Indeed the true way to compare flash light output is the amount of
energy in the flash of light.
This is independend of the way the light is directed, bundeled or difused.
The basic principle of this kind of flash guns is:
1. The voltage of the battery is boosted to a high voltage on a
capacitor, might be 500 volt
2. The (big) capacitor is discharged in to the flash bulb.
3. So all the light energy has to come frome the electrical energy in
the capacitor.
4. The energy in the capacitor = 0.5 x C x V x V could be: 0.5 x 2
milli Farad x 500Volt x 500 Volt = 125 Joule = 125 Watt seconds
5. If this energy lands in the light output with with an efficiency
of 80% visible light, this would lead to 100 Ws real light energy
and this could be called "effective Ws"
6. To know the Ws figure of your 540, you could open the gun and read
the value of the big capacitor and measure the voltage on it in
charged condition. :-)
Regards, Jos
Peter Loveday wrote:
guide number
Watt Seconds
Effective Watt Seconds
BCPS (Beam Candle Power Seconds)
16x the light. However its a largely irrelevant measure; GN is
completely dependant on any sort of reflectors, diffusers, zoom heads
etc etc. So on a white lightening, this figure changes with anything
you put on it. As does the 540, but this is less common of course.
This is of course also why the 540 changes GN with zoom position; the
Watt-Seconds would be the same no matter what; it is an absolute power.
Howeer even this is misleading, as it is the amount of power the flash
uses, not how much light it puts out. Different efficiencies etc can
make this vary somewhat; and it is often only truly useful when
comparing a range from one manufacturer.
I've not seen BCPS much, but I assume its the american or imperial
equivalent of Lux /Lumens/Candela? In any case, this is potentially
the most useful, but least quoted spec.
- Peter
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