You can see in Figure B, a guide number shot, that it's quite underexposed. According to the formula and guide number, it was exposed at f/22.

FIGURE B

This shot is underexposed due to an aperture that is too small. Click picture for a larger image.

Figure C shows the same shot, appropriately exposed by flash meter at f/8. That's quite a difference in range.

FIGURE C

The same shot, appropriately exposed by flash meter. Click picture for a larger image.

After testing some exposures under controlled conditions, I came to the conclusion that the published guide numbers for my SunPak units are either flat-out wrong, or both of the units have had identical intensity fall-off, which I think rather unlikely. I've determined that the actual guide number for full power with 400 speed film is 110, not 200.

I also learned something that had never occurred to me before, because I use a flash meter. When using a reflector umbrella, you will lose one full aperture stop due to the reflector's scattering of the light. Taking another look at our original example where we came up with an aperture setting of f/11, we would actually want to drop that to f/8 because of the use of a reflector umbrella, and then bracket the shot between f/8 and f/11.

Bear in mind, the underexposed example shot is not a fault of the guide number system, but rather the fault of an incorrect guide number. After determining what the true guide number was, and recomputing the math, taking into account the reflector umbrella, I came up with a result much closer to the flash meter. Not identical, but within one stop. When using the guide number system, you would bracket your shots anyway. Even if you choose not use a flash meter, you should at the very least verify the guide numbers for your flash units with a borrowed or rented meter.

That concludes this portion of our tour
The guide number system is a measurement of the light intensity of a flash or strobe at a given film speed. The higher the ASA/ISO of the film, the higher the GN will be. The higher the GN, the smaller the aperture needed for proper depth of field. By dividing the guide number by the distance the light travels to the subject, you can calculate the proper aperture for the best depth of field. Regardless of how many flash or strobe units you use, there's a mathematical formula to figure the appropriate aperture.

When using reflector umbrellas, you will also lose one full f/stop. We also learned that you can't always trust the published guide numbers and should verify them by meter prior to wasting film. If you're using more than one flash or strobe unit though, do yourself a favor and get a flash meter. In the long run, it will pay for itself in saved film and time. Good luck and good shooting.

[By the way, guide numbers are also somewhat applicable to digital photography. Many cameras shoot at film-speed equivalents. So, if you know that your camera is shooting digitally at the equivalent of 100 speed film, for example, you could then go on and use the guide numbers with some degree of confidence. --DG]

James Booth is a self-taught PC and Palm device user who dabbles in graphics and photography. He can be reached at lizardworks@mchsi.com.