Patent ID: 8184388

Claim:
An image forming optical system comprising: at least one cemented lens, wherein the cemented lens includes a first lens element e 1 , a second lens element e 2 , and a third lens element e 3 , and the first lens element e 1 is cemented to a surface on one side of the second lens element e 2 , and the third lens element e 3 is cemented to the other surface of the second lens element e 2 , and the first lens element e 1 is a positive lens, and a combined refracting power of the second lens element e 2 and the third lens element e 3 is negative, and the cemented lens satisfies the following conditional expression (1) 2.15 <Neff− 0.01 νeff< 15.0 (1) where, Neff denotes an effective refractive index when the cemented lens is assumed to be one lens, and is expressed by the following expression N eff = 1 P · F eff ⁢ ⁢ where ⁢ ⁢ P = ∑ i = 1 3 ⁢ 1 n i ⁢ f i ⁢ ⁢ 1 F eff = ∑ i = 1 3 ⁢ 1 f i and ν eff is an effective Abbe's number when the cemented lens is assumed to be one lens, and is expressed by the following expression v eff = 1 F eff · ∑ i = 1 3 ⁢ 1 f i · v d , i where, P denotes Petzval's sum, F eff is a combined focal length of the cemented lens when three lenses are in close contact as the cemented lens, and each lens element is let to be thin, n i is a refractive index of a lens element which forms the cemented lens, f i is a focal length of a lens element which forms the cemented lens, and ν d,i is Abbe's number of the lens element which forms the cemented lens, wherein the lens unit having at least the one cemented lens includes at least one positive lens et apart from the cemented lens, and the image forming optical system satisfies the following conditional expressions (2) and (3-2) 0.73< Neff−Ndet< 13 (2) 44.93≦ν det−νeff< 100 (3-2) where, Ndet and ν det denote a refractive index and Abbe's number respectively of the positive lens et.