CHAPTER 6 HOW TECHNOLOGY LETS THERE BE LIGHT^87
When you press the camera’s shutter button, the camera nor-
mally waits until the shutter is completely open. Then it sends a
signal to the flash gun. The circuitry inside the flash sends
another signal to the smaller of the two capacitors, telling it to
unleash its electrical charge.6
Killer Capacitors
A capacitor can retain a powerful, even fatal, jolt of electric-
ity long after a device has been turned off, unplugged, and
consigned to the trash bin. That’s why you see warnings on
electrical equipment not to open some areas even if the
device isn’t running.The charge on the trigger plate contributes electrons to the xenon gas
inside the tube, a process called ionization. At that moment, the
larger capacitor discharges its electricity, which races through the
xenon. The current excites the gas’s electrons, causing them to gain
energy. Immediately, the electrons shed that energy in the form of
photons, or light.8
That relatively small charge, called the trigger
charge, goes to a metal plate called the trigger
platelocated outside a tube of glass or fused quartz
that will in an instant produce the flash of light.7
If a flash isn’t built into a camera, flash attachments send and receive the signals necessary to
coordinate the camera and flash in as many as three ways. The most common is to fit the flash in a
hot shoe, mounted atop the camera, in line with the lens. The flash may also use a sync cable
connecting it to a camera’s PC terminal. (The “PC” has nothing to do with computers. The con-
nector is named for German shutter makers Prontor and Compur.) Finally, and not as common in
everyday shooting, the camera and flash communicate with one another through radio signals, or
it may be built into the camera. With any of the methods, when the flash is turned on, the first thing
it must do is send an electrical notice to the camera that it’s there. (Some flash units also send the
camera the Kelvin temperature of the light they produce to help the camera automatically set white
balance.) That knowledge may change the way the camera’s processor would otherwise expose a
photo based on only ambient lighting. Dedicated flashes are programmed to work with specific
cameras, and swap information that extends the ranges of what the flash and camera can do
when they combine their features.5
The current, now boosted
to thousands of volts,
is stored in two
capacitors, electrical
components that hold
large amounts of electric-
ity for later use, much like
a dam harnesses the
energy in a flowing river.
One of the capacitors
stores only a small
amount of the electricity.
The other, called an
electrolytic capacitor,
is designed to store and
quickly release the power-
ful charge destined to
create the flash of light.4