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.
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.