Watt-Seconds vs. Guide Numbers

Guide numbers are mostly used on speedlights and watts-second is being used on monolights. Does anybody know the computation from Guide Number to Watts-Second and vice versa?

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Watt-seconds (Ws) is a unit of energy. When reading flash unit specifications, the Ws rating is the amount of energy that can be stored in its capacitors and released when the flash is fired.

The Guide Number (GN), on the other hand, is a measure of light. It indicates how intense the light from a single flash is when falling on the subject.

There is no simple conversion formula between the two. While the energy that at most can be expended in a single burst of flash is proportional to the number of Ws' stored in the flash unit's capacitor, the actual intensity of light also depends on the efficiency of the light source, and any reflectors or diffusers in use.

However, as a rough rule of thumb, multiplying the square root of the Ws number by 4 (meters) or 13,2 (feet) usually gives a rough indicator of the GN in at ISO 100 for a standard parabolic reflector with a light spread matching a 35 mm FX lens. The inverse function computes the Ws from the GN with a 35 mm light spread in meters or feet.

GNm = SQRT(Ws) x 4
GNft = SQRT(Ws) x 13.2
 
Ws = (GNm/4)2
Ws = (GNft/13.2)2

Example: a studio flash with rated at 250 Ws may have a light output equivalent to a speedlight with a GN of 63 (meters) or 209 (feet)

Note that light spread affects the GN, but not the Ws rating. If two identical studio flashes both are rated at 150 Ws, and one is fitted with a reflector with an angle of illumination of about 45 degrees, and the other fitted with a reflector with an angle of illumination of about 90 degrees, the flash with the narrow cone reflector would have a higher GN than the one with the wide reflector.

In terms of energy consumption, a 60 watt tungsten bulb burning one second, a 600 watt halogen light being lit for 1/10th of a second, and a xenon tube electronic flash burst consuming 60000 watt for 1/1000 second all equals 60 Ws. However, a xenon tube is much more efficient converting energy into light than the other two, so even when fitted with the same reflector, the 60 WS electronic flash will produce more light than each of the other two. Translated into GNs, the GN of the electronic flash in this example would be greater than the GN of the tungsten bulb or the halogen light.

Measuring the GN of a flash is straightfoward. Just fire the flash and measure its output with an incident flash meter set to ISO 100. Multiply the aperture reading with the distance in meters or feet, and that's the GN of the flash in meters or feet.

There is no simple way to measure WS, so one have to rely on manufacturer's specifications. I suspect that some of the Ws ratings used to advertise studio flashes are not set by engineeing, but by the marketing department.

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