Patent ID: 8704936

Claim:
An image forming optical system of four lenses, comprising in order from an object side: a first lens having a positive refractive power, which is a biconvex lens; a second lens having a negative refractive power, of which, an image-side surface is a concave surface; a third lens having a positive refractive power, which is meniscus-shaped with a concave surface directed toward the object side; a fourth lens having a negative refractive power, of which, an image-side surface is a concave surface; and an aperture stop which is disposed nearest to the object side, wherein the image forming optical system satisfies the following conditional expressions (1) and (2) θ9<48 (1) TL/ 2 Y< 0.8 (2) where, θ9 denotes the maximum value (unit: degree) of an angle between a plane-normal over an entire area in an effective plane of the image-side surface of the fourth lens, TL denotes a distance from a surface vertex of the first lens up to an image plane, and Y denotes the maximum image height.