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I'd like to know how do battery-powered system flashes compare to studio units in terms of light output.

I know that AlienBees B400 has 400 "effective"  Ws, and that Canon Speedlite 580EX II has guide number 42 (meters) at ISO 100 and 50 mm zoom setting.

But how do I compare these two? Is there any way to convert guide numbers to watt-seconds?

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No, there is no way to convert guide numbers to watt-seconds. Watt-seconds is a measurement of how much energy is used by the flash, not how much light is put out. A significant portion of this energy is wasted as heat, infrared, ultraviolet, etc.

A 4 watt-second flash that is 100% efficient will put out the same amount of light as a 400 watt-second flash that is 1% efficient.

"Effective watt-seconds" are ill-defined as well, so are basically just as useless as watt-seconds.

By contrast, guide numbers are fairly well defined. They can be directly compared at a given beam spread (assuming manufacturers aren't stretching a bit. Ken Rockwell seems to think most flashes are over-rated by about a stop.)

However, the most accurate way to compare two flashes is through an actual scientifically defined unit like the lumen-second. From Wikipedia,

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the power of light perceived by the human eye.

As photographers, we're all keenly aware that time plays an important role in exposure. The longer the shutter is open, or the longer the light is on, the higher the exposure. Thus, lumen-seconds more directly translate to exposure than lumens do.

Here's a page from AlienBees' website which includes the specs on your B400 (7000 lumen-seconds) as well as a paragraph or two about how bogus "effective watt-seconds" are.

The Effective Wattseconds rating, however, is rather arbitrary and cannot be easily proven true or untrue, as it is merely used as a basis for inflated comparison of different flash systems.

I've looked around for a lumen-second rating of the 580 ex II, but can't seem to find one.

EDIT: David Hobby, master of the speed light, keeps saying 60 watt-seconds. (Note: that last link is an April Fools joke.)

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    \$\begingroup\$ FWIW, I think Ken Rockwell isn't too off base here. The Japan Camera Inspection Institute (which, as I understand it, manufacturers take seriously) allows +/- 1 GV from a metered reading. And of course there's not much point in quoting on the minus side. \$\endgroup\$
    – mattdm
    Dec 12, 2010 at 5:21
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    \$\begingroup\$ Wouldn't a Watt-second be a joule? Watts is joules per second. \$\endgroup\$
    – Nick Bedford
    Dec 12, 2010 at 11:48
  • \$\begingroup\$ one other thing to be aware of, especially with lower-end strobes (I know, for example, that the Vivitar 285HV does this; I haven't tested a 580EX -- and perhaps this even applies with studio units??) is that it's possible for different firings of the flash to offer different amounts of light, especially if you're shooting in rapid succession, and increasingly so the higher power you have set. I think some of the higher-end units have ways to avoid this (firing at the same intensity or not at all), and I'm fairly certain that some units at least do a better job of being uniform than others. \$\endgroup\$
    – lindes
    Dec 12, 2010 at 17:22
  • \$\begingroup\$ @Evan +1, great answer. But why would a lumen, which apparently measures human visual perception, be appropriate for an electronic sensor? For a sensor with linear response and equal sensitivity across the visible spectrum, it seems we should be interested in actual light flux without any adjustment for our vision. \$\endgroup\$
    – whuber
    Dec 12, 2010 at 18:05
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    \$\begingroup\$ Again FWIW, I don't mean to imply that Ken Rockwell is generally a good source of information. In fact, I think it's worth making a special note because I think he happens to be right on manufacturer-stated guide numbers. That is, although not everything he says is correct, not everything he says is wrong either. May be the stuck-clock-twice-a-day effect, but still. :) \$\endgroup\$
    – mattdm
    Jan 14, 2011 at 13:47
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FYI, Paul Buff does list an approximate guide number on his AlienBees spec page. You would have to set your 580 to a similar angle of coverage to compare the two.

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The only worthwhile way to compare lights, IMO, is to place them at the same distance from a subject, turn them all the way up then measure their power with an incident meter. This not only lets your record their effective, useful (for a photography) power output, but lets you easily compare them to figure out flash ratios. You can even use this method with various modifiers, which is how strobes in the real world are usually used.

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  • \$\begingroup\$ But what if you don't have both lights available to do this (e.g., when making a purchase)? Essentially, you're rejecting the use of standard units. \$\endgroup\$
    – Reid
    Jan 14, 2011 at 14:17
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    \$\begingroup\$ Rentals. But really, as with oh so many things in photography, the proof is in the pudding. \$\endgroup\$
    – Jędrek Kostecki
    Jan 14, 2011 at 14:40
  • \$\begingroup\$ If you do this make sure you have a comparable beam spread, otherwise you wont be measuring the useful power output. The best approach would be to use the light modifiers as you suggest at the end! \$\endgroup\$
    – Matt Grum
    Jan 14, 2011 at 15:31

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