In photography, there are two types of lenses: zoom lenses and prime lenses. Zoom lenses cover a range of focal lengths. If you’ve just bought a new DSLR, the kit lens that comes with your camera is almost certainly a zoom lens; most come with an 18-55mm lens, which means that lens covers every focal length between 18mm and 55mm. A prime lens, on the other hand, covers just one focal length. A 50mm lens is just a 50mm lens.