Flash zoom refers to how wide your flash will spread the light. Some flash units do not have a zoom option. In an automatic flash zoom mode, the flash will detects the focal length of the lens mounted on your camera body, and adjust the light spread the flash emits to match the angle of view of your lens.
To do that, the flash changes the distance between the actual flash tube (the light) and the diffuser (the plastic front of the flash). If you’re using a zoom lens, some flashes can re-adjust this distance as you zoom in or out to matching the new focal length. If you are using a prime lens then the flash obviously only needs one adjustment.
Different flash brands and models offer varying zoom range coverage. For example Canon’s 430 and 580 series offer a usable focal length range of 24mm to 105mm when used with a compatible EOS lens on a full frame (35mm) DSLR. If you’re using a smaller DX (also called APS-C) sensored camera, the usable range covered is 15mm to 65mm.
You can also set your flash to manual zoom mode. This way you can dictate the zoom zone and set your flash spread to the available zoom setting closest to your lens’ focal length. You can also use manual flash zoom for creative purposes by setting your flash zoom to a different viewing angle than that of your lens to control the spread of light. This allows you to zoom your flash in, and illuminate a specific part of your scene.
