Flash manufactures often (always) specify the power in watt-seconds (Ws), and not in the SI unit joules (J). See and example for the Godox studie flash QT series.
Since the unit conversion says that W = J / s, so Ws = (J / s) * s = J, thus the unit J could just as well have been used.
Is there any good reason that flash power is given in watt-seconds (Ws) and not joules (J) ?
Well, it's convention. And the convention makes sense since J is an energy unit associated with work and heat in general while Ws is in terms of power (specifically electrical power) and time which are units relevant to the photographer.
If I can shoot with 1kW of lighting at 1/10 sec, I can equivalently use a flash with 100Ws of energy (assuming similar efficiency of course, so this is just a first estimate).
This convention is very common with electrical energy, not just in photography: you will pay for electrical energy billed in units of kWh rather than units of MJ.
Watts is a measure of continuous power. A 100 watt bulb consumes 100 watts if for a minute, or if it burns for a week. It is a 100 watt rate either way. Watts is Not the same as total energy consumed or produced.
Watt seconds is the energy consumed for a period of time. 100 watts for one second is 100 watt seconds (100 x 1 = 100 ws). Joule is just another name for watt seconds, same thing. Our home electrical bill in North America is charged for the kilowatt-hours used (KWH), or I suppose in Europe it might be kilojoule or some such, but that converts to watt seconds of energy for the month.
Watt seconds is the total electrical energy consumed, it is NOT the energy of the light output (there is an efficiency factor in producing the light, an extremely rough number is that a flash or fluorescent light might be 20% efficient (ionized gas), and incandescent might be 2% efficient (heated metal). The rest of the energy goes to heat.
A 100 watt light bulb is seen by an open camera 1/100 second shutter as maybe 100 watts x 1/100 second duration = 1 watt second (regardless how long the bulb was on, camera is only concerned with what it sees but there is also this efficiency factor that determines the light output).
But a flash that consumes 100,000 watts from discharging its large capacitor in say 1/1000 second would be 100,000 x 1/1000 = 100 watt seconds.
The usual measure of it is that flash watt-seconds (joules) of energy = 1/2 CV², where C is capacitor size (farads) and V is volts. Again, the efficiency of the conversion to light affects the camera exposure.
Electronic flash evolved from giant units that plugged into wall outlets to heavy shoulder-borne battery compartments to in-camera units powered by AA size batteries. When I was a cub, I carried a “Little Giant” strobe loaded with “B” batteries. In those days portable radios used a twin voltage system. The “A” batteries were 1 ½ volt and the “B” batteries were high voltage packages. They were heavy, and there was danger when opening to change out the batteries. Anyway, the flash tube of even a modern unit requires high-voltage. The vintage versions didn’t step-up the voltage efficiently. Modern units thrive on solid state electronics.
In the early days, cameras had to be set manually. We used a “Guide Number” system. While still used today, this method has fallen by the wayside because cameras and electronic flash now use “chip logic” to set the exposure. The guide numbers were published. Say your unit had a guide number of 100 for 100 ISO film. We guestimated the subject distance, say 12 feet, divided 12 into 100, and set the aperture at f/8.
Finding a guide number that worked was the key. We used the published values or ran tests ourselves. The American Standards Association (ASA), forerunner to ISO, published the needed formulas.
Watt seconds = Capacitance in microfarads X voltage squared divided by 2,000,000. From this value, Effective Candle Power Seconds can be calculated.
From this answer, the Guide Number was calculated Guide number = square root of 0.63 X ECPS X ASA
ECPS = Effective Candle power seconds
ASA = Forerunner of ISO
These formulas, ASA PH 2.4-1953 became the standard adopted by all electronic flash manufacturers and film makers who published the data sheets enclosed with the film.
