Flash Guide Number

The flash guide number tells you - in a general sense - how powerful the flash is and hence, how much of an area it can illuminate.

Put another way:

  • If your goal is to take intimate portraits in a small room, you don't need flashes with huge guide numbers
  • If it's your intention to photograph the interior of cathedrals, then you do

These are clearly extreme examples - for many photographers the most common use for a flash will be somewhere in between.

While the information on this page probably won't help you zero in on the exact guide number you need, a better understanding of flash guide numbers will help clarify why one flash can cost $100 and another can cost $400.

Distance, Aperture and ISO

In order to understand how a flash guide number is calculated, you first have to understand two common digital SLR camera settings: aperture and ISO.

  1. Aperture is the width of the opening in the camera's lens - a wider aperture allows more light to land on the camera's sensor
  2. ISO indicates how much light the sensor can absorb - the higher the ISO number, the more light the sensor absorbs

Apertures are measured in f-stops, and there's a common scale for them: 8.011162232

From left to right the apertures are decreasing in size. This means that an aperture of f/1.4 is wide open while an aperture of f/32 is quite narrow.

There's also a common scale for ISO numbers:

100200400 80016003200

At ISO 100 the camera is not very sensitive to light and at ISO 3200 it is VERY sensitive and absorbs light quickly.

So what does this all have to do with flash guide numbers?

A Balanced Exposure

Regardless of the amount of available light, every photo that you take can turn out one of three ways:

  1. Under exposed — too dark
  2. Over exposed — too bright
  3. Correctly exposed — the right balance of dark and light

Ideally, you'd like to capture photos that look like #3 all the time - but this is sometimes difficult when the available light is very dim.

When there's not a lot of light to work with, you can:

  1. Open the lens aperture very wide
  2. Increase the ISO setting
  3. Enable the flash

When taking photos in dim light, you can sometimes get away without using a flash just by opening the lens aperture and increasing ISO. In this case, you're just setting the camera to absorb every ounce of available light to create a good exposure.

When you enable the flash, you're introducing a SECOND light source into the photo, and this light can either supplement the available light or overpower it completely.

The guide numer is an indication of a flash's ability to overpower ambient light and brighten your subject.

Flash Guide Number Formula

There's a mathematical formula for calculating flash guide numbers:

Guide Number = [Flash to Subject Distance] x [F-Stop]

Before we dig into some examples, it's important to note the following constant in the equation: ISO. When a guide number is calculated, it is often assumed that the ISO is set to 100, since increased ISO numbers will have an impact on the guide number calculation.

Now let's look at some examples.

When you're considering a new flash purchase, the only number that the manufacturer provides is the guide number - it's up to you to figure out the other two numbers in the equation.

The most obvious variable to consider first is flash to subject distance.

If we assume that the aperture is set to a constant value of f/4, then it quickly becomes apparent that flashes with higher guide numbers can effectively light subjects that are farther away:

Guide NumberF-StopFlash to Subject DistanceCalculation
40f/410 feet / 3 metersGN = 10 ft. x f/4 = 40
80f/420 feet / 6 metersGN = 20 ft. x f/4 = 80
120f/430 feet / 9.1 metersGN = 30 ft. x f/4 = 120
160f/440 feet / 12.2 metersGN = 40 ft. x f/4 = 160

Alternatively, if your flash to subject distance is a constant, then flashes with higher guide numbers allow you to use narrow lens apertures (which helps you achieve more depth in the image and keeps everything sharp):

Guide NumberF-StopFlash to Subject DistanceCalculation
40f/410 feet / 3 metersGN = 10 ft. x f/4 = 40
80f/810 feet / 3 metersGN = 10 ft. x f/8 = 80
110f/1110 feet / 3 metersGN = 10 ft. x f/11 = 110
160f/1610 feet / 3 metersGN = 10 ft. x f/16 = 160

So there you have it: you can leverage the extra power of a flash with a high guide number in one of two ways:

  1. By using a wide aperture, you can light subects that are farther away
  2. Staying the same distance from your subject, you're able to use a wider range of apertures

Bottom line: look for a flash with a high guide number if you think that you'll often use the flash to light subjects from a distance OR if you don't want your aperture choices to be limited every time you use a flash.

Guide Number Reporting

If you go shopping for an electronic flash online, you'll probably see it listed like this:

[Flash Name] with Guide Number (GN) of 141 ft. / 43m

Sometimes the ISO value will be stated, but if it isn't just remember that all guide numbers are calculated at ISO 100.

The only value ever reported as the guide number is the flash to subject distance in both feet and meters.

You'll note that the lens aperture used to calculate the guide number is left out of the reported value, which leads quite well into our next topic.

Guide Numbers = Marketing Hype

We now come to the only drawback of using flash guide numbers to evaluate and compare different electronic flash units.

Over time, flash manufacturers have come to rely on guide numbers to sell flashes a lot like camera manufacturers have relied on megapixels to sell cameras.

As such, they "optimize" the environments in which they test their flashes so that they can report a high guide number on the flash spec sheet.

Since you probably won't be using your flash to light subjects in a controlled environment, you'll discover that the "real" guide number of your flash doesn't match what the manufacturer states in their promotional literature.

So what's the takeaway here?

Use guide numbers to get a general sense of the power of an electronic flash, but if you're trying to compare two virtually identical units, don't get too hung up on one with a slightly higher guide number.

However, if you're looking at two different flashes and one reports a guide number of 72 while another has a guide number of 190, the one with 190 is clearly the more powerful flash (and hence much more expensive).

Related Links

Learn More About Digital SLR Cameras - Free!

The monthly newsletter is packed with digital SLR tips and advice. Current Issue
First NameE-mail Address 
Your privacy is respected and your information is NEVER shared with anyone.
previous pageSLR Flash Glossary Digital SLR Home Flash Recycle Ratenext page
What's New? On Facebook: On Twitter:
Home Lessons Free Newsletter DSLR Store About Contact Site Map Mirrorless Cameras