Speedlite Focal Length Settings
Most speedlites have the ability to focus the light into a narrower beam. This makes the speedlite more efficient and either allows the light to travel further or the speedlite to be used at a lower power setting to get the result you want.
Let’s look at an example. Let’s take a full-frame camera such as the Canon EOS 6D or the EOS 5D Mark IV fitted with a 24-70mm zoom lens and mount a Canon 600EX ii-RT Speedlite on top of the camera. The Canon 600EX ii-RT has a flash head zoom ranging from 20mm to 200mm. The speedlite head can be zoomed manually in steps of 20mm, 24mm, 28mm, 35mm, 50mm, 70mm, 80mm, 105mm, 135mm and 200mm. If we set the camera’s zoom lens to a 24mm focal distance and set the flash head zoom to the same setting of 24mm then when we take a photo with the flash turned on the light from the speedlite will cover exactly everything in the photo. i.e. the light from the speedlite will spread out just enough to cover everything that you can see through the viewfinder. The flash isn’t lighting everything else outside what you can see in the viewfinder because it’s not necessary and you would just be wasting light and hence flash power.
Obviously if something within the viewfinder scene is a long way away then the light from the flash won’t be bright enough to get a correct exposure but hopefully the main subject will be exposed correctly.
Now if you keep your flash head zoomed at 24mm (wide spread of light) and zoom your lens in to 70mm (narrower angle of view) everything in the viewfinder will still be covered by the flash but the flash is also lighting up much of the scene outside the image (outside what you can see in the viewfinder). If you are using a lens zoomed in to a focal length of 70mm then it’s much better to zoom the flash head into the same 70mm focal length or you’ll be wasting light around the edges. Makes sense doesn’t it. This is why most speedlites have an auto zoom setting and are able to communicate directly with your camera. When you zoom your lens to a particular setting then the speedlite will try and zoom into the same setting.
If you stay in manual zoom mode and zoom the speedlite to it’s narrowest beam (200mm) but you keep the camera’s lens at a focal length of 70mm then only a small central part of the image will be illuminated by the flash and you’ll get a spotlight effect.
Therefore a speedlite with a longer zoom range such as 200mm would be useful for photographers who plan to take photos of people at quite long distances using a 200mm lens. Great for paparazzi or event photographers where you can’t get close to the subject. It can also be used when shooting portraits outside during strong sunlight where a long lens and wide aperture are used to achieve a nice blurry background (bokeh) and the flash is used to fill in some go the hard and contrasty shadows caused by the direct sun (softens the lighting).
Of course if you haven’t got a speedlite with a 200mm zoom function a better method is to take the speedlite off you camera and move it closer to the subject. You will then have to manually zoom the flash head to the required zoom setting depending on the distance to the subject.
To produce a better quality softer light you can place your speedlite within a modifier or soft box. Since the speedlite head is no longer focusing a beam of light at the subject it’s best not to leave it on auto zoom. Instead it’s more important to fill your light modifier with a nice even spread of light to use a wider zoom setting of 20mm or 24mm to achieve better results.