Patent ID: 8289629

Claim:
An imaging optical system, characterized by comprising, in order from an object side thereof, a first lens group having positive refracting power, a stop, a second lens group having positive refracting power and a third lens group having negative refracting power, wherein: said first lens group comprises a cemented lens consisting of a first lens and a second lens; said second lens group comprises a third lens; said third lens group comprises a fourth lens, wherein said fourth lens has an object-side surface defined by a concave surface when viewed from the object side on and off an optical axis of the optical system, and an image-side surface defined by an aspheric surface that is convex when viewed from the object side on the optical axis and has one or more inflection points off the optical axis; said cemented lens is made up by cementing together said first lens formed of a resin layer and said second lens formed of a resin layer; said first lens is a positive lens; said second lens satisfies the following condition (5); and said cemented lens satisfies the following condition (6): 0.05â‰¦tâ‰¦0.2â€ƒâ€ƒ(5) 20 â‰¦v 2 âˆ’v 1â‰¦40â€ƒâ€ƒ(6) where t is a thickness (in mm) of said second lens, v 1 is an Abbe constant of said second lens, and v 2 is an Abbe constant of said first lens.