What are all those numbers?
“The tremendous number of speedlights in all shapes, sizes and prices exhibited at recent photo shows leaves even the electronic flash engineer confused. With all the claims, counter-claims and partial information. It is no wonder that the dealer is wary. On top of that, think of the poor customer. Confronted with the array of claims, and with no basis for comparison, it is amazing that so many have the courage to buy. What is more sorely needed are a few simple facts that can be used asa a basis of comparison. “
Though it sounds current, the above is a quote from an article by Harry L. Parker, president of the American Speedlight Corp. (ASCOR) in Photographic Trade News, May 1953. And it holds true today.
Let’s look at just one term, watt-seconds. What does that refer to? Is it related to watts that we commonly use to talk about continuous light? Does it tell us how much light a flash unit produces? Here are some quotes from publications by ASCOR, Norman, and Paul C. Bufff:
ASCOR (1950s): “MASS CONFUSION — For too long most people have rated speedlights (not the same thing as what we refer to as speed lights today, these were units that weighed hundreds of pounds and stored thousands of watt-seconds of energy in their capacitors: ED) in watt-seconds. Just what is a watt-second? If you really know, you are one of the unusual photographers. Then again, what photographer really cares? Watt-seconds is not a rating of light output or speedlight equipment. The use of watt-seconds, microfarads, and other terms has so confused the photographer that many are thoroughly baffled about electronic flash.”
Norman (1970s): “HOW MUCH LIGHT — For many years the common term used to describe the output of an electronic flash unit has been watt seconds. Unfortunately this is very misleading in that the term ‘watt seconds’ is used to describe the amount of energy store in the main discharge capacitors and is not directly related to the amount of actual light output.” (You can download a set of Norman flash guide pamphlets here.)
Paul C. Buff (1990s): Wattseconds defines the amount of electrical power supplied to the flashtube(s); not the light energy which the tubes emit. There are several design factors that affect how efficiently the flashtube converts electrical energy into useable light. These same factors also affect the flash duration. The same parameters which tend to decrease system efficiency also work to shorten flash duration, potentially to the point of inviting reciprocity-induced color errors…” (More on the Paul Buff site.) This was obviously written in the film days, whereas with digital we don’t have to worry about reciprocity departure and people are clamoring for short flash durations so they can splash milk on their subjects.
So, how do you compare the output from one flash/strobe to another? You pretty much cannot do it from the specs. You need to have both flashes in hand with the same modifier on each and do your own A/B comparisons. Luckily with digital we aren’t as beholden to super powerful flashes like we were in the film days working with 8x10 cameras at f/64 with a bellows factor added in requiring multiple pops from a 2400 ws box for a still life setup. For portrait photographers 200 ws would be my starting suggestion, and around 500 ws at the top end—and make sure that the strobe has the ability to dial down. I find more situations where I want less power from my strobes than more.
And everyone makes it difficult to tell just how much power you are using. There is a very popular line of flashes that use more speed light-like power descriptions: full, 1/2, 1/4, 1/8, 1/16, etc. This would be all well and good if all of their flashes had the same power. But what happens if you want to compare different models of their flashes on the same set? Flash A is 600ws, so 1/4 power is 150ws. Flash B is 320ws, to 1/4 power on that unit is only 80ws. Very frustrating if someone is trying to describe their lighting to you and says one light was at 1/32 power and the other was at 1/8 power. If the two lights aren’t the same, they could be putting out the same power as each other. Or the 1/8 power flash could be brighter than the 1/32 power flash. You have no way of knowing unless you know exactly which model(s) of flash units that person is using in which position.
Theoretically the number system of power levels should help compare output. As I understand it, when originally proposed it was to be based on a particular reflector and distance that could be replicated between brands. Say, for example a company had flashes ranging from 900ws to 3600ws with the same reflector and distance 10 would be the highest level (3600ws). Their 1800ws unit would only go to power level 9 (1 stop less), and there 900ws unit would top out at power level 8. Then another manufacturer would test their lights with a similar modifier/reflector at the same distance and even if the WS rating was different, say 600ws, if it put out the same amount of light as the original’s 900ws unit its top setting would be 8. Then you would know that any light that went to power level 10 had the same output. Any light set to 9.3 would be the same, etc., etc. However, in practice this has not caught on. Most flashes seem to just use 10 as their top level, no matter the actual output.
Oh, well… Pick a brand of lights (my choice is Interfit*) and enjoy them. Don’t compare them to others. Make the best photos you can. Light is light. Put your energy into the creative side.
*Yes, this is an affiliate link. If you purchase via this link or use the code “cornicello10” at checkout from Interfit you will get a 10% discount on your order (and free shipping on orders over $100) and I will get credit and may be compensated for directing you to them.