TECH TIPS
For years, photographers have used the watt-second rating of
electronic flash units to compare the output of different systems.
Yet other factors are at least as important as watt-second ratings,
when you're looking to get the most bang for the buck.
It's similar to shopping for a car. Simply choosing a vehicle with
the largest engine won't ensure the best performance or the fastest
speed. The car's weight, tires, design, and purpose all come into
play. The same holds true with electronic flash units.
The basic concept behind electronic flash is relatively simple.
Rechargeable batteries hold energy and, when triggered, release
stored energy to the flash tube.
And while mechanisms for control, safety, special features, and
performance increase the complexity, the heart of all
electronic-flash systems — whether small shoe-mount flashes
or large high-powered studio generators — is always the
same.
Capacitors inside the flash unit are the "rechargeable batteries"
that supply energy to the flash tube. Although not really
rechargeable batteries, they function in much the same way. The
primary difference is that rechargeable batteries charge and
discharge over a long period of time, while flash unit capacitors
charge and discharge over considerably shorter time frames.
What's Watt
To answer the initial question — what is a watt second
— it is a unit of stored energy, nothing more. Just as engine
size will not solely determine a car's efficiency, the amount of
stored energy will not be the only factor determining a flash's
performance. Choosing equipment based solely on watt seconds will
not give a true representation of the equipment's capability.
From the system design to the quality of components, every aspect
determines how much light is actually produced. Each of these
factors is significant. With respect to the watt-second rating, two
aspects are often overlooked: the design and quality of the flash
tube and reflector.
When we look at flash units in terms of watt seconds, we're not
speaking in terms of light produced. This is why it's impossible to
accurately assess the quantity of light until the stored energy in
the capacitors is converted into light. This happens at the flash
tube. Different flashes of similar energy storage capacity will
produce different amounts of light, based on the flash tube's
ability to convert energy from electricity to light.
This explains the frequently large disparity in the price of flash
tubes. After all, one of the principal laws of physics is that
energy cannot be created or destroyed; it can only be converted
from one form to another. Energy that reaches the flash tube, but
isn't converted into light, is converted into heat.
The same is true with light bulbs — the more efficient the
tube; the more light is produced per single watt of energy, and the
cooler the operating temperature.
The second often-overlooked contributing factor is the reflector.
Light leaves the flash tube in every direction. The reflector's
role is to give the photographer control over the "spill" of light
leaving a bare bulb. The angle and finish of the reflector
determine how the light is focused toward the subject. This also
determines the final output of the flash system, since the narrower
the angle and the more polished the reflector, the more light will
be directed toward the subject.
For these reasons, flash units of similar watt-second ratings
produce different amounts of light. There is no standard for flash
manufacturers to follow in terms of what the normal degree-angle
reflector should be, or what finish the surface should be.
This makes it difficult to compare flash units strictly by
reviewing their literature. For example, one company states power
in terms of "nominal " watt seconds. Looking at the information
presented, they state that "due to the efficiency of their units,
the 'nominal watt-second' rating is as compared to other brands of
flash. This means that while their 3200 watt-second pack is not
truly 3200, the manufacturer feels the unit puts out approximately
as much as other brands of 3200 watt-second packs. Of course, they
never state with which pack it is being compared.
How do you compare units? One method is to translate, as closely as
possible, the printed information from different manufacturers into
a common language. To do this, you will need to know: (1) at what
distance the units were tested, (2) at what film speed the units
were tested, and (3) the angle and finish of the reflectors
used.