What is the guide number of a flash and how to measure the power?


A cobra flash is an exceptional working companion, for the improvements it implies, not only in a photo studio but also in all work areas and all disciplines that lend themselves to it. But it would seem that some photographers do not dare to work with this accessory for the “complication in use that it implies” and for the terminology that it encompasses, of which the guide number is part, a term that ends up scaring us. Today you will discover what the guide number of a cobra flash is and how to measure the power of the latter.

What is the guide number of a flash cobra?

Previously, we saw in the blog the basic concepts to learn how to use a shoe flash, the types of flashes that exist and the aspects to take into account before acquiring one. A must post if you are considering buying an external flash.

We also explained to you which trigger modes a cobra flash can offer you, to practice the most varied disciplines and give creativity to your photo shoots. But today, we are going to detail what exactly is the guide number of a shoe flash, also called GN or guide number.

We also made some references to the concept of the guide number, as well as other advanced concepts on hot shoe flashes but today, we will deepen them. This seems very complicated to understand but it is not. The guide number of a flash refers to the power of the latter, which is a crucial element to know before choosing one flash over another.

Hot shoe flashes are essential accessories for anyone who wants to improve the lighting in their photos. The more powerful it is, the better the lighting will be on your photo shoots and therefore on your work too. This is undoubtedly the definitive step to take to become a better photographer.

Even though our cameras have a built-in flash in their design, it’s usually not enough as an external flash is always a great alternative. Although not all offer the same power and “distance” necessary, that’s why you need to know the guide number of your flash to determine the perfect exposure for each release.

The larger the guide number of a flash, the more powerful it will be. The power of the flash will allow you to illuminate the subject or the object in question according to different exposure configurations.

Relationship between guide number and flash zoom

Nowadays, most flashes on the market have a zoom head, such as the Gloxy GX-F1000 flash. These heads allow you to adapt the angle of the flash produced by the flash and then direct it to a specific point. This makes it easy to get the lighting and exposure you want.

When working with wide-angle lenses or with a reduced focal length, such as the DZOFilm Vespids, these flashes allow you to adapt the flash so that it is larger and covers the whole image, whereas if you use long focal lengths, as with a telephoto lens, the flash will direct the light to a specific plane, i.e. a smaller light angle. When the zoom of the cobra flash adapts to telephoto lenses, the guide number will increase since the flash will go further.

The greater the zoom value, the greater the guide number of a flash because we end up reducing the angle of the flash produced by the flash. Flash that focuses on moving towards the subject of the shot.

The guide number will vary depending on the focal length and how far away the subjects or objects are from the camera. Having the flash adapt to the focal length used will not only improve the lighting in your shots but also reduce vignetting and save energy.

Using a flash equipped with a zoom head gives great creative freedom with regard to the light in the shots. So imagine taking portraits with short focal lengths but with the zoom head, as if it were a telephoto lens. The flash light would produce a creative effect on the model’s face, while the rest of the scene would be dark, for a very attractive image.