Stump Ed S - Guide numbers & watt seconds

This is a question I asked over on that other forum we used to hang out on, but the replies got butchered when a user was removed along with all their postings.  So I'll ask it again here 

What's the difference between a watt-second and guide number, and why is studio lighting usually measured in w/s and shoe mounted flashes in gn?  And I've explanations on how to measure GN, but is there a way to measure w/s?

  OK- Here’s all the information.

Watt/Seconds are strictly an electrical value and is not a comprehensive rating on which to base exposure.  Watt/Seconds are derived from a mathematical formula based on a combination of voltage and capacitance that is applied to the flash tube at the moment of triggering allowing a pulse of high voltage to ionize the gas inside the tube thus causing a flash of light.   Professionals who are familiar with electronic flash equipment know that most consumer type small flash units and those built into cameras usually runt at about 30 to 80 watt/seconds, professional hand mono lights and power pack units go from 25-2,400 W/S and some heavy duty commercial units can go as high as tens of thousands of watt/seconds and require banks of external capacitors- this equipment is oftentimes custom built and used at indoor sports arenas and for photographing large objects such as automobiles in a studio setting where a lot of light is lost in bounce plus diffusion systems over a large area and lots of power is needed to enable a small working f/stop to maintain depth of field.  If I were shopping for a hand-held system for wedding work, a portrait studio setup or a system for photographing cars and trucks, the Watt/Second rating would get me in the ballpark as to what to look for.  A 2,400 W/S system working off a power pack may be overkill for a portrait studio because I prefer units of a much lesser power so that I can work at wider apertures for selective focus purposes and for a bit more softness.
A stronger system, however, is excellent for food and table top work using light modifiers.
There are also some cross-usages such as using a 2400 W/S unit as a fixed bounce fill lighting system in a portrait studio along with lower powered mono lights as mobile main and accent lights.

Seeing that there is no direct correlation between watt second output and exact exposures the question then is; what formula is used to determine exposure?  A bit of further explanation is in order.

Let’s use a simple set up as an example.  Se we set up a 100 W/S init in the center of a studio room and use the bare bulb mode- no reflector in place.  We then place an opaque black cloth over the flash tube and fire the unit; electronically speaking, we have used 100 W/S of energy but effectively, no light was seen- we have nevertheless expended 100 W/S of electrical energy.  We now remove the opaque cloth, fire the unit and our incident light flash meter tells us that at a distance of 10 feet from the light source we are getting a reading of f/5.6 worth an ISO setting of 100.  This reading was obtained with the flashtube oriented in a vertical position where the light is coming from the sides of the flash tube.  We then point the top of the flash tube at the meter and find that the reading is now f/2.8- we have now lost 2 stops because most of the light is emitted from the side of the tube so most of the light ended up on the ceiling and the floor and the least amount landed on the meter- where the subject is supposed to be.  Next we place a standard reflector on the flash head and get a reading of f/11 because the light that was striking the floor and ceiling is now hitting the reflector and being emitted as a beam of light over a certain angle of coverage.  Manufacturers often state this effective performance value as Beam Candle Power Seconds (B.C.P.S) or Effective Candle Power Seconds (E.C.P.S.).  This is a combination of factors which includes Watt/Second input and reflector efficiency. A unit cam be rated at (for example) say 2,300 E.C.P.S. at 60 degrees.  This would be a typical rating for a professional caliber professional hand held or camera mounted flash unit set at 100 W/S with a decent reflector in place for general use (wedding candids etc.)

OK- this is all very nice except we still have no exposure information so the next step is to determine the guide number.  There is a simple formula for this:
 You must find the square root of .063 X 1SO setting X E.C.P.S.

When you have done all the math and rounded things off you will arrive at a guide number of about 120.   This magic number takes care of the inverse square law and finally helps us achieve decent exposures with electronic flash; you simply divide the distance in feet, from the flash unit to the subject, into the guide number so that in this example at a distance of 10 feet the f/stop would be f/12 (between f/11 and f/16).  At 6 feet it would be f/20- just a bit before f/22 on your dial.  Theses are all, of course, for manual settings- if you prefer to use automatic of TTL flash modes, ostensibly your equipment should do you calculations for you but for very precise work, some photographers still prefer manual settings.

Now that you have some of the good news as to the simple math of flash lighting- here’s some bad news:  REAL guide number are determined by precise testing under YOUR usual shooting conditions and YOUR equipment- there are many variations in theses areas.  The formulas, however, get you well into the ballpark but it is up to each photographer to create the perfect guide numbers for each piece of flash equipment that the use.  As soon as you change or remove a reflector, bounce the light, use a light modifier, or operate at a different ISO rating, the entire guide number changes. 

Back to Watt/Seconds for a moment; once you have established a working system in your studio and or your location work you can use the Watt second adjustments for fast and east control of exposure variations and lighting ratios.  If you are working at 100 W/S and wish to increase the output of any adjustable unit, either individually or within a system, switching the unit to 200 W/S will give you 1 full f/stop more light- at 400W/S your will gain yet another f/stop and at 800 W/S yet another.  If you were to decrease the power to 50 W/S you would drop one f/stop and so on.  Double the power is +1 stop and halving the power is – 1 stop.

In every case, the manufacturer’s rating must be put to the test.  A published rating achieved under laboratory conditions or strictly by mathematical formula is not necessarily exact.  Even after comprehensive testing it may be required to adjust exposures in larger or smaller working areas or in brighter or darker colored rooms.  Even automatic settings may require some adjustment due to subject failure.  At the end of the day, good and consistent flash technique is half mathematics and half instinct based on experience.  It may sound complex but once you “get it” it becomes second nature.

I hope this helps to clear things up.   Ed S.  OH! By the way, Ryan- GREAT QUESTION!