A lens assembly includes a first lens group, a second lens group and an aperture. The first lens group includes two lenses with refractive power. The second lens group with a positive refractive power includes at least two lenses with refractive power. The aperture is disposed between the first lens group and the second lens group. The lens assembly includes 5 to 8 lenses with refractive power. D1 is the diameter of the lens surface of the first lens group farthest from the second lens group. LT is the length on the optical axis of the lens from the lens surface farthest from the imaging plane of the lens assembly to the lens surface closest to the imaging plane of the lens assembly. IMH is the maximum imaging height of lens assembly on the imaging plane, wherein the lens assembly satisfies conditions: 3.5