What is a Flash Unit's Guide Number?

The guide number (GN) for a flash unit measures how well it can light a subject at a specific ISO sensitivity, and with a precise angle of view (wide-angle vs. telephoto lens). To put it simply, a higher guide number means the flash is more powerful, all other things being equal.

Be careful when you are deciding on an external flash unit to use, whether it is a genuine Nikon Speedlight or an aftermarket unit. Simply comparing the GN is not enough. You must understand the settings the GN is based upon. Many flash units have zoom capability, and will be able to light up subjects farther away when zoomed out. However, imagine buying a flash unit from a manufacturer who publishes the GN based on a longer zoom position, and then comparing it to a different flash unit based on a shorter non-zoom position. The GN rating on the flash that is zoomed out would seem to be higher than the same unit not zoomed out. However, unless you are comparing flash unit guide numbers with exactly the same settings, it is truly like comparing apples and oranges.

For instance, to get an exact comparison of guide numbers, you’d have to know the following:

• Distance from flash head to the subject
• Aperture f-stop number in use on camera
• ISO sensitivity of your camera’s image sensor
• Angle-of-view setting on the flash’s zoom head
• Actual angle of view your lens provides (must match flash head)
• Temperature of ambient air

In reality, the camera has little to do with figuring the GN other than providing an aperture f-stop number and ISO sensitivity. So, how can you decide what GN is best without whipping out a scientific calculator? Just look at the flash unit advertisement to see what the GN is based on. Here are the most important figures:

• Flash “zoom” angle-of-view setting, (e.g. 18mm or 105mm, etc.)
• ISO sensitivity

If you see a flash unit advertised as GN 98, just realize that this is not enough information to make a decision. In this instance, the number 98 is the GN. It represents the number of feet from flash head to subject (98 feet). In countries that use the metric system, an equivalent GN is 30, which represents the number of meters from flash head to subject (30 meters). That number by itself is simply incomplete. Don’t buy a flash unit based solely on a GN like 98 or 100 or 111. Here’s why:

Let’s think about this for a second. Let’s say I were a manufacturer who was desperate to sell you a flash unit. I might stretch things a little bit. I might say my Super-Duper flash unit has a GN of 98 (feet) or 30 (meters), hoping you won’t ask about the settings I used to figure the number. Here is a comparison of two flash units with a so-called “comparable” guide number:

Super-Duper Flash Unit GN Information

• GN 98 (30)
• 80mm zoom-head setting
• ISO 200 sensitivity

Nikon SB-400 Flash Unit GN Information (real values)

• GN of 98 (30)
• 35mm zoom-head setting
• ISO 100 sensitivity

Nikon SB-400 Speedlight Flash Unit, GN of 98 (18)

Both of the flash units above have the same GN listed, so which one is really more powerful? The Nikon SB-400 will literally blow away the Super-Duper unit. Yet, the Super-Duper manufacturer lists the same GN! The Super-Duper unit must have its zoom head set to 80mm, a much narrower beam, and have twice the camera ISO sensitivity to equal the Nikon SB-400 unit. Mr. Super-Duper is hoping you won’t check the fine print at the bottom of the advertisement, so that you’ll think his much less powerful unit equals the Nikon SB-400. Surprisingly, there are flash unit manufacturers who will do exactly what I am describing.

What can you learn from this? The actual GN itself is not enough to make a decision on which flash unit to use. You must know what the GN is based on in order to make an informed decision. Take your time when buying a flash unit. You’re safe in sticking with Nikon’s Speedlights, since the ratings are well known and they’re designed to support all the features of your camera.

There are also excellent aftermarket flash units available from manufacturers like Vivitar, Sigma, Sunpak, Metz, Braun/Leitz, and others. Examine the underlying settings and not just the guide number itself. What the GN is based on is as important as the actual number.

For comparison purposes, the GN of the most Nikon pop-up Speedlights are from 39 to 56 (feet) or 12 to 17 (meters) at ISO 200. Nikon’s flagship Speedlight, the SB-910, is 157.5 (feet) or 48 (meters) at ISO 200. Obviously, the larger external flash unit has a lot more power and can light up subjects that are farther away.

Technical Guide Number Information (Geek alert)

For those technically minded among us, the GN is based on a specific formula: GN = distance × f/number. It is based on the inverse-square law, which states that doubling the GN requires four times more flash power. So, a flash with a GN of 100 is four times more powerful than a flash with a GN of 50. The guide number represents an exposure constant for a flash unit. For example, a GN of 80 feet at ISO 100 means that a subject 20 feet away can be completely illuminated with an aperture of f/4 (80 = 20 × 4) using a sensitivity of ISO 100. For the same guide number and an aperture of f/8, the light source should be 10 feet from the subject (80 = 10 × 8). Fortunately, your camera and flash combination are capable of figuring the correct values for you when you use TTL mode.

Keep on capturing time...
Darrell Young