The invention provides an external lens, which sequentially comprises a first lens, a second lens, a third lens, a fourth lens, a fifth lens, a sixth lens and a diaphragm from an object side to an imaging surface along an
optical axis, wherein the first lens has positive focal power, the object side surface of the first lens is a convex surface, and the image side surface of the first lens is a
concave surface; the second lens has focal power, the object side surface of the second lens is a
concave surface near the
optical axis, and the image side surface of the second lens is a convex surface near the
optical axis; the third lens has negative focal power, and the image side surface of the third lens is a
concave surface; the fourth lens has positive focal power, and the object side surface of the fourth lens is a convex surface; the fifth lens and the sixth lens form a cemented lens group, the fifth lens has negative focal power, and the image side surface of the fifth lens is a concave surface; the sixth lens has negative focal power, the object side surface of the sixth lens is a convex surface, and the image side surface of the sixth lens is a convex surface; the external lens satisfies the following conditional expressions: 0.1 < f / TTL < 0.2, wherein f represents the effective
focal length of the external lens, and TTL represents the total optical length of the external lens.