Patent ID: 7974016

Claim:
A fixed focus lens comprising, numbered in order of location from an object side to an image side, a first lens, a second lens, a third lens, and a fourth lens, the first lens being a biconvex lens with a positive refractive power, the second lens being a convex-concave lens with a negative refractive power, the third lens being a concave-convex lens with a positive refractive power, the fourth lens being an aspherical lens having a negative refractive power in the centre portion of the fourth lens and a positive refractive power at the periphery portion of the fourth lens, wherein the condition 1.0>|(1/r 2 −1/r 1 )|>0.6 is satisfied, where r 1 denotes the radius of curvature of a front surface of the second lens facing the object side and r 2 denotes the radius of curvature of a rear surface of the first lens facing the image side, and the condition 0.45>BFL/TTL>0.35 is satisfied, where BFL denotes the back focal length, the BFL is a distance from a rear surface of the fourth lens to an image plane at the image side, and TTL denotes the total length of the fixed focus lens.