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Choosing an External Flash (1): Flash Power

In the previous article, we learned about the benefits of having external flash. Now that you have decided to buy one, how do you determine which of the many models in the market is for you? In this article, find out what a “guide number” refers to and why it matters as we focus on understanding one of the most important factors to consider when buying a flash: Flash power. (Reported by: Masakatsu Nagayama)

Outdoor portrait shoot with external flash


FAQ #1: What should I consider when I choose an external flash?

A1: External flash units differ in power, size and weight. Choose one that best suits your intended purpose.

The external flashes in the market vary in size, weight and price. A basic rule of thumb is that those that are larger in size and higher in price will have a larger guide number (GN) (see FAQ #2) as well as a wider variety of features.

When choosing a flash, first of all, consider carefully how you want to use it.

If you plan to make use of more sophisticated techniques such as bounce flash photography and wireless firing, choose a mid- or high-spec external flash that comes with a rotatable flash head. Bounce flash photography requires more flash power than regular flash photography, so you might want to opt for a higher-spec flash for better results. 

Meanwhile, if portability is your priority, an entry-level unit would usually be the smallest and lightest. However, it will have relatively weaker flash power and a longer flash recycle time, which makes it less ideal for bounce photography and continuous shooting.

We will explore bounce photography and continuous shooting in the next article, and focus on flash power in this one.


FAQ #2: What is the “guide number”?

A2: It is a number that indicates how powerful the flash is.

The guide number (GN) provides a rough indication of the flash power. A larger GN represents a higher flash power.
The GN for the built-in flash of the EOS 77D is approximately 12, while that for the professional-class Speedlite 600EX II-RT is about 60.
You can find the guide number for the external flash inside the instruction manuals.


Speedlite 600EX II-RT, Speedlite 430EX III-RT, Speedlite 270EX II


The GN specifications for the respective Speedlite flash units are shown as follows:

Speedlite 600EX II-RT
Guide Number: Approx. 60 (metres) at ISO 100 and flash angle 200mm

Speedlite 430EX III-RT
Guide Number: Approx. 43 (metres) at ISO 100 and flash angle 105mm

Speedlite 270EX II
Guide Number: Approx. 27 (metres) at ISO 100 and flash angle 28mm

Note: The flash angle (angle of coverage) stated is usually the maximum flash angle of the flash in terms of 35mm film-equivalent (full-frame) focal length.


How is the GN calculated?

The formula for calculating GN is:

GN = Distance of subject from flash (metres) × f-number

For example, for GN60, during full flash output at ISO 100, a subject that is 10m away can be illuminated at f/6, and 5m away at f/12.

However,  the GN is not an absolute measure of flash power. Flash power varies with the ISO speed and angle of coverage (flash angle).
When you compare the GN of different flash units, make sure the base ISO speed setting is the same. You will also need to take the flash angle into account.
For Canon, as with most other manufacturers, the guide number is usually based on ISO 100.


FAQ #3: How far will the light from my external flash reach?

A3: This depends on the aperture and ISO speed settings.

To calculate the maximum flash-to-subject distance (i.e. how far the light from the flash can reach) at a given aperture setting, use the following formula:


Guide number (GN) ÷ f-number

For example, when using an external flash with GN40 and lens aperture set to f/4, the light emitted can reach a distance of 40 (GN) ÷ 4 (f-number) = 10m.

You can increase the GN by raising the ISO speed of the camera or narrowing the flash angle using the zoom feature of the flash unit.

In other words, to get the furthest flash-to-subject distance possible, use a powerful flash, the maximum aperture possible and a higher ISO speed.

Note, however, that the auto flash feature may not function correctly if the subject is located too far away. The effective normal flash range for the professional-class Speedlite 600EX II-RT is up to approximately 30m (when using an EF50mm f/1.4 lens, at ISO 100).


Zoom adjustment setting on the flash

External flashes with a zoom adjustment feature allows the flash angle to be adjusted manually or automatically. Light from the flash will reach a further distance if the flash angle is smaller. In the screen shown above, the zoom adjustment setting is set to the maximum for the focal length of 105mm. The numbers in the red box show that the flash can reach a distance of up to 6.0m at aperture f/7.1.


To learn more about how to choose from different Canon external flashes, check out:
Which Canon Speedlite Flash to Choose?


To learn more about individual Speedlites, go to the following articles:
Buyer's Quick Guide: Speedlite 270EX II
Buyer's Quick Guide: Speedlite 430EX III-RT
Buyer's Quick Guide: Speedlite 600EX II-RT
Canon Unveils the Speedlite 470EX-AI: World's First Speedlite with AI Bounce Technology


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Digital Camera Magazine

Digital Camera Magazine

A monthly magazine that believes that enjoyment of photography will increase the more one learns about camera functions. It delivers news on the latest cameras and features and regularly introduces various photography techniques.
Published by Impress Corporation

Masakatsu Nagayama

Masakatsu Nagayama

Runs an advertising studio, and started being a freelance photographer in 1998, where he has been active mainly in the advertising scene, shooting for magazines and online media. His speciality lies in snapshots of city life.