There are often occasions in photography when a sharp eye and quick
reactions are not sufficient to get the shot. Not only may the action take
place too rapidly for us to observe, but it may also occur at long and
unpredictable intervals. This is when we must turn for help to remote
sensing.
Remote sensing means, in effect, letting the subject take its own
picture. Although it is possible to actuate a camera mechanically, the
response will be slow. Because we are dealing here specifically with
fast-moving subjects, we must rely on electronics. Although it is possible
to purchase remote-sensing systems, it is surprisingly easy (and a lot
less expensive) to rise to the challenge and build your own equipment to
solve specific problems.
It is important to realize that no special skills or technical knowledge
are needed to undertake small electronic projects - just curiosity and
commonsense - while the resultant rewards and satisfaction are enormous.
Potentially there are many kinds of stimuli, used either alone or in
combination, that might serve for remote sensing, for example changes in
pressure, temperature, infrared radiation, magnetism or capacitance can
all be used in special circumstances. In practice, however, we need only
consider four categories.
Sound
Sound triggers are among the simplest to make and use, not least
because they can be readily improvised from old audio equipment such as
tape recorders. A sudden noise, for example a bursting balloon or an
airgun being fired, can be harnessed to fire either a camera or a strobe
light. By using the velocity of sound in air (about 1100 feet per second)
as a reference, sound triggers can also provide a way to measure short
time intervals. The chief drawback to the use of sound triggers is that
they can easily be set off unintentionally by sudden extraneous noises.
Vibration
Less familiar, but often extremely useful, are vibration sensors. Less
prone than sound sensors to accidental firing, they can be used to detect
the footfall of an insect on a leaf or the impact of a fly entering a
spider's web. In fact the uses are legion, from detecting the moment of
detonation - necessary in calculating when a shell will exit the barrel of
a cannon - to recording the moment of impact when a falling object hits
the floor.
Vibration sensors rely on the piezo-electrical properties of certain
materials, most notably piezo-electric plastic films, which respond
electrically to small mechanical forces.
Contact
The most basic of all sensors is a simple switch that relies on closing
a pair of electrical contacts. This can be fashioned from two sheets of
foil held apart by a spacer. Pressure on the sensor closes the contacts,
which then triggers either the camera or strobe.
Sensors of this sort can be used with small falling objects, which
trigger on impact. On a larger scale, a sensor of this kind could be used
to capture birds landing on a feeding station or detect animals walking
along a trail.
Light
The most versatile and useful of all remote-sensing devices are those
that rely on interruption of a light beam, not least because their
response is extremely rapid. Light sensors come in many forms, and are
generally incorporated together with a sound trigger in commercial sensing
systems.
Light sensors usually need to be aimed at a separate source some
distance away, and are often designed to work with invisible infrared
light. Some, however, combine source and sensor side by side and need only
a separate reflector. Ideally, sensing elements themselves should be small
and independent of their control unit to allow easy positioning in
confined spaces.