A lens assembly includes a first lens, a second lens, a third lens, a fourth lens and a fifth lens, all of which are arranged in order from an object side to an image side along an optical axis. The first lens includes a convex surface facing the image side. The second lens includes a concave surface facing the object side. The third lens is a biconvex lens with positive refractive power. The fourth lens is with negative refractive power and includes a concave surface facing the object side. The fifth lens is with positive refractive power and includes a convex surface facing the image side. The lens assembly satisfies: −20≤f1/f≤2, wherein f1 is an effective focal length of the first lens and f is an effective focal length of the lens assembly.