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Fundamental to HSP is a short exposure time. The way you go about achieving
this depends on the type of HSP you wish to pursue. The motion-picture method
can be achieved with extremely fast camera shutters, or high frequency stobe
lighting, both of which are normally beyond the scope and budget of the
backyard enthusiust. The snap-shot method can also be achieved with a fast
camera shutter, but more common is the use of a flash gun. In many respects,
the snap-shot method is the same as the motion-picture method, except that only
a single photograph is taken. This simplification allows the use of standard
equiptment. Only the snap-shot method shall be described from here on.
Shutter v's Flash
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The shutter speed on a Single Lens Reflex (SLR) camera can usually be adjusted,
with the shortest exposure time typically 1/1000s. However, many cameras have
dual wipe shutter blades that take more than 1ms to travel across the shutter
window. How then can these cameras take 1ms exposures? The solution comes by
having both shutter blades in motion at the same time. While the first blade
is opening the window, the second blade is closing it. This means that all
regions of the film are exposed for 1ms, but not all regions of the film
were recording the same 1ms in the clock. In most aplications this in not a
serious problem, though it is for HSP. Effectively, the process is evolving
across the photograph. Even if the shutter did open and close instantly, the
shortest possible exposure is still 1ms, which is an eternity with respect to
many processes.
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The solution comes not in the form of a faster shutter, but rather in the form
of a slower one. The B ( Bulb or Brief ) shutter speed on an SLR camera
allows the shutter to be held open whilest the button is held depressed. It
if this feature of an SLR camera that makes snap-shot HSP possible. However,
this is only half of the story. The other crucial component is a simple flash
gun with an automatic setting. These flash guns are capable of producing
enough light to correctly expose ordinary photographic film in a period of
just 30 microseconds. Should the only light source that exposes the film be
from the flash gun, then the exposure time for the photograph is 30
microseconds. The key to flash gun snap-shot HSP is therefore absolute
darkness. This allows the shutter to be opened for a long time without
exposing the film, while waiting for the flash gun to illuminate the target.
Once the flash is over, the shutter can be cloased, and the high speed
photograph is recorded on film.
Timing
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Now that we can take a photgraph with a 30 microsecond exposure time, how
do we determine when to fire the flash and take the picture. Afterall, the
process we wish to capture may last only a few hundred microseconds. This is
a whole new issue that has a variety of techniques that vary in both
success rate and complexity. While the human control system is the easiest
technique to use, it will require some simple electronics to reliably
obtain satisfactory results. These and other systems will be described later.