Doug Kerr
Well-known member
"Power" and watt-seconds
In "studio-type" flash systems, it is common to speak of setting the "power" of the unit, a property that is then denominated in the unit generally written "watt second", and usually abbreviated "WS" or "ws".
The quantity being spoken of, however, is not power (one clue being that power is not denominated in the unit watt-second, but rather the unit watt). Rather, the quantity being spoken of is the energy to which the storage capacitor is charged (a quantity that is in fact properly denominated in the unit watt-second, whose proper abbreviation is "W-s" - although the preferred unit today is in fact the joule).
But that is not really the quantity in which we are interested from a photographic exposure standpoint anyway. Ultimately, we are interested, for our complete flash "rig", the luminous intensity-time product in the direction toward the subject (which is denominated, preferably, in the unit candela-second).
However, this is not just a property of the flash unit itself (and its current output setting), but is greatly affected by the nature of the reflector and other "light modifiers" in use. If we wish to speak of the flash unit itself, we need to think in terms of its overall luminous flux-time product (which is preferably denominated in the unit lumen-second).
Now, for any given flash unit design, there is a direct (ordinarily approximately proportional) relationship between the stored energy and the overall luminous flux-time product. But that relationship is not the same for all flash units. It depends on many factors, which we can gather together as a single property, the "electro-optical conversion efficiency". This varies substantially between different products.
Thus two flash units, both set to an output of "1200 watt seconds", may deliver a significantly different overall luminous flux-time product. I have no idea how wide the spread is over today's units.
To cure this imprecision, the reasonable approach would be to adopt the use of the real photometric property (overall luminous flux-time product), denominated in the standard scientific unit (lumen-second). That would have been very "European".
But of course that would have been too "nerdly" for the US. Instead, the industry here some years ago adopted a system of "effective watt seconds". This was predicated on a certain uniform electro-optical conversion efficiency. Thus, two units having the same effective watt second rating could be expected to deliver the same overall luminous flux-time product.
The standard conversion efficiency adopted was lower than the actual efficiency of typical units at the time. As a result, the effective watt second rating would generally be numerically greater than the watt-second rating itself. This led to the complaint that this system was only a marketing scheme to "inflate the ratings". (Long-time radio engineers may recall a similar flap upon the adoption of the effective radiated power method of describing radio station radiation intensity.)
Even today, we will sometimes see flash unit manufacturers say, "Blitzkraft always specifies its flash units in term of true watt seconds, not the misleading 'effective watt seconds' values often seen elsewhere." In other words, they consistently use the familiar quantity, never mind that it is quite imprecise.
How does this relate to guide number?
Guide number is a way of specifying the luminous intensity-time product, in the direction of the axis, of a flash unit (the actual photometric property in which we are interested). It allows us to solve the "standard flash exposure equation" easily. If we take the guide number for the flash unit (this assumes a fixed output) and divide it by the distance from the flash to the subject, the result will be the f-number that would be recommended by the standard flash exposure equation.
For any given flash unit (and output setting), the guide number varies with the ISO sensitivity to be assumed. Often the guide number for ISO 100 is the basic one published. The guide number has the dimensionality of distance, and thus is often denominated in feet or meters (the manufacturer occasionally failing to mention which is meant).
Can we relate the watt-second value for a studio flash to guide number? Not really, since the watt-second rating does not tell us even the total luminous flux-time product, and certainly not the luminous intensity time product considering the particular reflector/light modifier setup in use.
For the masochists
Those who are interested in further information on this topic might want to read my technical article, "Characterizing the Output of Photographic Flash Units", available here:
http://dougkerr.net/Pumpkin/index.htm#PhotoflashOutput
Best regards,
Doug
In "studio-type" flash systems, it is common to speak of setting the "power" of the unit, a property that is then denominated in the unit generally written "watt second", and usually abbreviated "WS" or "ws".
The quantity being spoken of, however, is not power (one clue being that power is not denominated in the unit watt-second, but rather the unit watt). Rather, the quantity being spoken of is the energy to which the storage capacitor is charged (a quantity that is in fact properly denominated in the unit watt-second, whose proper abbreviation is "W-s" - although the preferred unit today is in fact the joule).
But that is not really the quantity in which we are interested from a photographic exposure standpoint anyway. Ultimately, we are interested, for our complete flash "rig", the luminous intensity-time product in the direction toward the subject (which is denominated, preferably, in the unit candela-second).
However, this is not just a property of the flash unit itself (and its current output setting), but is greatly affected by the nature of the reflector and other "light modifiers" in use. If we wish to speak of the flash unit itself, we need to think in terms of its overall luminous flux-time product (which is preferably denominated in the unit lumen-second).
Now, for any given flash unit design, there is a direct (ordinarily approximately proportional) relationship between the stored energy and the overall luminous flux-time product. But that relationship is not the same for all flash units. It depends on many factors, which we can gather together as a single property, the "electro-optical conversion efficiency". This varies substantially between different products.
Thus two flash units, both set to an output of "1200 watt seconds", may deliver a significantly different overall luminous flux-time product. I have no idea how wide the spread is over today's units.
To cure this imprecision, the reasonable approach would be to adopt the use of the real photometric property (overall luminous flux-time product), denominated in the standard scientific unit (lumen-second). That would have been very "European".
But of course that would have been too "nerdly" for the US. Instead, the industry here some years ago adopted a system of "effective watt seconds". This was predicated on a certain uniform electro-optical conversion efficiency. Thus, two units having the same effective watt second rating could be expected to deliver the same overall luminous flux-time product.
The standard conversion efficiency adopted was lower than the actual efficiency of typical units at the time. As a result, the effective watt second rating would generally be numerically greater than the watt-second rating itself. This led to the complaint that this system was only a marketing scheme to "inflate the ratings". (Long-time radio engineers may recall a similar flap upon the adoption of the effective radiated power method of describing radio station radiation intensity.)
Even today, we will sometimes see flash unit manufacturers say, "Blitzkraft always specifies its flash units in term of true watt seconds, not the misleading 'effective watt seconds' values often seen elsewhere." In other words, they consistently use the familiar quantity, never mind that it is quite imprecise.
How does this relate to guide number?
Guide number is a way of specifying the luminous intensity-time product, in the direction of the axis, of a flash unit (the actual photometric property in which we are interested). It allows us to solve the "standard flash exposure equation" easily. If we take the guide number for the flash unit (this assumes a fixed output) and divide it by the distance from the flash to the subject, the result will be the f-number that would be recommended by the standard flash exposure equation.
For any given flash unit (and output setting), the guide number varies with the ISO sensitivity to be assumed. Often the guide number for ISO 100 is the basic one published. The guide number has the dimensionality of distance, and thus is often denominated in feet or meters (the manufacturer occasionally failing to mention which is meant).
Can we relate the watt-second value for a studio flash to guide number? Not really, since the watt-second rating does not tell us even the total luminous flux-time product, and certainly not the luminous intensity time product considering the particular reflector/light modifier setup in use.
For the masochists
Those who are interested in further information on this topic might want to read my technical article, "Characterizing the Output of Photographic Flash Units", available here:
http://dougkerr.net/Pumpkin/index.htm#PhotoflashOutput
Best regards,
Doug
