Watt Seconds versus Guide numbers

[markbau]

Excuse my ignorance of what is probably a basic beginners question. Monolights power is given in watt seconds but a flash such as a camera mounted Metz give their power in guide numbers. Is there a way to convert a guide number into watt seconds? Specifically I have a Metz with a GN of 44, what is that in watt seconds?

 

[Dennis-B]

Excuse my ignorance of what is probably a basic beginners question. Monolights power is given in watt seconds but a flash such as a camera mounted Metz give their power in guide numbers. Is there a way to convert a guide number into watt seconds? Specifically I have a Metz with a GN of 44, what is that in watt seconds?

There's no direct correlation of watt-seconds to guide numbers. That's why most photographers who use mono- or powerpack- lights will use a flash meter to determine the correct exposure. In fact, the flash meter is a great tool for on-camera flashes. Guide number for electronic flashes are notoriously overrated.

In the print film days, there was enough slop in exposure to cover up under/over exposure errors, to a point. My earliest pro flash was a Honeywell 700. It's stated guide number of 80 for ISO 25 film was overstated by as much as 1.5 stops.

 

[Ian C]

Watt seconds (also called joules in the metric system) is a measure of stored electrical energy.

https://en.wikipedia.org/wiki/Joule


Photoflash guide number is a measure of light intensity at the subject.

https://en.wikipedia.org/wiki/Guide_number


There is no direct relationship between the stored electrical energy of an electronic flash unit and the light intensity delivered to the subject. Perhaps such a relationship could be established for one particular model of flash unit through experimentation. In general, this can’t be done due to the differences in the efficiencies among the many different flash units that exist.


The guide number system was initially devised for flashbulbs. Later, when electronic flash units came into use, the guide number ratings were extended to them as well.

 

[MattKing]

If you put the exact same reflectors/light modifiers on two different lights that differ only in terms of watt second rating, there may be a relationship between the two numbers.
But basically, guide numbers tell you about light, and watt seconds tell you about rough comparisons between two options.

 

[jtk]

.Watt second claims are useful to compare outputs of capacitors in studio-type lights, whose output is typically relatively slow. Guide number claims are useful to compare much less powerful lights, such as might be used in weddings or other snapshots and are much more instantaneous than studio-t;ype lights.

GN is handy for guessing exposure settings at short distances.

 

[wiltw]

It has been measured and published in the past, by a manufacturer of softboxes, that different brands of so-called '1000 watt-second' studio flash units output different intensities of light, one brand twice as bright as the other! The conversion of the same amount of electricity inside the storage capacitor goes thru wiring and flash tube conversion of electricity to light, and the light may or may not have a different efficiency reflector (one might be shiny while the other is matte) and the light is spread out in different angles of coverage (for example, one might cover 90 degrees spread, while the other has 120 degrees spread), resulting in different measured intensity of light...the conversion process from electricy to light has far too many variables for there to be a single 'conversion factor' to exist.

 

[markbau]

Thanks for the replies, I got my flash meter out and compared my monolight which says its 400WS to my Metz 44, the monolight was 1 & 2/3 stops brighter. Interestingly I discovered that my monolight was triggered by the Metz! I wasn't expecting that.

 

[reddesert]

There's a very approximate rule of thumb for W-s to guide number, but as others said the light intensity at the subject depends strongly on the reflector and light modifier. Monolights are typically used with a bigger modifier while speedlights have a small reflector built in, so it's hard to compare.

Most monolights have an optical slave trigger that you can switch on and off, so in a 2- or 3-light setup you only need to wire a sync to one of the lights (or use a remote trigger or trigger with on-camera flash, etc)

 

[AgX]

In a few cases a manufacturer of on-camera indicated also its electrical power (otherwise a look at/metering of the circuit would tell). But to establish a fixed relationship between both units one must take into account that reflectors already vary:

-) between makes/models of on-camera flashes

-) between makes/models of studio flashes

 

[ic-racer]

Perhaps analogy to a HiFi system. The watt output of the amplifier does not have an exact mathematical relationship to the db loudness of the speakers.

 

[RalphLambrecht]

Excuse my ignorance of what is probably a basic beginners question. Monolights power is given in watt seconds but a flash such as a camera mounted Metz give their power in guide numbers. Is there a way to convert a guide number into watt seconds? Specifically I have a Metz with a GN of 44, what is that in watt seconds?

I'm not aware of any mathematical conversion but challenged with the question, I would just measure the guide nmber wiyh a flashmeter: set the flash at a known distance(d) and measure the output in f/stop(N).then, use the relationship:N=GN/d.ergo GN=N•d.

 

[AgX]

But to establish a fixed relationship between both units one must take into account that reflectors already vary.

Not only the geometry and reflectivity of the reflector plays its part, but also the ability of the tube to transform the electrical energy into the wanted, photographically active light.

That this is no pure theorethical issue can be seen at spectral filters sometimes employed at the tube or reflector.