If you have ever shopped for a flash or read a review, you may have noticed a specification called the guide number (or GN, for short). The GN is a measurement of the flash’s power, which allows you to compare flash models. Even so, it’s hard to see how this number is useful to you. So today, let’s look at the GN to see how it identifies your flash and your camera settings using some math — instead of guessing and retrying until you get what you want. Using this information, you can use the guide number to calculate the best f/stop setting and determine the maximum range that’s effective for your flash.
Do I need to know this?
If you always use your flash in automatic mode, you may never need to know much about guide numbers. But in manual mode, if you don’t have a flash meter, understanding how guide numbers work is a powerful tool to get your lighting set up faster. Many older flashes have built-in guides to help calculate the aperture setting for a given distance. A quick look through Amazon or eBay will reveal many high quality, manual-only flash units. Knowing what GN means allows you to use these inexpensive flashes very effectively.
Using the guide number to determine aperture
You can download a spreadsheet I’ve made that will do much of the work for you. It also contains the GNs for a handful of popular flash heads. Start with understanding the math behind the spreadsheet to read the GN information for your own equipment.
Some flash devices simply have a single GN value, while others may have different numbers at different ranges and even ISO settings, so you need to pay attention when reading the GN. Let’s take a look at a typical flash setup. A Canon 580 EX II flash has a listed GN of 191. If you divide the GN by the distance in feet, you get a good estimate of the aperture to use. In this example, use a distance to the subject of 30 feet.
Guide Number / Distance = Aperture
191/ 30 = 6.3
The closest aperture setting on my Canon 30D to that is f5.6, so that would be a good aperture to use if the camera was set to ISO 100. If I was using a higher ISO number, then I could use a smaller aperture. The following chart shows the multiple factor for common ISO speeds:
ISO Multiplier Table
ISO 100: 1.0
ISO 200: 1.44
ISO 400: 2.07
ISO 800: 2.99
ISO 1600: 4.30
ISO 3200: 6.19
To expand on the previous calculation, add in the ISO to get more aperture options. Using the same guide number and distance while adding the ISO, see how this effects aperture setting:
(Guide Number / Distance) * ISO Multiplier = Aperture
(191/ 30) * 1.0 = 6.3
(191/ 30) * 1.44 = 9.1
(191 / 30) * 2.07 = 13.1
(191 / 30) * 2.99 =19.0
The most common question at this point is how to determine the distance to your subject without measuring it with a tape. Most lenses have a distance readout that shows you the distance to your subject once you focus. Just press your shutter halfway down while aiming at the subject, take your finger off the shutter, then look at the distance readout. You don’t need complete accuracy here since the aperture settings in your camera aren’t going to exactly match the calculations, but get as close as possible to avoid too much trial and error. Using this information, you should be able to get your light dialed in much faster than just by guessing.
Determining Maximum Flash Distance
Using a similar method, you can also determine the maximum distance to which your flash can be effective. This is extremely useful when trying to determine if you can light up a person on a stage from the back of an auditorium, for example. The calculation for this is the GN divided by the f/stop, and again the ISO value can be used here as well. In this example, use the GN of a Canon 580 EX II (58) and an f/stop of 5.6.
(Guide Number / f/stop) * ISO = Max Distance
(191/ 5.6) * 1.0 = 34′
(191 / 5.6) * 1.44 = 49′
(191 / 5.6) * 2.07= 70′
(191 / 5.6) * 2.99 = 102′
Since the f/stop and the ISO determine how much light enters the camera, you see how using a larger aperture (lower number) and a higher ISO can really affect the range of your flash. Let’s compare the difference between f/5.6 and f/2.8 as an example:
GN 191@ f/5.6 100 ISO = 34′
GN 191 @ f/5.6 200 ISO = 49′
GN 191 @ f/2.8 100 ISO = 68′
GN 191 @ f/2.8 200 ISO = 98′
These calculations help you determine if a long distance shot is even possible, or if you need to bring in additional light. If you are too far from the subject for your light to be effective, you could place your light closer to the subject and fire it with a wireless trigger, this would allow you to shoot from a distance but get the desired light effect.
Some Final Thoughts
I know this can seem a little intimidating at first, but understanding these concepts helps you get your lighting technique under control much faster and helps make sure you can get the shots you want. You can even use the calculations to build a custom “cheat sheet” for your particular flash to print out and keep as a handy reference. The spreadsheet has both the f/stop and maximum distance calculators. It also has a second worksheet that will create the cheat sheet for you by simply changing the guide number on the first line to match your flash device’s output.
Calculation Spreadsheet [ Download ]
Photo credits: Kerry Garrison