With zoom lenses, you can adjust the focal length by moving glass lenses inside the lens body, typically by using a rotating or sliding sleeve on the external barrel of the lens. A prime lens, on the other hand, has a fixed focal length. The name prime comes from the sole focal length, with only one measurement. A prime lens can be wide-angle, normal, or telephoto.
A standard zoom lens might include infinite gradations of focal length between 24 mm and 70 mm. A fixed prime lens has only one measurement, such as 50 mm. While a zoom lens can be adjusted to capture more of a scene or to focus on up-close details, a prime lens cannot. So why would you want a prime lens?
Despite the flexibility of zoom lenses, prime lenses remain popular because of their durability and lower price. Prime lenses also perform better in low-light conditions and are lightweight.