Do I need to know this?
If you are always using your flash in automatic modes, you may never need to know much about guide numbers but in manual modes, if you don’t have a flash meter, understanding how guide numbers work can be a powerful tool in helping you to get your lighting setup faster. Many older flashes even have built-in guides to help you calculate the aperture setting for a given distance.

Using the guide number to determine aperture
At the end of this article is a link to an Excel preadsheet that will do much of the work for you as well as containing the guide numbers for a handful of popular flash heads. First off you should understand the math behind it and how to read the guide number information for your own equipment.

Some flash devices will simply have a single guide number value while others may have different numbers at different ranges and even ISO settings so you need to pay attention when reading the guide number. Let’s take a look at a typical flash setup. A Canon 580 EX II flash has a listed guide number of 191, if we divide the guide number by the distance in feet we should get a good estimate of the aperture to use. In this example we will 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 an appropriate aperture to use if I was using 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, we can add in the ISO to give us more aperture options. Using the same guide number and distance while adding the ISO, let’s see how this affects our 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 now is how to determine the distance to your subject without measuring it with a tape. Most lenses will have a distance readout that will show you the distance to your subject once you focus on it. Just press your shutter half way down while aiming at the subject, take your finger off the shutter, and then look at the distance readout. We aren’t aiming at hyper-accuracy here since the aperture settings in your camera aren’t going to exactly match the calculations but we are trying to get as close as possible and avoid a lot of 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 we can also determine the maximum distance your flash can be effective at, 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 guide number divided by the f/stop, and again the ISO value can be used here as well. In this example we will use the guide number 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 will 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.

Summary
I know this can seem a little intimidating at first but understanding these concepts will help you get your lighting technique under control much faster and help 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 that can print out and keep as a handy reference. The spreadsheet that we have made available for you has both the f/stop and maximum distance calculators but 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 ]

Author: Kerry Garrison