a lens assembly comprises sequentially from an object side to an image side along an optical axis a first lens , a second lens , a stop , a third lens , a fourth lens and a fifth lens . the first lens is a biconvex lens with positive refractive power . the second lens is a meniscus lens with negative refractive power and the convex surface of second lens faces the object side . the third lens is a meniscus lens with negative refractive power and the convex surface of third lens faces the object side . the fourth lens is a meniscus lens with positive refractive power and the concave surface of fourth lens faces the object side . the fifth lens is a biconcave lens with negative refractive power . the lens assembly satisfies the following condition : 1 . 10 & lt ; d l1 / d st & lt ; 10 . 90 , wherein d l1 is an effective diameter of the first lens and d st is an effective diameter of the stop .