Flash zoom

…The focal length of your lens complicates this whole flash thing.…If you're using a very short focal length then you have a very wide field of view,…with a longer focal length you have a…narrower field of view, this is basic lens stuff.…With a narrower field of view though you don't…need light from the flash covering as wide an area,…whats more, you're probably shooting a subject that's farther…away so you need a longer reach with your flash.…Conversely, when you're shooting wide angle, you need the flash…to cover a very wide area.…If your flash coverage isn't wide enough to cover the whole area,…then you'll have a vignetting type darkness around the edges of your frame.…

These days most flashes have a built in zoom facility.…Optical elements within the flash head can zoom in or out…to focus the flash's output to a wider or narrower area.…When I turned the flash on just now,…you probably saw something inside the flash moving around.…If you're using a flash unit made by your camera…vendor, or one that's very compatible with your camera, then…