Guide Numbers and Watt-Seconds

Whenever I think about hauling lighting equipment to a location, the question comes up: Should I take a.c. strobes, extension cords and power bars, or will a couple of hot-shoe flashes, umbrellas and stands do it? Hmmmm...I know what I'd prefer.

The trouble is, hot-shoe flash output is (usually) expressed in Guide Numbers (GN) while strobes are rated in Watt-seconds. Sadly, there is no easy way to relate the two, so I made a quick setup to measure the output of each. I've always assumed that a 150 Watt-second strobe would blow away a flash in terms of light output, but I was about to be surprised.

Now, we should start with definitions. GN is the product of the distance of the flash to the subject and the f stop being used at a particular ISO setting (or film speed). Example: I set my flash meter at a sensitivity of 100 ISO and placed it at 10 feet from the flash. I fired the flash and read the f stop it was recommending. For a flash rated at GN 80 (ft - ISO 100), you would read f/8 on the meter (f/8 x 10 ft). Indeed, my Sigma EF 500 came in close at f/7.6 (hence a GN of 76).

Next, I set up the 150 Watt-second strobe at the same location as the flash, set it for full power, and blasted away. Here's the surprise: it read f/7.9 on the light meter, meaning its GN equivalent is 79, which is barely better than the Sigma. Admittedly, there are a couple of variables here. One is that the light meter is older and uncalibrated, and the Sigma's batteries may not have been at peak voltage. The point, however, is that on a relative basis you can determine how your strobe/flash sources compare. This is particularly helpful when the manufacturer of the flash doesn't provide the GN.

So GN is a measure of the amount of light available for proper exposure of a subject at a particular combination of distance from the flash and camera aperture setting, on a camera set at a particular ISO sensitivity. On the other hand, Watt-seconds are a measure of the electrical energy expended in the strobe tube (Watts are the rate at which energy is used, so multiplying by time gives energy used over that time period). This measure doesn't take into account the conversion efficiency of the tube from electrical energy to light energy, or the effect of the reflector behind it. Hence, GN and Watt-seconds can't be related mathematically.

Bottom line: measure the output of each with a flash meter so you know how they relate. Similarly, if you double the Watt-seconds you can expect the guide number to double, since it should add another stop to the flash meter reading.

Then, you write your results down, as I did, so that confusion doesn't ensue.