Patent ID: 7944627

Claim:
An image forming optical system comprising a first lens which is a biconvex lens, a second lens which is a biconcave lens, a third lens which is a positive meniscus lens convex to the image side and a fourth lens which is a negative meniscus lens convex to the object side, arranged from the object side to the image side, wherein a field stop is located on the object side of the image side plane of the first lens and assuming that fi represents an absolute value of focal length of the i-th lens, f represents an absolute value of focal length of the whole optical system, v di represents Abbe's number of the i-th lens and TTL represents a distance between the plane which is closer to the object between the object side plane of the first lens and the field stop plane and the image plane, the relations 0.425< f 1 /f 4 <0.7 (1) 25<v d2 <28 (2) 50<v d1 ,v d3 ,v d4 <57 (3) 0.595< f 2 /f 4 <1.05 (4) 1.5< f/f 1 <2.2 (5) f 1 <f 3 (6) 1.05< TTL/f< 1.18 (7) are satisfied.