Patent ID: 8427765

Claim:
A fixed focal length lens system comprising: a first lens of a positive refractive power; a second lens of a negative refractive power; an aperture diaphragm; a third lens of a negative refractive power; a fourth lens of a positive refractive power; and a fifth lens of a positive refractive power; wherein: the first lens, the second lens, the diaphragm, the third lens, the fourth lens, and the fifth lens are arranged sequentially from an object side to an image side; the first lens, the second lens, the third lens, and the fourth lens are meniscus lenses having concave surfaces facing the diaphragm; and the following inequalities are satisfied: 0.6 ≤  R 6 R 4  ≤ 1.0 where R 4 denotes the radius of curvature of the surface of the second lens facing the image side, and R 6 is the radius of curvature of the surface of the third lens facing the object; and 1.1 ≤ R 7 R 8 < 3.5 where R 7 represents the radius of curvature of the surface of the third lens facing the image side, and R 8 represents the radius of curvature of the surface of the fourth lens facing the object side; wherein when refractive indexes of the third lens, the fourth lens, and the fifth lens are n3, n4, n5, respectively, the following inequality is satisfied: 1.80<( n 3+ n 4 +n 5)/3.