A lens assembly including 4˜7 lenses with a refractive power is provided. D1 is the diameter of a lens surface farthest away from the image plane of the lens assembly. DL is the diameter of a lens surface closest to the image plane of the lens assembly. LT is the length on an optical axis of the lens from the lens surface farthest away from the image plane of the lens assembly to the lens surface closest to the image plane of the lens assembly. The lens assembly satisfies the following conditions: (1) 6 mm 0.6 or (2) 6 mm 0.4.