| Every flash has a "guide number". This value represents the amount of light produced and is used for metering in indoor photograph situations (where a flash is most often used). The guide number indicates the proper f-stop to use for photographing a subject at a given distance. This is most often found with either manual cameras or manual flashes.
Some newer camera and flash combinations are capable of sending out micro flashes before the photograph is taken that calculates the distance to the subjects and provides proper illumination for the subject in focus.
Guide Number / Distance = f-stop
Take for example three different flashes.
- Guide Number (in feet)
- On Camera Flash: 39 (Nikon N-80)
- Macro flash: 33 (SB-29)
- Hotshoe flash: 118 (SB-28)
- Distance to subject: 10 feet.
- Film speed: 100
Using the on-camera flash, an f/4 aperture would be recommended while the macro flash (less powerful) would need half a stop faster at f/3.4 while the hotshoe flash is much more powerful and could be stopped down to f/11.
If the subject is twice as far away the same light illuminates less (see inverse square law). The on camera flash would now need a f/1.8 as would the macro flash while the hotshoe flash would need f/5.6.
The macro flash is often less powerful because we are talking about a very different distance normally. With macro photography, you are classically working at about one foot away. Here, the flash at full power would allow the photographer to stop down to f/32 (good if you want the depth of field).
This can also be turned around to calculate the proper distance if a particular depth of field is desired. There, the f-stop is fixed and the guide number is too:
f-stop / Guide Number = Distance
As mentioned above, these values are for 100 speed film.
If using a faster or slower film the guide number changes. To calculate the amount to multiply the guide number by:
sqrt(Film speed / 100)
Thus, 200 speed film film has the guide number multiplied by 1.4 while 64 speed film is multiplied by 0.8.
Many flash heads can be twisted or tilted. This affects the amount of light that hits the subject. If the amount of tilt is greater than 30 degrees:
- 45 degrees: open up additional 1/2 stop
- 60 degrees: open up additional 1 stop
- 70 degrees: open up additional 2 stop
Often, photographers will 'bounce' a flash off a wall or ceiling to get lighting in a different direction or more diffuse.
0.7 * GN
f-stop = --------
FCD + CSD
"FCD" is the flash to ceiling distance and "CSD" is the ceiling to subject distance. This takes into account the light fall off from the indirect path.
As mentioned at the start, this value is calculated for typical indoor photography. Using a flash outdoors results in approximately a 70% decrease in effective light output. | 
