, one of
odd number lines and the other
of even, are interlaced to
form a frame. Each field is a
snapshot of the action at a
given moment. If the shots are
played back fast enough, the
brain sees a moving image.
Fields
per second- In australia
where the standard for
television screens have 625
lines(PAL), the CCD records 50
fields per second. With TV's
that uses 525 lines(NTSC),
video cameras record 60 fields
per second. With a 1/50 sec
shutter speed, each charge
supplied to the odd or even
lines lasts for 1/50 sec. The
CCD records an image
continuously, as 50 fields are
recorded a second.
With a high
speed shutter, such as 1/1000
sec, each charge is supplied
for only 1/1000 sec. As there
are still only 50 charges per
second, there are gaps of
19/1000 sec between each field
when no image is recorded.
When you
play back a moving subject
recorded with a high speed
shutter, these gaps cause it
to move like someone under a
strobe light.
With a high
speed shutter, each field is
blur-free, because most
subjects hardly move in so
short a time. Played back in
slow motion each frame can be
seen clearly. This is ideal
for analysing action, such as
your golf swing or tennis
serve.
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