Watt-Second to Guide Number Conversion: Comet 600 Electronic Flash Units

[laser]

I am going to list my trusty Comet 600 Studio Electronic Flash Units for sale on Photrio. They have a tremendous light output. Most people don't know how to interpret 600-watt-seconds. How do I convert this to a guide number?

Bob

 

[koraks]

I think this would depend on which light modifiers are fitted. And that this is also the reason why studio strobes are rated by the power delivered to the flash bulb as opposed onto the subject. I wouldn't hesitate to just list these with the 600Ws rating, which would be a relevant metric for this kind of item anyway.

 

[wiltw]

With the reflectors that would come with the sale of your gear, what f/stop would you ordinarily set at full power, if you put the light 10' from the subject, and you had ISO 100 film loaded?

Guide Number = Distance * f/stop​

 

[Mr Bill]

I see that you've already posted your ad with some working exposure info so anything further I might say is moot. But it would not be a disservice to potential buyers to add a link to photographer David Hobby's excellent website, Strobist. Especially the part found by doing a search for "strobist monobloc."

And... if you know, it might be worth mentioning if the sync voltage is ok/not ok for digital cameras. Best of luck with the sale.

 

[BrianShaw]

With the reflectors that would come with the sale of your gear, what f/stop would you ordinarily set at full power, if you put the light 10' from the subject, and you had ISO 100 film loaded?

Guide Number = Distance * f/stop​

This.. very easily and quickly determinable with a flash meter.

 

[Chan Tran]

This.. very easily and quickly determinable with a flash meter.

And this number almost always lower than the published number if they do list it.

 

[Ian C]

Watt-seconds (joules in the SI system) is a measure of stored electrical energy in the capacitor or multiple capacitors of an electronic flash unit.

Guide number is compact way of representing the light output of a flashbulb or electronic flash unit. The relationship uses distance and aperture number formula as given in post #3.

There is no fixed relationship that connects these two ideas. Such a relationship could probably be established for a particular make and model of flash unit through testing, but it would apply to that model only. It’s not a useful idea in general.

Where watt-second/joule rating might make sense is in comparing two power packs to each other. Power pack-cable-and flash head flash sets used to be the standard in studios using large format cameras with small apertures and slow films.

In such a setup, a 4800 WS/joule power pack could deliver roughly twice the flash power as a 2400 WS/joule pack, roughly a 1-stop increase with both units set to full power.

You could easily test the unit with a flash meter to determine its guide number.

 

[wiltw]

Watt-seconds (joules in the SI system) is a measure of stored electrical energy in the capacitor or multiple capacitors of an electronic flash unit.

Guide number is compact way of representing the light output of a flashbulb or electronic flash unit. The relationship uses distance and aperture number formula as given in post #3.

There is no fixed relationship that connects these two ideas. Such a relationship could probably be established for a particular make and model of flash unit through testing, but it would apply to that model only. It’s not a useful idea in general.

Where watt-second/joule rating might make sense is in comparing two power packs to each other. Power pack-cable-and flash head flash sets used to be the standard in studios using large format cameras with small apertures and slow films.

In such a setup, a 4800 WS/joule power pack could deliver roughly twice the flash power as a 2400 WS/joule pack, roughly a 1-stop increase with both units set to full power.

You could easily test the unit with a flash meter to determine its guide number.

And Chimera, the softbox manufacturer, published a test report decades ago, testing 1000 watt-second power packs with heads, from a very wide variety of manufacturers. If you compared Dynalite 1000 w-s to (I forget which brand...(the guilty are not identified to protect their reputation!) , the Dynalite metered exposure was one f/stop brighter in spite of the fact that its heads projected light across a wider area than the other brand heads! Thus proving that one CANNOT even compare different manufacturer units based upon watt-second rating! The relative output (doubling) holds true within a single manufacturers packs compared to one another, but you risk false assumptions when comparing the rating of two different brands (or even two different model heads from the same manufaurer).

 

[koraks]

The relative output (doubling) holds true within a single manufacturers packs compared to one another

I doubt even that, really. It relies on the same efficiency across a variety of flash tubes and discharge circuits. I'm pretty sure there are bound to be differences within the different systems of even the same supplier.