How to Use a Flash

How to Use a Hotshoe Flash

Flash features are great but, how do you actually use a flash? Keep reading to learn how.

In a previous post I went through some of the most common and recommended features we may find in a hotshoe flash. It was not a comprehensive list, but more of a general view of hotshoe flashes. 

But, how do you actually use a flash on your camera’s hotshoe? It’s actually a matter of remembering a formula and some simple calculations, as you’ll see later in this post.

Even though I use other flashes, for this post and explanation I’m going to use a simple flash which doesn’t zoom, doesn’t tilt nor swivel and its powered by the camera, Fujifilm’s EF-X8.

Guide Number

While going through the common flash features, I explained that the main feature or characteristic in a flash is its Guide Number (GN). This is a measure of the flash power, and it’s described as the theoretical coverage distance of a flash under specific conditions.

The usual conditions used to describe this coverage distance are:

  • The flash is set to maximum power
  • The flash’s zoom is set to its longest setting
  • The camera is set to ISO 100
  • The lens has an aperture of f/1

That last bit may seem odd, because f/1 lenses are not common, but we need to remember these are all theoretical conditions. Once we use our own camera settings, we’ll be able to determine the actual coverage of a flash, use a special formula.

The Formula

The formula that describes how to determine the actual coverage distance goes like this:

Distance (d) = GN/ Lens Aperture (f)

Since the EF-X8 that I mentioned before has a GN of 8 meters (26 ft), and I plan to shoot a few images using a lens set at f/1.4, the actual coverage distance will be 8m/1.4 = 5.7m (18 ft).

In the example below, Jessica (my model) is standing 5.7m away from my camera and flash, so I set the camera to ISO 100, 1/250 s and pressed the shutter button.

How to Use Flash - 01
ISO 100, f/1.4, 1/250s, Flash at 1/1 (Full Power), Model 5.7m away

As you can see, she is correctly exposed, even though the rest of the scene looks a bit dark.

Now that I look at the scene, I want to shoot closer to Jessica, in order to make medium shot portraits. Considering the lens I’m using (35mm), she needs to be about 2 m away from the camera in order for the frame to be medium shot. 

In this case I know the distance, but I need to know the aperture to use and expose my model correctly. The formula changes a bit:

Lens Aperture (f) = GN/Distance (d)

So, the aperture I need to use is 8m/2m = f/4. So I set the aperture, press the shutter button and Jessica is correctly exposed.

How to Use Flash - 02
ISO 100, f/4, 1/250s, Flash at 1/1 (Full Power), Model 2m away

Power Settings?

But wait, what if I want to shoot at f/1.4, with my model 2 meters away from me, because I want a medium shot with a shallow depth of field? Well, it’s time for us to adjust the flash’s power settings.

Up to this point we’ve used the flash at maximum power, expressed as 1/1. Considering the current conditions (aperture) we can make a table that explains how we can use a lower power setting and how we need to compensate the lack of light coming from the flash with a wider aperture.

In this case (GN = 8 m, d = 2 m), the table goes like this.


According to the table, if I want to shoot at f/1.4, the flash power should be set to ⅛. So I set the flash power, opened the lens aperture and pressed the shutter again. Jessica is correctly exposed, again! 🙂

How to Use Flash - 03
ISO 100, f/1.4, 1/250s, Flash at 1/8 Power, Model 2 m away

As you can see using a flash is not super complex, but it takes time to calculate. Over time and with a lot of practice, you’ll be able to see a scene and determine the lens aperture and power setting of your flash intuitively.

My question for you is: Have you used a flash this way? What was your experience? Please, leave a comment below.

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