Before the days of automatic through-the-lens (TTL) flash units, exposure calculations had to be done using the Guide Number (GN) for the flash unit. When you read the manual for your new Speedlight you made a note of the GN, right?
The GN is the relative measure of the flash unit’s power or output. The larger the GN, the more powerful the flash unit, the farther it can throw light. The manufacturer determines the GN by thoroughly testing the flash at various f/stops. GN is usually listed in feet for ISO 100 film or ISO 100 setting on digital cameras.
To determine proper exposure think of the following formula: GN = f/stop x distance (flash to subject) Using a flash with a GN of 110, shooting at ISO 100 and 10 feet away from the subject, the proper f/stop would be f/11. Shutter speed is set at your camera’s flash-sync speed (you’ll find this in your camera manual). The Nikon F100 has a sync speed of 1/250. So the settings using a ISO100 film would be f/11 @ 1/250 second.
If your shooting a film or setting other than ISO 100 you have to do some calculating. Doubling your ISO does not necessarily double the effective range of the flash. Based on the example above, changing to ISO 200 the GN would not be 220. Moving from ISO100 to ISO 200 is one stop. Since we gained a stop of light the new f/stop is f16. The GN for ISO 200 would be 160 or GN = f16 x 10.
By knowing your flash unit’s GN, the distance from flash to subject and the simple equation GN=f/stop x distance, you can calculate the correct exposure every time. This is also useful for those with TTL flashes as you can determine the maximum distance for a given f/stop.