Patent ID: 7095570

Claim:
An image forming optical system comprising: in order from an object side, a first lens which is a negative lens having a concave surface directed toward an object side, an aperture stop, a second lens which is a double convex lens, a third lens which is a negative meniscus lens having a convex surface directed toward an image side, and a fourth lens which is a positive lens, wherein a surface at an object side of the fourth lens is an aspherical surface, a radius of curvature of which decreases from a center portion toward a circumferential portion and a surface at an image side of the fourth lens is an aspherical surface, a radius of curvature of which increases from a center portion toward a circumferential portion, and the following condition is satisfied: 0.05<φ m/φp< 0.80 where φm represents the power of the fourth lens at the position of the maximum light height and φp represents the power of the fourth lens at the position of the praxis.