The Guide Number (GN)

The starting point is the Guide N or GN number of the Flash. Sort of a manual exposure data point published, provided by the manufacturer who tends to exaggerate and sometimes some lie a lot in their favor. But, we like the police with a lie detector soon sort out the.....indiscretions. 

Every flash model has its guide number, and even they may vary based on age, usage as the flash gets older. New flashes may be stronger. As the plastic degausses or shows age, just like the headlamps on your car turning frosty or yellow, the capacitor may weaken, this may pollute, dilute, and change the output.  The GN of a flash is usually given at ISO 100. If a different ISO is being used, you have to adjust it.

EXAMPLE: Lets use a simple Vivitar 285HV which suggests a guide number of 120 for ISO 100 at ten feet.  Simple.   Divide 10 into 120 and you get F12. Digitals can do F12. In the old days we just used F11.  Any photographer who has used a Vivitar 285 HV found the flash underexposed, about one stop, and he adjusted the guide number down to compensate.  Most used 90 or so.  What does this mean in manual shooting? You got better more saturated shots with F9 or actually F8 than stopped down at F11. More light got to the subject and made it back to the film.  The correction, a photographer decision created more saturated brilliant pictures.  You are in command. 

VARIATIONS - Read the manual, the manuals of many newer flashes have started to provide guide numbers for various focal lengths. This can help more accurately determine the aperture to be used.  The focal length can change the Guide Number. You can bet the manufacturer advertises at the best looking number for his product. It may not produce the effects you want at your focal length.