The 18-55mm kit lens is an f/3.5-5.6 lens. What this means is that the largest aperture the lens is capable of is f/3.5 @ 18mm if you zoom out to 55mm the largest aperture your lens is capable of is f/5.6, so you will never get this lens to f/2.8, but at wider FOV you can open up about 1 1/3 stop, however you'll get best results from the kit lens stopped down to around f/8, it doesn't do all that well wide open.