Effective watt-seconds - a shorter story

My treatise on the effective watt-second system of denoting flash unit output was pretty lengthy. I thought a (slightly) more concise version might be useful.

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The watt-second rating of a flash unit tells us the amount of electrical energy stored in the capacitor which feeds the flash tube when it is triggered.

We treat this rating as an indicator of the luminous output of the flash unit, but it is only approximately that. Units with the same watt second rating may have significantly different luminous outputs (a result of their having differing "conversion efficiencies").

We would be better off if we thought in terms of the actual luminous output, which in technical work is stated in lumen-seconds. But that never became the custom among "working photographers".

The "effective watt-second" system was devised as a way to indicate the true luminous output of a flash unit using familiar words. The effective watt-second rating of a flash unit is in fact a precise, consistent measure of the luminous output of the flash unit. It is exactly the lumen-second measure of luminous output divided by 7.5.

Thus two flash units both (accurately) rated at 1750 effective watt seconds will have the same luminous output.

The scale of the effective watt-second is such that, for typical modern flash unit designs (that is, with typical conversion efficiencies), a unit with a watt-second rating of 800 watt-seconds might have a luminous output rating of perhaps 1700 to 2100 effective watt-seconds (depending on its conversion efficiency).


That difference in numbers (800 vs. 1700 to 2100) is neither "peculiar", "wrong", nor "dishonest". The watt-second and the effective watt-second are two different units, used to describe two different properties, not the same unit used to describe the same property under two different "standards".

That having been said, the fact that the two units involve two of the same words certainly is an invitation to misunderstanding. This is aggravated if a manufacturer is careless in using the effective watt-second system. For example, we might see this in the specification for a flash unit:

Effective watt-seconds: 800 watt-seconds

which would be more properly stated:

Effective watt-seconds: 800

The effective watt-second system is not attractive, especially since there is a much better way to specify the same property (luminous output): in lumen seconds.

But we must be careful not to be sucked into the claim that the effective watt-second system is a scheme to allow manufacturers to inflate the ratings of their flash units. We make it that if we see "effective watt-seconds" and carelessly think "watt-seconds".

We must similarly avoid the claim that the effective watt-second rating is meaningless, not an accurate indicator of anything. That's just not so - quite the opposite. It is in fact the watt-second rating that is not an accurate indicator of anything.

Best regards,

Doug

I have made this a separate post as it comprises a recommendation rather than information.

Thinking in terms of the watt-second ratings of flash units is a poor substitute for thinking in terms of luminous output, something like thinking of an automotive engine in terms of displacement when we are really interested in horsepower.

The effective watt-second system provides a bona fide description of luminous output, but the nature of the term itself opens the door to numerous misunderstandings. These even come to bite the very manufacturers who, for one reason or another, use the system (some of them denouncing it on the same page of their specifications).

I urge that we begin thinking in the "capability" of flash units in terms of their luminous output, stated in lumen-seconds.

If the manufacturers of interest to us do not state that for their flash units, we should press them until they do.

Then, we should become acquainted with the rated luminous output of our favorite flash units (in lumen-seconds). If the manufacture has not provided that value, denominated in lumen-seconds, but has provided an effective watt-seconds rating, we are in good shape. Just multiply the effective watt-second rating by 7.5 to get the rating of luminous output in lumen-seconds. Then remember that, and discard the effective watt-second number.

Then, as you shoot, and come to learn how much "soup" is typically needed for different tasks, reflect on the luminous output - not the watt-second rating - of the units used in your "successful recipes". Soon, your "knowledge base" will come to be in terms of the actual property of importance - the luminous output, and in terms of the "proper" technical unit, one not having a built-in seed of misunderstanding.

Photographers have at many times in history had to go through something like this (change from specifying aperture in the "Uniform System" to specifying it in terms of f-number, for example), and have survived it. And aircraft machinists survived the change in stating inch dimensions on drawings in terms of fractions to decimal presentation.

We will never get there if we don't start.

Best regards,

Doug

Yes, the watt-second is as notoriously non-standard as mpg is for cars. But it's all there is, in practical terms, and it does have just about as much comparative merit as is needed for its marketplace. Bigger numbers = brighter light. The more substantive characteristics of strobes, such as cycle times and color consistency, can be really hard to grab.

But we have a simple solution: Buy or rent the best. ProPhoto, Elinchrome, Hensel, et.al. all deliver outstanding highly-engineered product quality in a range of strobe products, albeit at premium prices. They're usually easy to rent in most cities and even biggish towns. With so many "professional" photographers going belly-up these days many very good lighting kits are also easy to find at discounts on forum b/s boards and eBay. I've seen some real doozy bargains lately, such as $15,000+ kits going for $2,000-$4,000.

If you can't afford the best you probably don't really need the best. That's where products like "Alien Bees" come into the picture. Good enough for occasional, relatively undemanding amateur use.

Addendum: Doug suggests that manufacturers of strobe systems be pressured to produce equipment that performs more precisely to the w/s specification. Actually, the top brands already do a very good job, Doug. Rent a top-grade ProPhoto or Elinchrome kit and I think you'll have little complaint.

That aside, pressure won't come from amateur photographers. They don't buy or rent much of this grade of gear. This spec pressure on the top strobe manufacturers actually occurred quite some time ago and largely through the major rental shops in the U.S. who were beginning to see their professional customers bitch. When you spend $100,000+ on rentals each year you get heard.

Ken,

I agree that i've never been disappointed renting Profoto or Hensel. Elinchrom has the advantage of being silent. But then my bargain basement Lumedyne are also silent. The advantage for me with Lumedyne is that I can use them AC or battery powered and now I connect several to a Jet fighter battery and have endless power.

The great thing is that one can travel anywhere and find one of the premier brands for rent and it takes all of 10 minutes to switch one's mindset from one make to another.

Most of what's missing in a cheaper pack is not in the pack but in the photgrapher.

Asher