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An
Introduction to Pictorial High Speed Photography
In the last century photographers were accustomed to exposures gauged
in minutes. For them the term "high speed" implied something far
different from its meaning today, when exposures are routinely measured in
thousandths of a second. Indeed, modern CCD cameras can record 100 million
frames in one second and it is even possible to work with exposure times of
less than 1 picosecond (one trillionth of a second)! Clearly high speed,
despite implied precision, is a relative term, and wholly subjective . It
is, therefore, necessary to define the sense in which we use it here.
As Humpty Dumpty said, "when I use a word it means what I want
it to mean - neither more nor less". Accordingly, we are
using "high speed photography" to mean imaging with
exposure times in the microsecond (one-millionth of a second) range.
Nanosecond and picosecond imaging we prefer to call "ultra-high
speed photography", leaving its practice to others.
Most of our ( TC
Nature) subjects are "natural" phenomena - animal
movement and events mediated by gravitational and surface tension forces.
Because they can be recorded on regular filmstock and in full color, we
call this "Pictorial High Speed Photography", to
distinguish it from purely scientific or analytical imaging.
The photography of fast-moving objects is a somewhat technical
undertaking and today relies heavily on electronics. It comes as a
considerable surprise, therefore, to learn that high speed imaging is
almost as old as photography itself. In 1839 H.W. Fox Talbot recorded his
first image at Lacock Abbey with an exposure lasting several hours. Four
years later Daguerre introduced practical photography to an astonished
public with exposures lasting only minutes. In 1851, just 13 years after
the very first photograph was taken, Fox Talbot used light from an
electric spark to freeze movement with an exposure of ten millionths of a
second - a remarkable feat indeed.
Nevertheless, even after one hundred and fifty years high-speed
photography still appears cloaked in mystery, an arcane art practiced in
secret by a mysterious, technological elite. This popular view of
high-speed photography has long been fostered by its practitioners in
order to safeguard their investment in custom-built equipment and hard-won
technical knowledge, but today these walls of deliberate obfuscation are
collapsing. Any determined and practical-minded photographer wishing to
create innovative high-speed images can now do so. Indeed, a few
inspirational science teachers are showing high-school students how to
create dramatic high-speed images by combining minimal equipment with
maximum imagination. Foremost among them is Dr. Loren Winters, whose web sites
are a revelation to those who believe
that high-speed photography can only be undertaken in well-equipped
University science labs.
The Basic Principles
The special problems associated with Pictorial High Speed Photography
may be conveniently considered under three main headings, each of which
will be discussed in turn:
Detection:
Because the events to be recorded are too rapid or transitory for the
eye to perceive, and often take place at long, unpredictable intervals,
they must be detected remotely. This is usually done using sound or
vibration sensors, or by the interruption of a light beam. In some
situations multiple sensors are used, for example to ensure that the
system only fires when the subject is correctly positioned in frame, is in
focus and is travelling in the desired direction.
To learn more, go to Detection
Synchronization:
Once detected, there will be a brief but unavoidable delay before the
image can be recorded. Although the time required for a sensor to respond
or a flash to fire can generally be ignored, mechanical camera components
such as the mirror and shutter introduce a real and significant delay.
Precise measurement, or at least control, of the interval between
detection and imaging is often needed to synchronize flash and action.
To learn more, go to Synchronization
Imaging:
Finally the action must be recorded on film. There are two ways to do
this. Either a very fast shutter speed must be used on the camera or the
action must be frozen by using a very brief pulse of light. Use of a
shutter imposes a limitation of about 1/8000-second and requires a
powerful light source lasting longer than the time the shutter is open.
More often, movement is frozen using a brief pulse of light from an
electronic flash or strobe that occurs during the time the shutter is
open. This means that the camera itself is rarely a limiting factor in
high-speed imaging.
To learn more, go to Imaging
For a detailed description of how one particular shot was obtained go
to: Case Study
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